What uses are there for placement new

Question

Has anyone here ever used C   s  placement new   If so  what for  It looks to me like it would only be useful on memory-mapped hardware

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It s useful if you want to separate allocation from initialization  STL uses placement new to create container elements

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I ve used it to construct objects allocated on the stack via alloca     shameless plug  I blogged about it here

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Script engines can use it in the native interface to allocate native objects from scripts  See Angelscript  www angelcode com angelscript  for examples

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I ve used it for storing objects with memory mapped files  The specific example was an image database which processed vey large numbers of large images  more than could fit in memory

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I ve used it to create objects based on memory containing messages received from the network

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I think this has not been highlighted by any answer  but another good example and usage for the new placement is to reduce the memory fragmentation  by using memory pools   This is specially useful in embedded and high availability systems  In this last case it s specially important because for a system that has to run 24 365 days it s very important to have no fragmentation  This problem has nothing to do with memory leakage    Even when a very good malloc implementation is used  or similar memory management function  it s very difficult to deal with fragmentation for a long time  At some point if you don t manage cleverly the memory reservation release calls you could end up with a lot of small gaps that are difficult to reuse  assign to new reservations   So  one of the solutions that are used in this case is to use a memory pool to allocate before hand the memory for the application objects  After-wards each time you need memory for some object you just use the new placement to create a new object on the already reserved memory    This way  once your application starts you already have all the needed memory reserved  All the new memory reservation release goes to the allocated pools  you may have several pools  one for each different object class   No memory fragmentation happens in this case since there will no gaps and your system can run for very long periods  years  without suffering from fragmentation    I saw this in practice specially for the VxWorks RTOS since its default memory allocation system suffers a lot from fragmentation  So allocating memory through the standard new malloc method was basically prohibited in the project  All the memory reservations should go to a dedicated memory pool

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Placement new allows you to construct an object in memory that s already allocated  You may want to do this for optimization when you need to construct multiple instances of an object  and it is faster not to re-allocate memory each time you need a new instance   Instead  it might be more efficient to perform a single allocation for a chunk of memory that can hold multiple objects  even though you don t want to use all of it at once  DevX gives a good example   Standard C   also supports placement new operator  which constructs an object on a pre-allocated buffer  This is useful when building a memory pool  a garbage collector or simply when performance and exception safety are paramount  there s no danger of allocation failure since the memory has already been allocated  and constructing an object on a pre-allocated buffer takes less time    char  buf    new char sizeof string       pre-allocated buffer string  p   new  buf  string  quot hi quot          placement new string  q   new string  quot hi quot                ordinary heap allocation  You may also want to be sure there can be no allocation failure at a certain part of critical code  for instance  in code executed by a pacemaker    In that case you would want to allocate memory earlier  then use placement new within the critical section  Deallocation in placement new You should not deallocate every object that is using the memory buffer   Instead you should delete   only the original buffer   You would have to then call the destructors of your classes manually   For a good suggestion on this  please see Stroustrup s FAQ on  Is there a  quot placement delete quot

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It s used by std  vector lt  gt  because std  vector lt  gt  typically allocates more memory than there are objects in the vector lt  gt

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I have one more idea  it is valid for C  11   Let s look at the following example   include  lt cstddef gt   include  lt cstdio gt   int main         struct alignas 0x1000  A           char data 0x1000               printf  quot max align t   zu n quot   alignof max align t         A a      printf  quot a   p n quot    amp a        A  ptr   new A      printf  quot ptr   p n quot   ptr       delete ptr     With C  11 standard  GCC gives the following output  max align t  16 a  0x7ffd45e6f000 ptr  0x1fe3ec0  ptr is not aligned properly  With C  17 standard and further  GCC gives the following output  max align t  16 a  0x7ffc924f6000 ptr  0x9f6000  ptr is aligned properly  As I know  C   standard didn t support over-aligned new before C  17 came  and if your structure has alignment greater than max align t  you can have problems  To bypass this issue in C  11  you can use aligned alloc   include  lt cstddef gt   include  lt cstdlib gt   include  lt cstdio gt   include  lt new gt   int main         struct alignas 0x1000  A           char data 0x1000               printf  quot max align t   zu n quot   alignof max align t         A a      printf  quot a   p n quot    amp a        void  buf   aligned alloc alignof A   sizeof A        if  buf    nullptr            printf  quot aligned alloc   failed n quot            exit 1             A  ptr   new buf  A        printf  quot ptr   p n quot   ptr       ptr- gt  A        free ptr      ptr is aligned in this case  max align t  16 a  0x7ffe56b57000 ptr  0x2416000

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See the fp h file in the xll project at http   xll codeplex com It solves the  unwarranted chumminess with the compiler  issue for arrays that like to carry their dimensions around with them   typedef struct  FP       unsigned short int rows      unsigned short int columns      double array 1             Actually  array rows  columns       FP

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It s actually kind of required to implement any kind of data structure that allocates more memory than minimally required for the number of elements inserted  i e   anything other than a linked structure which allocates one node at a time    Take containers like unordered map  vector  or deque  These all allocate more memory than is minimally required for the elements you ve inserted so far to avoid requiring a heap allocation for every single insertion  Let s use vector as the simplest example   When you do   vector lt Foo gt  vec      Allocate memory for a thousand Foos  vec reserve 1000         that doesn t actually construct a thousand Foos  It simply allocates reserves memory for them  If vector did not use placement new here  it would be default-constructing Foos all over the place as well as having to invoke their destructors even for elements you never even inserted in the first place   Allocation    Construction  Freeing    Destruction  Just generally speaking to implement many data structures like the above  you cannot treat allocating memory and constructing elements as one indivisible thing  and you likewise cannot treat freeing memory and destroying elements as one indivisible thing   There has to be a separation between these ideas to avoid superfluously invoking constructors and destructors unnecessarily left and right  and that s why the standard library separates the idea of std  allocator  which doesn t construct or destroy elements when it allocates frees memory   away from the containers that use it which do manually construct elements using placement new and manually destroy elements using explicit invocations of destructors         I hate the design of std  allocator but that s a different subject I ll avoid ranting about   -D      So anyway  I tend to use it a lot since I ve written a number of general-purpose standard-compliant C   containers that could not be built in terms of the existing ones  Included among them is a small vector implementation I built a couple decades ago to avoid heap allocations in common cases  and a memory-efficient trie  doesn t allocate one node at a time   In both cases I couldn t really implement them using the existing containers  and so I had to use placement new to avoid superfluously invoking constructors and destructors on things unnecessary left and right   Naturally if you ever work with custom allocators to allocate objects individually  like a free list  then you d also generally want to use placement new  like this  basic example which doesn t bother with exception-safety or RAII    Foo  foo   new free list allocate    Foo           foo- gt  Foo    free list free foo

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Head Geek  BINGO   You got it totally - that s exactly what it s perfect for   In many embedded environments  external constraints and or the overall use scenario forces the programmer to separate the allocation of an object from its initialization   Lumped together  C   calls this  instantiation   but whenever the constructor s action must be explicitly invoked WITHOUT dynamic or automatic allocation  placement new is the way to do it   It s also the perfect way to locate a global C   object that is pinned to the address of a hardware component  memory-mapped I O   or for any static object that  for whatever reason  must reside at a fixed address

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Script engines can use it in the native interface to allocate native objects from scripts  See Angelscript  www angelcode com angelscript  for examples

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The one place I ve run across it is in containers which allocate a contiguous buffer and then fill it with objects as required  As mentioned  std  vector might do this  and I know some versions of MFC CArray and or CList did this  because that s where I first ran across it   The buffer over-allocation method is a very useful optimization  and placement new is pretty much the only way to construct objects in that scenario  It is also used sometimes to construct objects in memory blocks allocated outside of your direct code   I have used it in a similar capacity  although it doesn t come up often  It s a useful tool for the C   toolbox  though

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I ve used it to create a Variant class  i e  an object that can represent a single value that can be one of a number of different types    If all of the value-types supported by the Variant class are POD types  e g  int  float  double  bool  then a tagged C-style union is sufficient  but if you want some of the value-types to be C   objects  e g  std  string   the C union feature won t do  as non-POD datatypes may not be declared as part of a union   So instead I allocate a byte array that is big enough  e g  sizeof the largest data type I support   and use placement new to initialize the appropriate C   object in that area when the Variant is set to hold a value of that type     And placement delete beforehand when switching away from a different non-POD data type  of course

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It is useful if you are building a kernel - where do you place the kernel code you read from disk or the pagetable  You need to know where to jump to   Or in other  very rare circumstances such as when you have loads of allocated room and want to place a few structures behind each other  They can be packed this way without the need for the offsetof   operator  There are other tricks for that too  though   I also believe some STL implementations make use of placement new  like std  vector  They allocate room for 2 n elements that way and don t need to always realloc

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Placement new allows you to construct an object in memory that s already allocated  You may want to do this for optimization when you need to construct multiple instances of an object  and it is faster not to re-allocate memory each time you need a new instance   Instead  it might be more efficient to perform a single allocation for a chunk of memory that can hold multiple objects  even though you don t want to use all of it at once  DevX gives a good example   Standard C   also supports placement new operator  which constructs an object on a pre-allocated buffer  This is useful when building a memory pool  a garbage collector or simply when performance and exception safety are paramount  there s no danger of allocation failure since the memory has already been allocated  and constructing an object on a pre-allocated buffer takes less time    char  buf    new char sizeof string       pre-allocated buffer string  p   new  buf  string  quot hi quot          placement new string  q   new string  quot hi quot                ordinary heap allocation  You may also want to be sure there can be no allocation failure at a certain part of critical code  for instance  in code executed by a pacemaker    In that case you would want to allocate memory earlier  then use placement new within the critical section  Deallocation in placement new You should not deallocate every object that is using the memory buffer   Instead you should delete   only the original buffer   You would have to then call the destructors of your classes manually   For a good suggestion on this  please see Stroustrup s FAQ on  Is there a  quot placement delete quot

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It may be handy when using shared memory  among other uses    For example  http   www boost org doc libs 1 51 0 doc html interprocess synchronization mechanisms html interprocess synchronization mechanisms conditions conditions anonymous example

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It s also useful when you want to re-initialize global or statically allocated structures    The old C way was using memset   to set all elements to 0  You cannot do that in C   due to vtables and custom object constructors   So I sometimes use the following   static Mystruct m    for                 re-initialize the structure  Note the use of placement new         and the extra parenthesis after Mystruct to force initialization       new   amp m  Mystruct             do-some work that modifies m s content

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It is useful if you are building a kernel - where do you place the kernel code you read from disk or the pagetable  You need to know where to jump to   Or in other  very rare circumstances such as when you have loads of allocated room and want to place a few structures behind each other  They can be packed this way without the need for the offsetof   operator  There are other tricks for that too  though   I also believe some STL implementations make use of placement new  like std  vector  They allocate room for 2 n elements that way and don t need to always realloc

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Has anyone here ever used C   s  quot placement new quot   If so  what for  It looks to me like it would only be useful on memory-mapped hardware   It s really useful when needing to copy out  pass as outputs    non-copyable objects  ex  where operator    has been automatically deleted because the class contains a const member  OR non-trivially-copyable objects  where using memcpy   is undefined behavior      from within a function  This  obtaining these non-copyable or non-trivially-copyable objects from a function  could aid in unit testing that function  by allowing you to see that a certain data object now looks a certain way after being processed by that function  OR it could simply be a part of your normal API for whatever use you see fit  Let s go over these examples and explain in detail what I mean and how  quot placement new quot  can be used to solve these problems  TLDR  Note  I ve tested every single line of code in this answer  It works  It is valid  It does not violate the C   standard  Placement new is   The replacement in C   for   when operator     the assignment operator  is deleted  and you need to  quot copy quot   actually copy-construct  a therefore otherwise non-copyable object into a given memory location  The replacement in C   for memcpy   when your object is not trivially-copyable  meaning that using memcpy   to copy this non-trivially-copyable object  quot may be undefined quot    Important  a  quot non-copyable quot  object is NOT truly non-copyable  It is simply not copyable via the   operator is all  which is a call to a class s underlying operator    overload function  This means that when you do B   C   what is actually happening is a call to B operator  C    and when you do A   B   C   what is actually happening is A operator  B operator  C     Therefore   quot non-copyable quot  objects are only copyable via other means  such as via the class s copy constructor  since  again  the class has no operator    method   quot Placement new quot  can be used to call any one of the many constructors which may exist in a class in order to construct an object into a desired pre-allocated memory location  Since  quot placement new quot  syntax allows for calling any constructor in a class  this includes passing it an existing instance of a class in order to have placement new call a class s copy-constructor to copy-construct a new object from the passed-in object into another place in memory  Copy-constructing one object into another place in memory   is a copy  This action creates a copy of that original object  When done  you CAN have two objects  instances   that are byte-identical  literally byte for byte  depending on your copy constructor s implementation   located in two separate places in memory  That  by definition  is a copy  It just wasn t done using the class s operator    method is all  Therefore  one might define a class as being  quot non-copyable quot  if it has no operator    method  but it is still very-much copyable  legally  according to the C   standard and mechanisms provided by C    safely and without undefined behavior  using its copy constructor and placement new syntax  as demonstrated below  Reminder  all lines of code below work  You can run much of the code right here  incl  many of the code blocks below  although it may require some commenting uncommenting blocks of code since it is not cleanly set up into separate examples  1  What is a non-copyable object  A non-copyable object cannot be copied with the   operator  operator    function   That s it  It can still be legally copied  however  See the really important note just above  Non-copyable class Example 1  Here  copy-constructing is fine  but copying is prohibited since we have explicitly deleted the assignment operator  Trying to do nc2   nc1  results in this compile-time error   error  use of deleted function    NonCopyable1 amp  NonCopyable1  operator  const NonCopyable1 amp        Here s the full example   include  lt stdio h gt   class NonCopyable1   public      int i   5          Delete the assignment operator to make this class non-copyable      NonCopyable1 amp  operator  const NonCopyable1 amp  other    delete      int main         printf  quot Hello World n quot             NonCopyable1 nc1      NonCopyable1 nc2      nc2   nc1       copy assignment  compile-time error      NonCopyable1 nc3   nc1     copy constructor  works fine       return 0     Non-copyable class Example 2  Here  copy-constructing is fine  but copying is prohibited since the class contains a const member  which can t be written to  supposedly  since there are work-arounds obviously   Trying to do nc2   nc1  results in this compile-time error   error  use of deleted function    NonCopyable1 amp  NonCopyable1  operator  const NonCopyable1 amp      note     NonCopyable1 amp  NonCopyable1  operator  const NonCopyable1 amp      is implicitly deleted because the default definition would be ill-formed  error  non-static const member    const int NonCopyable1  i     can   t use default assignment operator   Full example   include  lt stdio h gt   class NonCopyable1   public      const int i   5     classes with  const  members are non-copyable by default     int main         printf  quot Hello World n quot             NonCopyable1 nc1      NonCopyable1 nc2      nc2   nc1       copy assignment  compile-time error      NonCopyable1 nc3   nc1     copy constructor  works fine       return 0     So  if a class is non-copyable  you can NOT do the following to get a copy of it as an output  The line outputData   data  will cause compilation to fail with the previous error messages shown in the last example just above   include  lt functional gt    include  lt stdio h gt   class NonCopyable1   public      const int i     classes with  const  members are non-copyable by default         Constructor to custom-initialize  i      NonCopyable1 int val   5    i val                    nothing else to do               Some class which  perhaps asynchronously  processes data  You attach a     callback  which gets called later      - Also  this may be a shared library over which you have no or little     control  so you cannot easily change the prototype of the callable callback     function   class ProcessData   public      void attachCallback std  function lt void void  gt  callable                callback    callable                 void callCallback                 callback            private      std  function lt void void  gt  callback       int main         printf  quot Hello World n quot             NonCopyable1 outputData     we need to receive back data through this object     printf  quot outputData i  before     i n quot   outputData i      is 5          ProcessData processData         Attach a lambda function as a callback  capturing  outputData  by         reference so we can receive back the data from inside the callback via         this object even though the callable prototype returns  void   is a          void void   callable function       processData attachCallback   amp outputData                          int someRandomData   999              NonCopyable1 data someRandomData                  NOT ALLOWED SINCE COPY OPERATOR  Assignment operator  WAS                 AUTO-DELETED since the class has a  const  data member              outputData   data                   processData callCallback           verify we get 999 here  NOT 5      printf  quot outputData i  after     i n quot   outputData i         return 0     One solution  memcpy the data into the outputData  This is perfectly acceptable in C  but not always ok in C    Cppreference com states  emphasis added    If the objects are potentially-overlapping or not TriviallyCopyable  the behavior of memcpy is not specified and may be undefined   and   Notes Objects of trivially-copyable types that are not potentially-overlapping subobjects are the only C   objects that may be safely copied with std  memcpy or serialized to from binary files with std  ofstream  write   std  ifstream  read      https   en cppreference com w cpp string byte memcpy  So  let s just be safe and ensure an object is trivially copyable before copying it with memcpy    Replace this part above      processData attachCallback   amp outputData                          int someRandomData   999              NonCopyable1 data someRandomData                  NOT ALLOWED SINCE COPY OPERATOR  Assignment operator  WAS                 AUTO-DELETED since the class has a  const  data member              outputData   data                with this  Notice the usage of memcpy   to copy out the data this time  and std  is trivially copyable to ensure  at compile time  this type truly is safe to copy with memcpy             added to top       include  lt cstring gt      for  memcpy         include  lt type traits gt     for  std  is trivially copyable lt  gt             Attach a lambda function as a callback  capturing  outputData  by         reference so we can receive back the data from inside the callback via         this object even though the callable prototype returns  void   is a          void void   callable function       processData attachCallback   amp outputData                          int someRandomData   999              NonCopyable1 data someRandomData               static assert std  is trivially copyable lt NonCopyable1 gt   value   quot NonCopyable1 must  quot                   quot be a trivially-copyable type in order to guarantee that  memcpy    is safe  quot                   quot to use on it  quot                memcpy  amp outputData   amp data  sizeof data                 Sample program output now that it can compile  amp  run  It works   Hello World outputData i  before    5 outputData i  after    999   To be extra safe  however  you should manually call the destructor of the object you re overwriting before overwriting it  like this  BEST MEMCPY   SOLUTION      processData attachCallback   amp outputData                          int someRandomData   999              NonCopyable1 data someRandomData               static assert std  is trivially copyable lt NonCopyable1 gt   value   quot NonCopyable1 must  quot                   quot be a trivially-copyable type in order to guarantee that  memcpy    is safe  quot                   quot to use on it  quot                outputData  NonCopyable1       manually call destructor before overwriting this object             memcpy  amp outputData   amp data  sizeof data                 If the static assert   above fails  however  then you shouldn t use memcpy    An always-safe and better C   alternative  therefore  is to use  quot placement new quot   Here  we simply copy-construct data right into the memory region occupied by outputData  That s what this  quot placement new quot  syntax does for us  It does NOT dynamically allocate memory  like the new operator normally does  Normally  the new operator first dynamically allocates memory on the heap and then constructs an object into that memory by calling the object s constructor  However  placement new does NOT do the allocation part  Instead  it simply skips that part and constructs an object into memory at an address you specify  YOU have to be the one to allocate that memory  either statically or dynamically  beforehand  and YOU have to ensure that memory is properly aligned for that object  see alignof and alignas and the Placement new example here   it will be in this case since we explicitly created the outputData object as an object  calling it s constructor with NonCopyable1 outputData    and YOU have to ensure that the memory buffer pool is large enough to hold the data you are about to construct into it  So  the generic placement new syntax is this     Call T  s specified constructor below  constructing it as an object right into    the memory location pointed to by  ptr to buffer   No dynamic memory allocation    whatsoever happens at this time  The object  T  is simply constructed into this    address in memory  T  ptr to T   new ptr to buffer  T optional input args to T s constructor    In our case  it will look like this  calling the copy constructor of the NonCopyable1 class  which we ve already proven repeatedly above is valid even when the assignment copy operator is deleted     copy-construct  data  right into the address at   amp outputData   using placement new syntax new  amp outputData  NonCopyable1 data     Our final attachCallback lambda now looks like this  with the placement new syntax in place of memcpy    Notice that the check to ensure the object is trivially copyable is no longer required whatsoever      gt  BEST C   SOLUTION ALL-AROUND--AVOIDS MEMCPY BY COPY-CONSTRUCTING DIRECTLY INTO THE TARGET MEMORY LOCATION USING PLACEMENT NEW   lt      USE THIS           processData attachCallback   amp outputData                          int someRandomData   999              NonCopyable1 data someRandomData               outputData  NonCopyable1       manually call destructor before overwriting this object                copy-construct  data  right into the address at   amp outputData   using placement new syntax             new  amp outputData  NonCopyable1 data                    Assume that  data  will be further manipulated and used below now  but we needed                its state at this moment in time                   Note also that under the most trivial of cases  we could have also just called                out custom constructor right here too  like this  You can call whatever                constructor you want                 new  amp outputData  NonCopyable1 999                                    2  What is a non-trivially-copyable object  A non-trivially-copyable object may be one which contains virtual methods and things  as this can lead to the class having to track  quot vee pointers quot   vptr  and  quot vee tables quot   vtbls   to point to the proper virtual implementation in memory  Read more about that here  Dr  Dobb s  quot Storage Layout of Polymorphic Objects quot   However  even in this case  so long as you re memcpy  ing from the same process to the same process  ie  within the same virtual memory space   and NOT between processes  and NOT deserializing from disk to RAM  it seems to me that memcpy   would technically work fine and produce no bugs  and I ve proven this in a handful of examples to myself   but it technically seems to be behavior which is not defined by the C   standard  so therefore it is undefined behavior  so therefore it can t be relied upon 100  from compiler to compiler  and from one version of C   to the next  so   it is undefined behavior and you shouldn t memcpy   in that case  In other words  if the static assert std  is trivially copyable lt NonCopyable1 gt   value   check fails above  do NOT use memcpy    You must use  quot placement new quot  instead  One way to get that static assert to fail is to simply declare or define a custom copy assignment operator in your class definition for your NonCopyable1 class  like this     Custom copy assignment operator declaration  NonCopyable1 amp  operator  const NonCopyable1 amp  other       OR      Custom copy assignment operator definition  NonCopyable1 amp  operator  const NonCopyable1 amp  other           Check for    and don t allow    self assignment          ie  only copy the contents from the other object         to this object if it is not the same object  ie  if it is not         self-assignment       if this     amp other                    copy all non-const members manually here  if the class had any  ex             j   other j             k   other k             etc             Do deep copy of data via any member   pointers    if such members exist               the assignment function   operator      expects you to return the         contents of your own object  the left side   passed by reference  so         that constructs such as  test1   test2   test3   are valid         See this reference  from Stanford  p11  here          http   web stanford edu class archive cs cs106b cs106b 1084 cs106l handouts 170 Copy Constructor Assignment Operator pdf             MyClass one  two  three              three   two   one      return  this       For more examples on custom copy constructors  assignment operators  etc  and the  quot Rule of Three quot  and  quot Rule of Five quot   see my hello world repository and example here   So  now that we have a custom assignment operator  the class is no longer trivially copyable  and this code      processData attachCallback   amp outputData                          int someRandomData   999              NonCopyable1 data someRandomData               static assert std  is trivially copyable lt NonCopyable1 gt   value   quot NonCopyable1 must  quot                   quot be a trivially-copyable type in order to guarantee that  memcpy    is safe  quot                   quot to use on it  quot                outputData  NonCopyable1       manually call destructor before overwriting this object             memcpy  amp outputData   amp data  sizeof data                 will produce this error   main cpp  In lambda function  main cpp 151 13  error  static assertion failed  NonCopyable1 must be a trivially-copyable type in order to guarantee that  memcpy    is safe to use on it               static assert std  is trivially copyable lt NonCopyable1 gt   value   quot NonCopyable1 must  quot                               So  you MUST  really should  use  quot placement new quot  instead  like this  as previously described above      processData attachCallback   amp outputData                          int someRandomData   999              NonCopyable1 data someRandomData               outputData  NonCopyable1       manually call destructor before overwriting this object                copy-construct  data  right into the address at   amp outputData   using placement new syntax             new  amp outputData  NonCopyable1 data                 More on pre-allocating a buffer memory pool for usage with  quot placement new quot  If you re really just going to use placement new to copy-construct right into a memory pool shared memory pre-allocated object space anyway  there s no need to use NonCopyable1 outputData  to construct a useless instance into that memory that we have to destroy later anyway  Instead  you can just use a memory pool of bytes  The format is like this   From   quot Placement new quot  section here  https   en cppreference com w cpp language new     within any scope          char buf sizeof T        Statically allocate memory large enough for any object of                              type  T   it may be misaligned         OR  to force proper alignment of your memory buffer for your object of type  T           you may specify memory alignment with  alignas    like this instead      alignas alignof T   char buf sizeof T        T  tptr   new buf  T     Construct a  T  object  placing it directly into your                               pre-allocated storage at memory address  buf       tptr- gt  T                 You must   manually   call the object s destructor                               Leaving scope here auto-deallocates your statically-allocated                               memory  buf    So  in my example above  this statically-allocated output buffer     This constructs an actual object here  calling the  NonCopyable1  class s    default constructor  NonCopyable1 outputData    would become this     This is just a statically-allocated memory pool  No constructor is called     Statically allocate an output buffer properly aligned  and large enough     to store 1 single  NonCopyable1  object  alignas alignof NonCopyable1   uint8 t outputData sizeof NonCopyable1    NonCopyable1  outputDataPtr    NonCopyable1    amp outputData 0     and then you d read the contents of the outputData object through the outputDataPtr pointer  The former method  NonCopyable1 outputData   is best if a constructor for this class exists which requires no input parameters you do NOT have access to at the time of the creation of this buffer  and if you only intend to store this one data type into this buffer  whereas the latter uint8 t buffer method is best if you either A  do NOT have access to all of the input parameters required to even construct the object at the location you need to create this buffer  OR B  if you plan to store multiple data types into this memory pool  perhaps for communicating between threads  modules  processes  etc  in a union-sort of way  More on C   and why it makes us jump through these hoops in this case So  this whole  quot placement new quot  thing in C    and the need for it  took me a lot of study and a long time to wrap my mind around it  After thinking about it  it has occurred to me that the paradigm of C  where I come from  is to manually allocate some memory  then stick some stuff into it  These are intended to be separate actions when dealing with both static and dynamic memory allocation  remember  you can t even set default values for structs    There is no concept of a constructor or destructor  and even getting the behavior of a scope-based destructor which gets automatically called as a variable exits a given scope is a pain-in-the-butt and requires some fancy gcc extension   attribute      cleanup   my variable    magic as I demonstrate in my answer here  Arbitrarily copying from one object to another  however  is super easy  Just copy the objects around  This is contrasted to the paradigm of C    which is RAII  Resource Acquisition is Initialization   This paradigm focuses on objects being ready for use the instant they are created  To accomplish this  they rely on constructors and destructors  This means that creating an object like this  NonCopyable1 data someRandomData    doesn t just allocate memory for that object  it also calls the object s constuctor and constructs  places  that object right into that memory  It tries to do multiple things in one  So  in C    memcpy   and the assignment operator     AKA  operator    function  are explicitly more limited by the nature of C    This is why we have to go through the hoops of this weird  quot copy-construct my object into a given memory location via placement new quot  process in C   instead of just creating a variable and copying stuff into it later  or memcpy  ing stuff into it later if it contains a const member  like we would do in C  C   really tries to enforce RAII  and this is in part how they do it  You can use std  optional lt  gt   emplace   instead As of C  17  you can use std  optional lt  gt  as a wrapper for this too  The modern C   emplace   functions of various containers and wrappers do what we manually did above with  quot placement new quot   see also my answer here and the quote about how std  vector lt T Allocator gt   emplace back  quot typically uses placement-new to construct the element in-place quot    std  optional statically allocates a buffer large enough for the object you want to put into it  It then either stores that object  or a std  nullopt  same as      which means it does not hold that object  To replace one object in it with another object  just call the emplace   method on the std  optional object  This does the following   Constructs the contained value in-place  If  this already contains a value before the call  the contained value is destroyed by calling its destructor   So  it first manually calls the destructor on an existing object already inside it  if an existing object is already inside it  then it does the equivalent of  quot placement new quot  to copy-construct a new object  which you provide it  into that memory space  So  this output buffer  NonCopyable1 outputData       OR  alignas alignof NonCopyable1   uint8 t outputData sizeof NonCopyable1    NonCopyable1  outputDataPtr    NonCopyable1    amp outputData 0     now becomes this    include  lt optional gt   std  optional lt NonCopyable1 gt  outputData   std  nullopt   and this  quot placement new quot  copy-constructing into that output buffer  processData attachCallback   amp outputData                  int someRandomData   999          NonCopyable1 data someRandomData           outputData  NonCopyable1       manually call destructor before overwriting this object            copy-construct  data  right into the address at   amp outputData   using placement new syntax         new  amp outputData  NonCopyable1 data             now becomes this emplace  ment of new data into that buffer  Notice that the manual call to the destructor is no longer necessary since std  optional lt  gt   emplace   already handles calling the destructor on any already-existing object for us   processData attachCallback   amp outputData                  int someRandomData   999          NonCopyable1 data someRandomData              emplace  data  right into the  outputData  object         outputData emplace data            Now  to get the data out of outputData  simply dereference it with    or call  value   on it  Ex     verify we get 999 here  if  outputData has value          printf  quot   outputData  i  after     i n quot     outputData  i          OR      printf  quot outputData value   i  after     i n quot   outputData value   i     else        printf  quot outputData has value   is false  quot       Sample output   Hello World   outputData  i  after    999 outputData value   i  after    999   Run this full example code here  References  amp  additional  EXCELLENT reading          some of the most-useful and simplest  quot placement new quot  examples Iv e ever seen   https   www geeksforgeeks org placement-new-operator-cpp   great example  https   en cppreference com w cpp language new -- gt  see the  quot Placement new quot  section and example here   I helped write the example   How do I make this C   object non-copyable   makes the really important point that calling the placement new line calls the object s constructor as it constructs it   Line  3  Fred  f   new place  Fred     essentially just calls the constructor Fred  Fred    This means  quot the this pointer in the Fred constructor will be equal to place quot    http   www cs technion ac il users yechiel c  -faq placement-new html  http   www cs technion ac il users yechiel c  -faq memory-pools html   Dr  Dobb s  quot Storage Layout of Polymorphic Objects quot   good pre-C  11 intro to the C    quot Rule of Three quot   http   web stanford edu class archive cs cs106b cs106b 1084 cs106l handouts 170 Copy Constructor Assignment Operator pdf My  quot hello world quot  example and repository  demoing custom copy constructors  assignment operators  etc   related to the C    quot Rule of Three quot     quot Rule of Five quot     quot Rule of Zero quot     quot Rule of 0 3 5 quot   https   github com ElectricRCAircraftGuy eRCaGuy hello world blob master cpp copy c

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Script engines can use it in the native interface to allocate native objects from scripts  See Angelscript  www angelcode com angelscript  for examples

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The one place I ve run across it is in containers which allocate a contiguous buffer and then fill it with objects as required  As mentioned  std  vector might do this  and I know some versions of MFC CArray and or CList did this  because that s where I first ran across it   The buffer over-allocation method is a very useful optimization  and placement new is pretty much the only way to construct objects in that scenario  It is also used sometimes to construct objects in memory blocks allocated outside of your direct code   I have used it in a similar capacity  although it doesn t come up often  It s a useful tool for the C   toolbox  though

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Here is the killer use for the C   in-place constructor  aligning to a cache line  as well as other powers of 2 boundaries  Here is my ultra-fast pointer alignment algorithm to any power of 2 boundaries with 5 or less single-cycle instructions      Quickly aligns the given pointer to a power of two boundary IN BYTES   return An aligned pointer of typename T   brief Algorithm is a 2 s compliment trick that works by masking off the desired number in 2 s compliment and adding them to the pointer   param pointer The pointer to align   param boundary byte count The boundary byte count that must be an even power of 2   warning Function does not check if the boundary is a power of 2     template  lt typename T   char gt  inline T  AlignUp void  pointer  uintptr t boundary byte count      uintptr t value   reinterpret cast lt uintptr t gt  pointer     value        value    1   amp   boundary byte count - 1      return reinterpret cast lt T  gt  value      struct Foo   Foo          char buffer sizeof  Foo    64   Foo  foo   new  AlignUp lt Foo gt   buffer  64   Foo       Now doesn t that just put a smile on your face   -   I     C  1x

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Generally  placement new is used to get rid of allocation cost of a  normal new      Another scenario where I used it is a place where I wanted to have access to the pointer to an object that was still to be constructed  to implement a per-document singleton

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Script engines can use it in the native interface to allocate native objects from scripts  See Angelscript  www angelcode com angelscript  for examples

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Placement new is also very useful when serialising  say with boost  serialization   In 10 years of c   this is only the second case I ve needed placement new for  third if you include interviews

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The one place I ve run across it is in containers which allocate a contiguous buffer and then fill it with objects as required  As mentioned  std  vector might do this  and I know some versions of MFC CArray and or CList did this  because that s where I first ran across it   The buffer over-allocation method is a very useful optimization  and placement new is pretty much the only way to construct objects in that scenario  It is also used sometimes to construct objects in memory blocks allocated outside of your direct code   I have used it in a similar capacity  although it doesn t come up often  It s a useful tool for the C   toolbox  though

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Placement new allows you to construct an object in memory that s already allocated  You may want to do this for optimization when you need to construct multiple instances of an object  and it is faster not to re-allocate memory each time you need a new instance   Instead  it might be more efficient to perform a single allocation for a chunk of memory that can hold multiple objects  even though you don t want to use all of it at once  DevX gives a good example   Standard C   also supports placement new operator  which constructs an object on a pre-allocated buffer  This is useful when building a memory pool  a garbage collector or simply when performance and exception safety are paramount  there s no danger of allocation failure since the memory has already been allocated  and constructing an object on a pre-allocated buffer takes less time    char  buf    new char sizeof string       pre-allocated buffer string  p   new  buf  string  quot hi quot          placement new string  q   new string  quot hi quot                ordinary heap allocation  You may also want to be sure there can be no allocation failure at a certain part of critical code  for instance  in code executed by a pacemaker    In that case you would want to allocate memory earlier  then use placement new within the critical section  Deallocation in placement new You should not deallocate every object that is using the memory buffer   Instead you should delete   only the original buffer   You would have to then call the destructors of your classes manually   For a good suggestion on this  please see Stroustrup s FAQ on  Is there a  quot placement delete quot

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I ve seen it used as a slight performance hack for a  dynamic type  pointer  in the section  Under the Hood        But here is the tricky trick I used to get fast performance for small types  if the value being held can fit inside of a void   I don t actually bother allocating a new object  I force it into the pointer itself using placement new

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I ve used it to construct objects allocated on the stack via alloca     shameless plug  I blogged about it here

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We use it with custom memory pools  Just a sketch   class Pool   public      Pool        implementation details irrelevant           virtual  Pool        ditto            virtual void  allocate size t       virtual void deallocate void          static Pool  misc pool     return misc pool p     global MiscPool for general use          class ClusterPool   public Pool                class FastPool   public Pool                class MapPool   public Pool                class MiscPool   public Pool                    elsewhere     void  pnew new size t size       return Pool  misc pool  - gt allocate size      void  pnew new size t size  Pool  pool p       if   pool p          return Pool  misc pool  - gt allocate size           else         return pool p- gt allocate size           void pnew delete void  p       Pool  hp   Pool  find pool p         note  if p    0  then Pool  find pool p  will return 0     if  hp          hp- gt deallocate p              elsewhere     class Obj   public        misc ctors  dtors  etc         just a sampling of new del operators    void  operator new size t s                return pnew new s        void  operator new size t s  Pool  hp      return pnew new s  hp        void operator delete void  dp              pnew delete dp        void operator delete void  dp  Pool        pnew delete dp         void  operator new   size t s              return pnew new s        void  operator new   size t s  Pool  hp    return pnew new s  hp        void operator delete   void  dp            pnew delete dp        void operator delete   void  dp  Pool      pnew delete dp            elsewhere     ClusterPool  cp   new ClusterPool arg1  arg2         Obj  new obj   new  cp  Obj arg a  arg b          Now you can cluster objects together in a single memory arena  select an allocator which is very fast but does no deallocation  use memory mapping  and any other semantic you wish to impose by choosing the pool and passing it as an argument to an object s placement new operator

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It s useful if you want to separate allocation from initialization  STL uses placement new to create container elements

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Here is the killer use for the C   in-place constructor  aligning to a cache line  as well as other powers of 2 boundaries  Here is my ultra-fast pointer alignment algorithm to any power of 2 boundaries with 5 or less single-cycle instructions      Quickly aligns the given pointer to a power of two boundary IN BYTES   return An aligned pointer of typename T   brief Algorithm is a 2 s compliment trick that works by masking off the desired number in 2 s compliment and adding them to the pointer   param pointer The pointer to align   param boundary byte count The boundary byte count that must be an even power of 2   warning Function does not check if the boundary is a power of 2     template  lt typename T   char gt  inline T  AlignUp void  pointer  uintptr t boundary byte count      uintptr t value   reinterpret cast lt uintptr t gt  pointer     value        value    1   amp   boundary byte count - 1      return reinterpret cast lt T  gt  value      struct Foo   Foo          char buffer sizeof  Foo    64   Foo  foo   new  AlignUp lt Foo gt   buffer  64   Foo       Now doesn t that just put a smile on your face   -   I     C  1x

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We use it with custom memory pools  Just a sketch   class Pool   public      Pool        implementation details irrelevant           virtual  Pool        ditto            virtual void  allocate size t       virtual void deallocate void          static Pool  misc pool     return misc pool p     global MiscPool for general use          class ClusterPool   public Pool                class FastPool   public Pool                class MapPool   public Pool                class MiscPool   public Pool                    elsewhere     void  pnew new size t size       return Pool  misc pool  - gt allocate size      void  pnew new size t size  Pool  pool p       if   pool p          return Pool  misc pool  - gt allocate size           else         return pool p- gt allocate size           void pnew delete void  p       Pool  hp   Pool  find pool p         note  if p    0  then Pool  find pool p  will return 0     if  hp          hp- gt deallocate p              elsewhere     class Obj   public        misc ctors  dtors  etc         just a sampling of new del operators    void  operator new size t s                return pnew new s        void  operator new size t s  Pool  hp      return pnew new s  hp        void operator delete void  dp              pnew delete dp        void operator delete void  dp  Pool        pnew delete dp         void  operator new   size t s              return pnew new s        void  operator new   size t s  Pool  hp    return pnew new s  hp        void operator delete   void  dp            pnew delete dp        void operator delete   void  dp  Pool      pnew delete dp            elsewhere     ClusterPool  cp   new ClusterPool arg1  arg2         Obj  new obj   new  cp  Obj arg a  arg b          Now you can cluster objects together in a single memory arena  select an allocator which is very fast but does no deallocation  use memory mapping  and any other semantic you wish to impose by choosing the pool and passing it as an argument to an object s placement new operator

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Head Geek  BINGO   You got it totally - that s exactly what it s perfect for   In many embedded environments  external constraints and or the overall use scenario forces the programmer to separate the allocation of an object from its initialization   Lumped together  C   calls this  instantiation   but whenever the constructor s action must be explicitly invoked WITHOUT dynamic or automatic allocation  placement new is the way to do it   It s also the perfect way to locate a global C   object that is pinned to the address of a hardware component  memory-mapped I O   or for any static object that  for whatever reason  must reside at a fixed address

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It s used by std  vector lt  gt  because std  vector lt  gt  typically allocates more memory than there are objects in the vector lt  gt

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I have an idea too  C   does have zero-overhead principle  But exceptions do not follow this principle  so sometimes they are turned off with compiler switch  Let s look to this example   include  lt new gt   include  lt cstdio gt   include  lt cstdlib gt   int main         struct A           A                 printf  quot A   n quot                       A                 printf  quot  A   n quot                      char data 1000000000000000000           some very big number             try           A  result   new A            printf  quot new passed   p n quot   result           delete result        catch  std  bad alloc            printf  quot new failed n quot             We allocate a big struct here  and check if allocation is successful  and delete it  But if we have exceptions turned off  we can t use try block  and unable to handle new   failure  So how we can do that  Here is how   include  lt new gt   include  lt cstdio gt   include  lt cstdlib gt   int main         struct A           A                 printf  quot A   n quot                       A                 printf  quot  A   n quot                      char data 1000000000000000000           some very big number             void  buf   malloc sizeof A        if  buf    nullptr            A  result   new buf  A            printf  quot new passed   p n quot   result           result- gt  A            free result         else           printf  quot new failed n quot              Use simple malloc Check if it is failed in a C way If it successful  we use placement new Manually call the destructor  we can t just call delete  call free  due we called malloc  UPD  Useless wrote a comment which opened to my view the existence of new nothrow   which should be used in this case  but not the method I wrote before  Please don t use the code I wrote before  Sorry

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I ve used it for storing objects with memory mapped files  The specific example was an image database which processed vey large numbers of large images  more than could fit in memory

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It is useful if you are building a kernel - where do you place the kernel code you read from disk or the pagetable  You need to know where to jump to   Or in other  very rare circumstances such as when you have loads of allocated room and want to place a few structures behind each other  They can be packed this way without the need for the offsetof   operator  There are other tricks for that too  though   I also believe some STL implementations make use of placement new  like std  vector  They allocate room for 2 n elements that way and don t need to always realloc

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It s actually kind of required to implement any kind of data structure that allocates more memory than minimally required for the number of elements inserted  i e   anything other than a linked structure which allocates one node at a time    Take containers like unordered map  vector  or deque  These all allocate more memory than is minimally required for the elements you ve inserted so far to avoid requiring a heap allocation for every single insertion  Let s use vector as the simplest example   When you do   vector lt Foo gt  vec      Allocate memory for a thousand Foos  vec reserve 1000         that doesn t actually construct a thousand Foos  It simply allocates reserves memory for them  If vector did not use placement new here  it would be default-constructing Foos all over the place as well as having to invoke their destructors even for elements you never even inserted in the first place   Allocation    Construction  Freeing    Destruction  Just generally speaking to implement many data structures like the above  you cannot treat allocating memory and constructing elements as one indivisible thing  and you likewise cannot treat freeing memory and destroying elements as one indivisible thing   There has to be a separation between these ideas to avoid superfluously invoking constructors and destructors unnecessarily left and right  and that s why the standard library separates the idea of std  allocator  which doesn t construct or destroy elements when it allocates frees memory   away from the containers that use it which do manually construct elements using placement new and manually destroy elements using explicit invocations of destructors         I hate the design of std  allocator but that s a different subject I ll avoid ranting about   -D      So anyway  I tend to use it a lot since I ve written a number of general-purpose standard-compliant C   containers that could not be built in terms of the existing ones  Included among them is a small vector implementation I built a couple decades ago to avoid heap allocations in common cases  and a memory-efficient trie  doesn t allocate one node at a time   In both cases I couldn t really implement them using the existing containers  and so I had to use placement new to avoid superfluously invoking constructors and destructors on things unnecessary left and right   Naturally if you ever work with custom allocators to allocate objects individually  like a free list  then you d also generally want to use placement new  like this  basic example which doesn t bother with exception-safety or RAII    Foo  foo   new free list allocate    Foo           foo- gt  Foo    free list free foo

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Placement new is also very useful when serialising  say with boost  serialization   In 10 years of c   this is only the second case I ve needed placement new for  third if you include interviews

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I ve used it to construct objects allocated on the stack via alloca     shameless plug  I blogged about it here

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I ve used it in real-time programming  We typically don t want to perform any dynamic allocation  or deallocation  after the system starts up  because there s no guarantee how long that is going to take    What I can do is preallocate a large chunk of memory  large enough to hold any amount of whatever that the class may require   Then  once I figure out at runtime how to construct the things  placement new can be used to construct objects right where I want them  One situation I know I used it in was to help create a heterogeneous circular buffer   It s certainly not for the faint of heart  but that s why they make the syntax for it kinda  gnarly

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See the fp h file in the xll project at http   xll codeplex com It solves the  unwarranted chumminess with the compiler  issue for arrays that like to carry their dimensions around with them   typedef struct  FP       unsigned short int rows      unsigned short int columns      double array 1             Actually  array rows  columns       FP

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I ve used it to construct objects allocated on the stack via alloca     shameless plug  I blogged about it here

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I ve seen it used as a slight performance hack for a  dynamic type  pointer  in the section  Under the Hood        But here is the tricky trick I used to get fast performance for small types  if the value being held can fit inside of a void   I don t actually bother allocating a new object  I force it into the pointer itself using placement new

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Has anyone here ever used C   s  quot placement new quot   If so  what for  It looks to me like it would only be useful on memory-mapped hardware   It s really useful when needing to copy out  pass as outputs    non-copyable objects  ex  where operator    has been automatically deleted because the class contains a const member  OR non-trivially-copyable objects  where using memcpy   is undefined behavior      from within a function  This  obtaining these non-copyable or non-trivially-copyable objects from a function  could aid in unit testing that function  by allowing you to see that a certain data object now looks a certain way after being processed by that function  OR it could simply be a part of your normal API for whatever use you see fit  Let s go over these examples and explain in detail what I mean and how  quot placement new quot  can be used to solve these problems  TLDR  Note  I ve tested every single line of code in this answer  It works  It is valid  It does not violate the C   standard  Placement new is   The replacement in C   for   when operator     the assignment operator  is deleted  and you need to  quot copy quot   actually copy-construct  a therefore otherwise non-copyable object into a given memory location  The replacement in C   for memcpy   when your object is not trivially-copyable  meaning that using memcpy   to copy this non-trivially-copyable object  quot may be undefined quot    Important  a  quot non-copyable quot  object is NOT truly non-copyable  It is simply not copyable via the   operator is all  which is a call to a class s underlying operator    overload function  This means that when you do B   C   what is actually happening is a call to B operator  C    and when you do A   B   C   what is actually happening is A operator  B operator  C     Therefore   quot non-copyable quot  objects are only copyable via other means  such as via the class s copy constructor  since  again  the class has no operator    method   quot Placement new quot  can be used to call any one of the many constructors which may exist in a class in order to construct an object into a desired pre-allocated memory location  Since  quot placement new quot  syntax allows for calling any constructor in a class  this includes passing it an existing instance of a class in order to have placement new call a class s copy-constructor to copy-construct a new object from the passed-in object into another place in memory  Copy-constructing one object into another place in memory   is a copy  This action creates a copy of that original object  When done  you CAN have two objects  instances   that are byte-identical  literally byte for byte  depending on your copy constructor s implementation   located in two separate places in memory  That  by definition  is a copy  It just wasn t done using the class s operator    method is all  Therefore  one might define a class as being  quot non-copyable quot  if it has no operator    method  but it is still very-much copyable  legally  according to the C   standard and mechanisms provided by C    safely and without undefined behavior  using its copy constructor and placement new syntax  as demonstrated below  Reminder  all lines of code below work  You can run much of the code right here  incl  many of the code blocks below  although it may require some commenting uncommenting blocks of code since it is not cleanly set up into separate examples  1  What is a non-copyable object  A non-copyable object cannot be copied with the   operator  operator    function   That s it  It can still be legally copied  however  See the really important note just above  Non-copyable class Example 1  Here  copy-constructing is fine  but copying is prohibited since we have explicitly deleted the assignment operator  Trying to do nc2   nc1  results in this compile-time error   error  use of deleted function    NonCopyable1 amp  NonCopyable1  operator  const NonCopyable1 amp        Here s the full example   include  lt stdio h gt   class NonCopyable1   public      int i   5          Delete the assignment operator to make this class non-copyable      NonCopyable1 amp  operator  const NonCopyable1 amp  other    delete      int main         printf  quot Hello World n quot             NonCopyable1 nc1      NonCopyable1 nc2      nc2   nc1       copy assignment  compile-time error      NonCopyable1 nc3   nc1     copy constructor  works fine       return 0     Non-copyable class Example 2  Here  copy-constructing is fine  but copying is prohibited since the class contains a const member  which can t be written to  supposedly  since there are work-arounds obviously   Trying to do nc2   nc1  results in this compile-time error   error  use of deleted function    NonCopyable1 amp  NonCopyable1  operator  const NonCopyable1 amp      note     NonCopyable1 amp  NonCopyable1  operator  const NonCopyable1 amp      is implicitly deleted because the default definition would be ill-formed  error  non-static const member    const int NonCopyable1  i     can   t use default assignment operator   Full example   include  lt stdio h gt   class NonCopyable1   public      const int i   5     classes with  const  members are non-copyable by default     int main         printf  quot Hello World n quot             NonCopyable1 nc1      NonCopyable1 nc2      nc2   nc1       copy assignment  compile-time error      NonCopyable1 nc3   nc1     copy constructor  works fine       return 0     So  if a class is non-copyable  you can NOT do the following to get a copy of it as an output  The line outputData   data  will cause compilation to fail with the previous error messages shown in the last example just above   include  lt functional gt    include  lt stdio h gt   class NonCopyable1   public      const int i     classes with  const  members are non-copyable by default         Constructor to custom-initialize  i      NonCopyable1 int val   5    i val                    nothing else to do               Some class which  perhaps asynchronously  processes data  You attach a     callback  which gets called later      - Also  this may be a shared library over which you have no or little     control  so you cannot easily change the prototype of the callable callback     function   class ProcessData   public      void attachCallback std  function lt void void  gt  callable                callback    callable                 void callCallback                 callback            private      std  function lt void void  gt  callback       int main         printf  quot Hello World n quot             NonCopyable1 outputData     we need to receive back data through this object     printf  quot outputData i  before     i n quot   outputData i      is 5          ProcessData processData         Attach a lambda function as a callback  capturing  outputData  by         reference so we can receive back the data from inside the callback via         this object even though the callable prototype returns  void   is a          void void   callable function       processData attachCallback   amp outputData                          int someRandomData   999              NonCopyable1 data someRandomData                  NOT ALLOWED SINCE COPY OPERATOR  Assignment operator  WAS                 AUTO-DELETED since the class has a  const  data member              outputData   data                   processData callCallback           verify we get 999 here  NOT 5      printf  quot outputData i  after     i n quot   outputData i         return 0     One solution  memcpy the data into the outputData  This is perfectly acceptable in C  but not always ok in C    Cppreference com states  emphasis added    If the objects are potentially-overlapping or not TriviallyCopyable  the behavior of memcpy is not specified and may be undefined   and   Notes Objects of trivially-copyable types that are not potentially-overlapping subobjects are the only C   objects that may be safely copied with std  memcpy or serialized to from binary files with std  ofstream  write   std  ifstream  read      https   en cppreference com w cpp string byte memcpy  So  let s just be safe and ensure an object is trivially copyable before copying it with memcpy    Replace this part above      processData attachCallback   amp outputData                          int someRandomData   999              NonCopyable1 data someRandomData                  NOT ALLOWED SINCE COPY OPERATOR  Assignment operator  WAS                 AUTO-DELETED since the class has a  const  data member              outputData   data                with this  Notice the usage of memcpy   to copy out the data this time  and std  is trivially copyable to ensure  at compile time  this type truly is safe to copy with memcpy             added to top       include  lt cstring gt      for  memcpy         include  lt type traits gt     for  std  is trivially copyable lt  gt             Attach a lambda function as a callback  capturing  outputData  by         reference so we can receive back the data from inside the callback via         this object even though the callable prototype returns  void   is a          void void   callable function       processData attachCallback   amp outputData                          int someRandomData   999              NonCopyable1 data someRandomData               static assert std  is trivially copyable lt NonCopyable1 gt   value   quot NonCopyable1 must  quot                   quot be a trivially-copyable type in order to guarantee that  memcpy    is safe  quot                   quot to use on it  quot                memcpy  amp outputData   amp data  sizeof data                 Sample program output now that it can compile  amp  run  It works   Hello World outputData i  before    5 outputData i  after    999   To be extra safe  however  you should manually call the destructor of the object you re overwriting before overwriting it  like this  BEST MEMCPY   SOLUTION      processData attachCallback   amp outputData                          int someRandomData   999              NonCopyable1 data someRandomData               static assert std  is trivially copyable lt NonCopyable1 gt   value   quot NonCopyable1 must  quot                   quot be a trivially-copyable type in order to guarantee that  memcpy    is safe  quot                   quot to use on it  quot                outputData  NonCopyable1       manually call destructor before overwriting this object             memcpy  amp outputData   amp data  sizeof data                 If the static assert   above fails  however  then you shouldn t use memcpy    An always-safe and better C   alternative  therefore  is to use  quot placement new quot   Here  we simply copy-construct data right into the memory region occupied by outputData  That s what this  quot placement new quot  syntax does for us  It does NOT dynamically allocate memory  like the new operator normally does  Normally  the new operator first dynamically allocates memory on the heap and then constructs an object into that memory by calling the object s constructor  However  placement new does NOT do the allocation part  Instead  it simply skips that part and constructs an object into memory at an address you specify  YOU have to be the one to allocate that memory  either statically or dynamically  beforehand  and YOU have to ensure that memory is properly aligned for that object  see alignof and alignas and the Placement new example here   it will be in this case since we explicitly created the outputData object as an object  calling it s constructor with NonCopyable1 outputData    and YOU have to ensure that the memory buffer pool is large enough to hold the data you are about to construct into it  So  the generic placement new syntax is this     Call T  s specified constructor below  constructing it as an object right into    the memory location pointed to by  ptr to buffer   No dynamic memory allocation    whatsoever happens at this time  The object  T  is simply constructed into this    address in memory  T  ptr to T   new ptr to buffer  T optional input args to T s constructor    In our case  it will look like this  calling the copy constructor of the NonCopyable1 class  which we ve already proven repeatedly above is valid even when the assignment copy operator is deleted     copy-construct  data  right into the address at   amp outputData   using placement new syntax new  amp outputData  NonCopyable1 data     Our final attachCallback lambda now looks like this  with the placement new syntax in place of memcpy    Notice that the check to ensure the object is trivially copyable is no longer required whatsoever      gt  BEST C   SOLUTION ALL-AROUND--AVOIDS MEMCPY BY COPY-CONSTRUCTING DIRECTLY INTO THE TARGET MEMORY LOCATION USING PLACEMENT NEW   lt      USE THIS           processData attachCallback   amp outputData                          int someRandomData   999              NonCopyable1 data someRandomData               outputData  NonCopyable1       manually call destructor before overwriting this object                copy-construct  data  right into the address at   amp outputData   using placement new syntax             new  amp outputData  NonCopyable1 data                    Assume that  data  will be further manipulated and used below now  but we needed                its state at this moment in time                   Note also that under the most trivial of cases  we could have also just called                out custom constructor right here too  like this  You can call whatever                constructor you want                 new  amp outputData  NonCopyable1 999                                    2  What is a non-trivially-copyable object  A non-trivially-copyable object may be one which contains virtual methods and things  as this can lead to the class having to track  quot vee pointers quot   vptr  and  quot vee tables quot   vtbls   to point to the proper virtual implementation in memory  Read more about that here  Dr  Dobb s  quot Storage Layout of Polymorphic Objects quot   However  even in this case  so long as you re memcpy  ing from the same process to the same process  ie  within the same virtual memory space   and NOT between processes  and NOT deserializing from disk to RAM  it seems to me that memcpy   would technically work fine and produce no bugs  and I ve proven this in a handful of examples to myself   but it technically seems to be behavior which is not defined by the C   standard  so therefore it is undefined behavior  so therefore it can t be relied upon 100  from compiler to compiler  and from one version of C   to the next  so   it is undefined behavior and you shouldn t memcpy   in that case  In other words  if the static assert std  is trivially copyable lt NonCopyable1 gt   value   check fails above  do NOT use memcpy    You must use  quot placement new quot  instead  One way to get that static assert to fail is to simply declare or define a custom copy assignment operator in your class definition for your NonCopyable1 class  like this     Custom copy assignment operator declaration  NonCopyable1 amp  operator  const NonCopyable1 amp  other       OR      Custom copy assignment operator definition  NonCopyable1 amp  operator  const NonCopyable1 amp  other           Check for    and don t allow    self assignment          ie  only copy the contents from the other object         to this object if it is not the same object  ie  if it is not         self-assignment       if this     amp other                    copy all non-const members manually here  if the class had any  ex             j   other j             k   other k             etc             Do deep copy of data via any member   pointers    if such members exist               the assignment function   operator      expects you to return the         contents of your own object  the left side   passed by reference  so         that constructs such as  test1   test2   test3   are valid         See this reference  from Stanford  p11  here          http   web stanford edu class archive cs cs106b cs106b 1084 cs106l handouts 170 Copy Constructor Assignment Operator pdf             MyClass one  two  three              three   two   one      return  this       For more examples on custom copy constructors  assignment operators  etc  and the  quot Rule of Three quot  and  quot Rule of Five quot   see my hello world repository and example here   So  now that we have a custom assignment operator  the class is no longer trivially copyable  and this code      processData attachCallback   amp outputData                          int someRandomData   999              NonCopyable1 data someRandomData               static assert std  is trivially copyable lt NonCopyable1 gt   value   quot NonCopyable1 must  quot                   quot be a trivially-copyable type in order to guarantee that  memcpy    is safe  quot                   quot to use on it  quot                outputData  NonCopyable1       manually call destructor before overwriting this object             memcpy  amp outputData   amp data  sizeof data                 will produce this error   main cpp  In lambda function  main cpp 151 13  error  static assertion failed  NonCopyable1 must be a trivially-copyable type in order to guarantee that  memcpy    is safe to use on it               static assert std  is trivially copyable lt NonCopyable1 gt   value   quot NonCopyable1 must  quot                               So  you MUST  really should  use  quot placement new quot  instead  like this  as previously described above      processData attachCallback   amp outputData                          int someRandomData   999              NonCopyable1 data someRandomData               outputData  NonCopyable1       manually call destructor before overwriting this object                copy-construct  data  right into the address at   amp outputData   using placement new syntax             new  amp outputData  NonCopyable1 data                 More on pre-allocating a buffer memory pool for usage with  quot placement new quot  If you re really just going to use placement new to copy-construct right into a memory pool shared memory pre-allocated object space anyway  there s no need to use NonCopyable1 outputData  to construct a useless instance into that memory that we have to destroy later anyway  Instead  you can just use a memory pool of bytes  The format is like this   From   quot Placement new quot  section here  https   en cppreference com w cpp language new     within any scope          char buf sizeof T        Statically allocate memory large enough for any object of                              type  T   it may be misaligned         OR  to force proper alignment of your memory buffer for your object of type  T           you may specify memory alignment with  alignas    like this instead      alignas alignof T   char buf sizeof T        T  tptr   new buf  T     Construct a  T  object  placing it directly into your                               pre-allocated storage at memory address  buf       tptr- gt  T                 You must   manually   call the object s destructor                               Leaving scope here auto-deallocates your statically-allocated                               memory  buf    So  in my example above  this statically-allocated output buffer     This constructs an actual object here  calling the  NonCopyable1  class s    default constructor  NonCopyable1 outputData    would become this     This is just a statically-allocated memory pool  No constructor is called     Statically allocate an output buffer properly aligned  and large enough     to store 1 single  NonCopyable1  object  alignas alignof NonCopyable1   uint8 t outputData sizeof NonCopyable1    NonCopyable1  outputDataPtr    NonCopyable1    amp outputData 0     and then you d read the contents of the outputData object through the outputDataPtr pointer  The former method  NonCopyable1 outputData   is best if a constructor for this class exists which requires no input parameters you do NOT have access to at the time of the creation of this buffer  and if you only intend to store this one data type into this buffer  whereas the latter uint8 t buffer method is best if you either A  do NOT have access to all of the input parameters required to even construct the object at the location you need to create this buffer  OR B  if you plan to store multiple data types into this memory pool  perhaps for communicating between threads  modules  processes  etc  in a union-sort of way  More on C   and why it makes us jump through these hoops in this case So  this whole  quot placement new quot  thing in C    and the need for it  took me a lot of study and a long time to wrap my mind around it  After thinking about it  it has occurred to me that the paradigm of C  where I come from  is to manually allocate some memory  then stick some stuff into it  These are intended to be separate actions when dealing with both static and dynamic memory allocation  remember  you can t even set default values for structs    There is no concept of a constructor or destructor  and even getting the behavior of a scope-based destructor which gets automatically called as a variable exits a given scope is a pain-in-the-butt and requires some fancy gcc extension   attribute      cleanup   my variable    magic as I demonstrate in my answer here  Arbitrarily copying from one object to another  however  is super easy  Just copy the objects around  This is contrasted to the paradigm of C    which is RAII  Resource Acquisition is Initialization   This paradigm focuses on objects being ready for use the instant they are created  To accomplish this  they rely on constructors and destructors  This means that creating an object like this  NonCopyable1 data someRandomData    doesn t just allocate memory for that object  it also calls the object s constuctor and constructs  places  that object right into that memory  It tries to do multiple things in one  So  in C    memcpy   and the assignment operator     AKA  operator    function  are explicitly more limited by the nature of C    This is why we have to go through the hoops of this weird  quot copy-construct my object into a given memory location via placement new quot  process in C   instead of just creating a variable and copying stuff into it later  or memcpy  ing stuff into it later if it contains a const member  like we would do in C  C   really tries to enforce RAII  and this is in part how they do it  You can use std  optional lt  gt   emplace   instead As of C  17  you can use std  optional lt  gt  as a wrapper for this too  The modern C   emplace   functions of various containers and wrappers do what we manually did above with  quot placement new quot   see also my answer here and the quote about how std  vector lt T Allocator gt   emplace back  quot typically uses placement-new to construct the element in-place quot    std  optional statically allocates a buffer large enough for the object you want to put into it  It then either stores that object  or a std  nullopt  same as      which means it does not hold that object  To replace one object in it with another object  just call the emplace   method on the std  optional object  This does the following   Constructs the contained value in-place  If  this already contains a value before the call  the contained value is destroyed by calling its destructor   So  it first manually calls the destructor on an existing object already inside it  if an existing object is already inside it  then it does the equivalent of  quot placement new quot  to copy-construct a new object  which you provide it  into that memory space  So  this output buffer  NonCopyable1 outputData       OR  alignas alignof NonCopyable1   uint8 t outputData sizeof NonCopyable1    NonCopyable1  outputDataPtr    NonCopyable1    amp outputData 0     now becomes this    include  lt optional gt   std  optional lt NonCopyable1 gt  outputData   std  nullopt   and this  quot placement new quot  copy-constructing into that output buffer  processData attachCallback   amp outputData                  int someRandomData   999          NonCopyable1 data someRandomData           outputData  NonCopyable1       manually call destructor before overwriting this object            copy-construct  data  right into the address at   amp outputData   using placement new syntax         new  amp outputData  NonCopyable1 data             now becomes this emplace  ment of new data into that buffer  Notice that the manual call to the destructor is no longer necessary since std  optional lt  gt   emplace   already handles calling the destructor on any already-existing object for us   processData attachCallback   amp outputData                  int someRandomData   999          NonCopyable1 data someRandomData              emplace  data  right into the  outputData  object         outputData emplace data            Now  to get the data out of outputData  simply dereference it with    or call  value   on it  Ex     verify we get 999 here  if  outputData has value          printf  quot   outputData  i  after     i n quot     outputData  i          OR      printf  quot outputData value   i  after     i n quot   outputData value   i     else        printf  quot outputData has value   is false  quot       Sample output   Hello World   outputData  i  after    999 outputData value   i  after    999   Run this full example code here  References  amp  additional  EXCELLENT reading          some of the most-useful and simplest  quot placement new quot  examples Iv e ever seen   https   www geeksforgeeks org placement-new-operator-cpp   great example  https   en cppreference com w cpp language new -- gt  see the  quot Placement new quot  section and example here   I helped write the example   How do I make this C   object non-copyable   makes the really important point that calling the placement new line calls the object s constructor as it constructs it   Line  3  Fred  f   new place  Fred     essentially just calls the constructor Fred  Fred    This means  quot the this pointer in the Fred constructor will be equal to place quot    http   www cs technion ac il users yechiel c  -faq placement-new html  http   www cs technion ac il users yechiel c  -faq memory-pools html   Dr  Dobb s  quot Storage Layout of Polymorphic Objects quot   good pre-C  11 intro to the C    quot Rule of Three quot   http   web stanford edu class archive cs cs106b cs106b 1084 cs106l handouts 170 Copy Constructor Assignment Operator pdf My  quot hello world quot  example and repository  demoing custom copy constructors  assignment operators  etc   related to the C    quot Rule of Three quot     quot Rule of Five quot     quot Rule of Zero quot     quot Rule of 0 3 5 quot   https   github com ElectricRCAircraftGuy eRCaGuy hello world blob master cpp copy c

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Generally  placement new is used to get rid of allocation cost of a  normal new      Another scenario where I used it is a place where I wanted to have access to the pointer to an object that was still to be constructed  to implement a per-document singleton

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The one place I ve run across it is in containers which allocate a contiguous buffer and then fill it with objects as required  As mentioned  std  vector might do this  and I know some versions of MFC CArray and or CList did this  because that s where I first ran across it   The buffer over-allocation method is a very useful optimization  and placement new is pretty much the only way to construct objects in that scenario  It is also used sometimes to construct objects in memory blocks allocated outside of your direct code   I have used it in a similar capacity  although it doesn t come up often  It s a useful tool for the C   toolbox  though

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I ve used it in real-time programming  We typically don t want to perform any dynamic allocation  or deallocation  after the system starts up  because there s no guarantee how long that is going to take    What I can do is preallocate a large chunk of memory  large enough to hold any amount of whatever that the class may require   Then  once I figure out at runtime how to construct the things  placement new can be used to construct objects right where I want them  One situation I know I used it in was to help create a heterogeneous circular buffer   It s certainly not for the faint of heart  but that s why they make the syntax for it kinda  gnarly

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I ve used it to create objects based on memory containing messages received from the network

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It is useful if you are building a kernel - where do you place the kernel code you read from disk or the pagetable  You need to know where to jump to   Or in other  very rare circumstances such as when you have loads of allocated room and want to place a few structures behind each other  They can be packed this way without the need for the offsetof   operator  There are other tricks for that too  though   I also believe some STL implementations make use of placement new  like std  vector  They allocate room for 2 n elements that way and don t need to always realloc

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Generally  placement new is used to get rid of allocation cost of a  normal new      Another scenario where I used it is a place where I wanted to have access to the pointer to an object that was still to be constructed  to implement a per-document singleton

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I ve used it to create objects based on memory containing messages received from the network

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It s useful if you want to separate allocation from initialization  STL uses placement new to create container elements

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I ve seen it used as a slight performance hack for a  dynamic type  pointer  in the section  Under the Hood        But here is the tricky trick I used to get fast performance for small types  if the value being held can fit inside of a void   I don t actually bother allocating a new object  I force it into the pointer itself using placement new

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I ve used it for storing objects with memory mapped files  The specific example was an image database which processed vey large numbers of large images  more than could fit in memory

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I ve used it in real-time programming  We typically don t want to perform any dynamic allocation  or deallocation  after the system starts up  because there s no guarantee how long that is going to take    What I can do is preallocate a large chunk of memory  large enough to hold any amount of whatever that the class may require   Then  once I figure out at runtime how to construct the things  placement new can be used to construct objects right where I want them  One situation I know I used it in was to help create a heterogeneous circular buffer   It s certainly not for the faint of heart  but that s why they make the syntax for it kinda  gnarly

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I think this has not been highlighted by any answer  but another good example and usage for the new placement is to reduce the memory fragmentation  by using memory pools   This is specially useful in embedded and high availability systems  In this last case it s specially important because for a system that has to run 24 365 days it s very important to have no fragmentation  This problem has nothing to do with memory leakage    Even when a very good malloc implementation is used  or similar memory management function  it s very difficult to deal with fragmentation for a long time  At some point if you don t manage cleverly the memory reservation release calls you could end up with a lot of small gaps that are difficult to reuse  assign to new reservations   So  one of the solutions that are used in this case is to use a memory pool to allocate before hand the memory for the application objects  After-wards each time you need memory for some object you just use the new placement to create a new object on the already reserved memory    This way  once your application starts you already have all the needed memory reserved  All the new memory reservation release goes to the allocated pools  you may have several pools  one for each different object class   No memory fragmentation happens in this case since there will no gaps and your system can run for very long periods  years  without suffering from fragmentation    I saw this in practice specially for the VxWorks RTOS since its default memory allocation system suffers a lot from fragmentation  So allocating memory through the standard new malloc method was basically prohibited in the project  All the memory reservations should go to a dedicated memory pool

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Placement new allows you to construct an object in memory that s already allocated  You may want to do this for optimization when you need to construct multiple instances of an object  and it is faster not to re-allocate memory each time you need a new instance   Instead  it might be more efficient to perform a single allocation for a chunk of memory that can hold multiple objects  even though you don t want to use all of it at once  DevX gives a good example   Standard C   also supports placement new operator  which constructs an object on a pre-allocated buffer  This is useful when building a memory pool  a garbage collector or simply when performance and exception safety are paramount  there s no danger of allocation failure since the memory has already been allocated  and constructing an object on a pre-allocated buffer takes less time    char  buf    new char sizeof string       pre-allocated buffer string  p   new  buf  string  quot hi quot          placement new string  q   new string  quot hi quot                ordinary heap allocation  You may also want to be sure there can be no allocation failure at a certain part of critical code  for instance  in code executed by a pacemaker    In that case you would want to allocate memory earlier  then use placement new within the critical section  Deallocation in placement new You should not deallocate every object that is using the memory buffer   Instead you should delete   only the original buffer   You would have to then call the destructors of your classes manually   For a good suggestion on this  please see Stroustrup s FAQ on  Is there a  quot placement delete quot

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I ve used it to create objects based on memory containing messages received from the network

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I ve used it to create a Variant class  i e  an object that can represent a single value that can be one of a number of different types    If all of the value-types supported by the Variant class are POD types  e g  int  float  double  bool  then a tagged C-style union is sufficient  but if you want some of the value-types to be C   objects  e g  std  string   the C union feature won t do  as non-POD datatypes may not be declared as part of a union   So instead I allocate a byte array that is big enough  e g  sizeof the largest data type I support   and use placement new to initialize the appropriate C   object in that area when the Variant is set to hold a value of that type     And placement delete beforehand when switching away from a different non-POD data type  of course

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We use it with custom memory pools  Just a sketch   class Pool   public      Pool        implementation details irrelevant           virtual  Pool        ditto            virtual void  allocate size t       virtual void deallocate void          static Pool  misc pool     return misc pool p     global MiscPool for general use          class ClusterPool   public Pool                class FastPool   public Pool                class MapPool   public Pool                class MiscPool   public Pool                    elsewhere     void  pnew new size t size       return Pool  misc pool  - gt allocate size      void  pnew new size t size  Pool  pool p       if   pool p          return Pool  misc pool  - gt allocate size           else         return pool p- gt allocate size           void pnew delete void  p       Pool  hp   Pool  find pool p         note  if p    0  then Pool  find pool p  will return 0     if  hp          hp- gt deallocate p              elsewhere     class Obj   public        misc ctors  dtors  etc         just a sampling of new del operators    void  operator new size t s                return pnew new s        void  operator new size t s  Pool  hp      return pnew new s  hp        void operator delete void  dp              pnew delete dp        void operator delete void  dp  Pool        pnew delete dp         void  operator new   size t s              return pnew new s        void  operator new   size t s  Pool  hp    return pnew new s  hp        void operator delete   void  dp            pnew delete dp        void operator delete   void  dp  Pool      pnew delete dp            elsewhere     ClusterPool  cp   new ClusterPool arg1  arg2         Obj  new obj   new  cp  Obj arg a  arg b          Now you can cluster objects together in a single memory arena  select an allocator which is very fast but does no deallocation  use memory mapping  and any other semantic you wish to impose by choosing the pool and passing it as an argument to an object s placement new operator

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It s also useful when you want to re-initialize global or statically allocated structures    The old C way was using memset   to set all elements to 0  You cannot do that in C   due to vtables and custom object constructors   So I sometimes use the following   static Mystruct m    for                 re-initialize the structure  Note the use of placement new         and the extra parenthesis after Mystruct to force initialization       new   amp m  Mystruct             do-some work that modifies m s content

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It s used by std  vector lt  gt  because std  vector lt  gt  typically allocates more memory than there are objects in the vector lt  gt

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I ve used it in real-time programming  We typically don t want to perform any dynamic allocation  or deallocation  after the system starts up  because there s no guarantee how long that is going to take    What I can do is preallocate a large chunk of memory  large enough to hold any amount of whatever that the class may require   Then  once I figure out at runtime how to construct the things  placement new can be used to construct objects right where I want them  One situation I know I used it in was to help create a heterogeneous circular buffer   It s certainly not for the faint of heart  but that s why they make the syntax for it kinda  gnarly

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It s used by std  vector lt  gt  because std  vector lt  gt  typically allocates more memory than there are objects in the vector lt  gt

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It may be handy when using shared memory  among other uses    For example  http   www boost org doc libs 1 51 0 doc html interprocess synchronization mechanisms html interprocess synchronization mechanisms conditions conditions anonymous example

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We use it with custom memory pools  Just a sketch   class Pool   public      Pool        implementation details irrelevant           virtual  Pool        ditto            virtual void  allocate size t       virtual void deallocate void          static Pool  misc pool     return misc pool p     global MiscPool for general use          class ClusterPool   public Pool                class FastPool   public Pool                class MapPool   public Pool                class MiscPool   public Pool                    elsewhere     void  pnew new size t size       return Pool  misc pool  - gt allocate size      void  pnew new size t size  Pool  pool p       if   pool p          return Pool  misc pool  - gt allocate size           else         return pool p- gt allocate size           void pnew delete void  p       Pool  hp   Pool  find pool p         note  if p    0  then Pool  find pool p  will return 0     if  hp          hp- gt deallocate p              elsewhere     class Obj   public        misc ctors  dtors  etc         just a sampling of new del operators    void  operator new size t s                return pnew new s        void  operator new size t s  Pool  hp      return pnew new s  hp        void operator delete void  dp              pnew delete dp        void operator delete void  dp  Pool        pnew delete dp         void  operator new   size t s              return pnew new s        void  operator new   size t s  Pool  hp    return pnew new s  hp        void operator delete   void  dp            pnew delete dp        void operator delete   void  dp  Pool      pnew delete dp            elsewhere     ClusterPool  cp   new ClusterPool arg1  arg2         Obj  new obj   new  cp  Obj arg a  arg b          Now you can cluster objects together in a single memory arena  select an allocator which is very fast but does no deallocation  use memory mapping  and any other semantic you wish to impose by choosing the pool and passing it as an argument to an object s placement new operator

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It s useful if you want to separate allocation from initialization  STL uses placement new to create container elements

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I ve used it for storing objects with memory mapped files  The specific example was an image database which processed vey large numbers of large images  more than could fit in memory

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Generally  placement new is used to get rid of allocation cost of a  normal new      Another scenario where I used it is a place where I wanted to have access to the pointer to an object that was still to be constructed  to implement a per-document singleton

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I have one more idea  it is valid for C  11   Let s look at the following example   include  lt cstddef gt   include  lt cstdio gt   int main         struct alignas 0x1000  A           char data 0x1000               printf  quot max align t   zu n quot   alignof max align t         A a      printf  quot a   p n quot    amp a        A  ptr   new A      printf  quot ptr   p n quot   ptr       delete ptr     With C  11 standard  GCC gives the following output  max align t  16 a  0x7ffd45e6f000 ptr  0x1fe3ec0  ptr is not aligned properly  With C  17 standard and further  GCC gives the following output  max align t  16 a  0x7ffc924f6000 ptr  0x9f6000  ptr is aligned properly  As I know  C   standard didn t support over-aligned new before C  17 came  and if your structure has alignment greater than max align t  you can have problems  To bypass this issue in C  11  you can use aligned alloc   include  lt cstddef gt   include  lt cstdlib gt   include  lt cstdio gt   include  lt new gt   int main         struct alignas 0x1000  A           char data 0x1000               printf  quot max align t   zu n quot   alignof max align t         A a      printf  quot a   p n quot    amp a        void  buf   aligned alloc alignof A   sizeof A        if  buf    nullptr            printf  quot aligned alloc   failed n quot            exit 1             A  ptr   new buf  A        printf  quot ptr   p n quot   ptr       ptr- gt  A        free ptr      ptr is aligned in this case  max align t  16 a  0x7ffe56b57000 ptr  0x2416000

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I ve seen it used as a slight performance hack for a  dynamic type  pointer  in the section  Under the Hood        But here is the tricky trick I used to get fast performance for small types  if the value being held can fit inside of a void   I don t actually bother allocating a new object  I force it into the pointer itself using placement new

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I have an idea too  C   does have zero-overhead principle  But exceptions do not follow this principle  so sometimes they are turned off with compiler switch  Let s look to this example   include  lt new gt   include  lt cstdio gt   include  lt cstdlib gt   int main         struct A           A                 printf  quot A   n quot                       A                 printf  quot  A   n quot                      char data 1000000000000000000           some very big number             try           A  result   new A            printf  quot new passed   p n quot   result           delete result        catch  std  bad alloc            printf  quot new failed n quot             We allocate a big struct here  and check if allocation is successful  and delete it  But if we have exceptions turned off  we can t use try block  and unable to handle new   failure  So how we can do that  Here is how   include  lt new gt   include  lt cstdio gt   include  lt cstdlib gt   int main         struct A           A                 printf  quot A   n quot                       A                 printf  quot  A   n quot                      char data 1000000000000000000           some very big number             void  buf   malloc sizeof A        if  buf    nullptr            A  result   new buf  A            printf  quot new passed   p n quot   result           result- gt  A            free result         else           printf  quot new failed n quot              Use simple malloc Check if it is failed in a C way If it successful  we use placement new Manually call the destructor  we can t just call delete  call free  due we called malloc  UPD  Useless wrote a comment which opened to my view the existence of new nothrow   which should be used in this case  but not the method I wrote before  Please don t use the code I wrote before  Sorry

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