In what cases do I use malloc and or new

Question

I see in C   there are multiple ways to allocate and free data and I understand that when you call malloc you should call free and when you use the new operator you should pair with delete and it is a mistake to mix the two  e g  Calling free   on something that was created with the new operator   but I m not clear on when I should use malloc  free and when I should use new  delete in my real world programs   If you re a C   expert  please let me know any rules of thumb or conventions you follow in this regard

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Always use new in C    If you need a block of untyped memory  you can use operator new directly   void  p   operator new size          operator delete p

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new will initialise the default values of the struct and correctly links the references in it to itself   E g   struct test s       int some strange name   1      int  amp easy   some strange name      So new struct test s will return an initialised structure with a working reference  while the malloc ed version has no default values and the intern references aren t initialised

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malloc   is used to dynamically assign memory in C while the same work is done by new   in c    So you cannot mix coding conventions of 2 languages  It would be good if you asked for difference between calloc and malloc

User · Answer

Always use new in C    If you need a block of untyped memory  you can use operator new directly   void  p   operator new size          operator delete p

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There are a few things which new does that malloc doesn   t    new constructs the object by calling the constructor of that object new doesn   t require typecasting of allocated memory  It doesn   t require an amount of memory to be allocated  rather it requires a number of  objects to be constructed    So  if you use malloc  then you need to do above things explicitly  which is not always practical  Additionally  new can be overloaded but malloc can   t be

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There is one big difference between malloc and new  malloc allocates memory  This is fine for C  because in C  a lump of memory is an object   In C    if you re not dealing with POD types  which are similar to C types  you must call a constructor on a memory location to actually have an object there  Non-POD types are very common in C    as many C   features make an object automatically non-POD   new allocates memory and creates an object on that memory location  For non-POD types this means calling a constructor   If you do something like this   non pod type  p    non pod type   malloc sizeof  p     The pointer you obtain cannot be dereferenced because it does not point to an object  You d need to call a constructor on it before you can use it  and this is done using placement new    If  on the other hand  you do   non pod type  p   new non pod type      You get a pointer that is always valid  because new created an object   Even for POD types  there s a significant difference between the two   pod type  p    pod type   malloc sizeof  p   std  cout  lt  lt  p- gt foo    This piece of code would print an unspecified value  because the POD objects created by malloc are not initialised   With new  you could specify a constructor to call  and thus get a well defined value   pod type  p   new pod type    std  cout  lt  lt  p- gt foo     prints 0   If you really want it  you can use use new to obtain uninitialised POD objects  See this other answer for more information on that   Another difference is the behaviour upon failure  When it fails to allocate memory  malloc returns a null pointer  while new throws an exception   The former requires you to test every pointer returned before using it  while the later will always produce valid pointers   For these reasons  in C   code you should use new  and not malloc  But even then  you should not use new  in the open   because it acquires resources you need to release later on  When you use new you should pass its result immediately into a resource managing class   std  unique ptr lt T gt  p   std  unique ptr lt T gt  new T        this won t leak

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The new and delete operators can operate on classes and structures  whereas malloc and free only work with blocks of memory that need to be cast   Using new delete will help to improve your code as you will not need to cast allocated memory to the required data structure

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If you work with data that doesn t need construction destruction and requires reallocations  e g   a large array of ints   then I believe malloc free is a good choice as it gives you realloc  which is way faster than new-memcpy-delete  it is on my Linux box  but I guess this may be platform dependent   If you work with C   objects that are not POD and require construction destruction  then you must use the new and delete operators   Anyway  I don t see why you shouldn t use both  provided that you free your malloced memory and delete objects allocated with new  if can take advantage of the speed boost  sometimes a significant one  if you re reallocing large arrays of POD  that realloc can give you   Unless you need it though  you should stick to new delete in C

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Use malloc and free only for allocating memory that is going to be managed by c-centric libraries and APIs  Use new and delete  and the    variants  for everything that you control

User · Answer

From the C   FQA Lite       16 4  Why should I use new instead of   trustworthy old malloc         FAQ  new delete call the   constructor destructor  new is type   safe  malloc is not  new can be   overridden by a class       FQA  The virtues of new mentioned by   the FAQ are not virtues  because   constructors  destructors  and   operator overloading are garbage  see   what happens when you have no garbage   collection    and the type safety   issue is really tiny here  normally   you have to cast the void  returned by   malloc to the right pointer type to   assign it to a typed pointer variable    which may be annoying  but far from    unsafe         Oh  and using trustworthy old malloc   makes it possible to use the equally   trustworthy  amp  old realloc  Too bad we   don t have a shiny new operator renew or something       Still  new is not bad enough to   justify a deviation from the common   style used throughout a language  even   when the language is C    In   particular  classes with non-trivial   constructors will misbehave in fatal   ways if you simply malloc the objects    So why not use new throughout the   code  People rarely overload operator   new  so it probably won t get in your   way too much  And if they do overload   new  you can always ask them to stop    Sorry  I just couldn t resist

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Rare case to consider using malloc free instead of new delete is when you re allocating and then reallocating  simple pod types  not objects  using realloc as there is no similar function to realloc in C    although this can be done using a more C   approach

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The short answer is  don t use malloc for C   without a really good reason for doing so  malloc has a number of deficiencies when used with C    which new was defined to overcome   Deficiencies fixed by new for C   code   malloc is not typesafe in any meaningful way  In C   you are required to cast the return from void   This potentially introduces a lot of problems    include  lt stdlib h gt   struct foo     double d 5        int main       foo  f1   malloc 1      error  no cast   foo  f2   static cast lt foo  gt  malloc sizeof foo       foo  f3   static cast lt foo  gt  malloc 1       No error  bad    It s worse than that though  If the type in question is POD  plain old data  then you can semi-sensibly use malloc to allocate memory for it  as f2 does in the first example   It s not so obvious though if a type is POD  The fact that it s possible for a given type to change from POD to non-POD with no resulting compiler error and potentially very hard to debug problems is a significant factor  For example if someone  possibly another programmer  during maintenance  much later on were to make a change that caused foo to no longer be POD then no obvious error would appear at compile time as you d hope  e g    struct foo     double d 5     virtual  foo            would make the malloc of f2 also become bad  without any obvious diagnostics  The   example here is trivial  but it s possible to accidentally introduce non-PODness much further away  e g  in a base class  by adding a non-POD member   If you have C  11 boost you can use is pod to check that this assumption is correct and produce an error if it s not    include  lt type traits gt   include  lt stdlib h gt   foo  safe foo malloc       static assert std  is pod lt foo gt   value   foo must be POD      return static cast lt foo  gt  malloc sizeof foo         Although boost is unable to determine if a type is POD without C  11 or some other compiler extensions  malloc returns NULL if allocation fails  new will throw std  bad alloc  The behaviour of later using a NULL pointer is undefined  An exception has clean semantics when it is thrown and it is thrown from the source of the error  Wrapping malloc with an appropriate test at every call seems tedious and error prone   You only have to forget once to undo all that good work   An exception can be allowed to propagate to a level where a caller is able to sensibly process it  where as NULL is much harder to pass back meaningfully  We could extend our safe foo malloc function to throw an exception or exit the program or call some handler    include  lt type traits gt   include  lt stdlib h gt   void my malloc failed handler     foo  safe foo malloc       static assert std  is pod lt foo gt   value   foo must be POD      foo  mem   static cast lt foo  gt  malloc sizeof foo       if   mem         my malloc failed handler            or throw           return mem     Fundamentally malloc is a C feature and new is a C   feature  As a result malloc does not play nicely with constructors  it only looks at allocating a chunk of bytes  We could extend our safe foo malloc further to use placement new    include  lt stdlib h gt   include  lt new gt   void my malloc failed handler     foo  safe foo malloc       void  mem   malloc sizeof foo      if   mem         my malloc failed handler            or throw           return new  mem foo       Our safe foo malloc function isn t very generic - ideally we d want something that can handle any type  not just foo  We can achieve this with templates and variadic templates for non-default constructors    include  lt functional gt   include  lt new gt   include  lt stdlib h gt   void my malloc failed handler     template  lt typename T gt  struct alloc     template  lt typename    Args gt    static T  safe malloc Args amp  amp     args        void  mem   malloc sizeof T        if   mem           my malloc failed handler              or throw               return new  mem T std  forward args                Now though in fixing all the issues we identified so far we ve practically reinvented the default new operator  If you re going to use malloc and placement new then you might as well just use new to begin with

User · Answer

malloc   is used to dynamically assign memory in C while the same work is done by new   in c    So you cannot mix coding conventions of 2 languages  It would be good if you asked for difference between calloc and malloc

User · Answer

The new and delete operators can operate on classes and structures  whereas malloc and free only work with blocks of memory that need to be cast   Using new delete will help to improve your code as you will not need to cast allocated memory to the required data structure

User · Answer

new vs malloc    1  new is an operator  while malloc   is a function   2  new calls constructors  while malloc   does not   3  new returns exact data type  while malloc   returns void     4  new never returns a NULL  will throw on failure  while malloc   returns NULL  5  Reallocation of memory not handled by new while malloc   can

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Rare case to consider using malloc free instead of new delete is when you re allocating and then reallocating  simple pod types  not objects  using realloc as there is no similar function to realloc in C    although this can be done using a more C   approach

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To answer your question  you should know the difference between malloc and new  The difference is simple   malloc allocates memory  while new allocates memory AND calls the constructor of the object you re allocating memory for   So  unless you re restricted to C  you should never use malloc  especially when dealing with C   objects  That would be a recipe for breaking your program   Also the difference between free and delete is quite the same  The difference is that delete will call the destructor of your object in addition to freeing memory

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If you have C code you want to port over to C    you might leave any malloc   calls in it   For any new C   code  I d recommend using new instead

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From a lower perspective  new will initialize all the memory before giving the memory whereas malloc will keep the original content of the memory

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Always use new in C    If you need a block of untyped memory  you can use operator new directly   void  p   operator new size          operator delete p

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From a lower perspective  new will initialize all the memory before giving the memory whereas malloc will keep the original content of the memory

User · Answer

If you have C code you want to port over to C    you might leave any malloc   calls in it   For any new C   code  I d recommend using new instead

User · Answer

If you have C code you want to port over to C    you might leave any malloc   calls in it   For any new C   code  I d recommend using new instead

User · Answer

The short answer is  don t use malloc for C   without a really good reason for doing so  malloc has a number of deficiencies when used with C    which new was defined to overcome   Deficiencies fixed by new for C   code   malloc is not typesafe in any meaningful way  In C   you are required to cast the return from void   This potentially introduces a lot of problems    include  lt stdlib h gt   struct foo     double d 5        int main       foo  f1   malloc 1      error  no cast   foo  f2   static cast lt foo  gt  malloc sizeof foo       foo  f3   static cast lt foo  gt  malloc 1       No error  bad    It s worse than that though  If the type in question is POD  plain old data  then you can semi-sensibly use malloc to allocate memory for it  as f2 does in the first example   It s not so obvious though if a type is POD  The fact that it s possible for a given type to change from POD to non-POD with no resulting compiler error and potentially very hard to debug problems is a significant factor  For example if someone  possibly another programmer  during maintenance  much later on were to make a change that caused foo to no longer be POD then no obvious error would appear at compile time as you d hope  e g    struct foo     double d 5     virtual  foo            would make the malloc of f2 also become bad  without any obvious diagnostics  The   example here is trivial  but it s possible to accidentally introduce non-PODness much further away  e g  in a base class  by adding a non-POD member   If you have C  11 boost you can use is pod to check that this assumption is correct and produce an error if it s not    include  lt type traits gt   include  lt stdlib h gt   foo  safe foo malloc       static assert std  is pod lt foo gt   value   foo must be POD      return static cast lt foo  gt  malloc sizeof foo         Although boost is unable to determine if a type is POD without C  11 or some other compiler extensions  malloc returns NULL if allocation fails  new will throw std  bad alloc  The behaviour of later using a NULL pointer is undefined  An exception has clean semantics when it is thrown and it is thrown from the source of the error  Wrapping malloc with an appropriate test at every call seems tedious and error prone   You only have to forget once to undo all that good work   An exception can be allowed to propagate to a level where a caller is able to sensibly process it  where as NULL is much harder to pass back meaningfully  We could extend our safe foo malloc function to throw an exception or exit the program or call some handler    include  lt type traits gt   include  lt stdlib h gt   void my malloc failed handler     foo  safe foo malloc       static assert std  is pod lt foo gt   value   foo must be POD      foo  mem   static cast lt foo  gt  malloc sizeof foo       if   mem         my malloc failed handler            or throw           return mem     Fundamentally malloc is a C feature and new is a C   feature  As a result malloc does not play nicely with constructors  it only looks at allocating a chunk of bytes  We could extend our safe foo malloc further to use placement new    include  lt stdlib h gt   include  lt new gt   void my malloc failed handler     foo  safe foo malloc       void  mem   malloc sizeof foo      if   mem         my malloc failed handler            or throw           return new  mem foo       Our safe foo malloc function isn t very generic - ideally we d want something that can handle any type  not just foo  We can achieve this with templates and variadic templates for non-default constructors    include  lt functional gt   include  lt new gt   include  lt stdlib h gt   void my malloc failed handler     template  lt typename T gt  struct alloc     template  lt typename    Args gt    static T  safe malloc Args amp  amp     args        void  mem   malloc sizeof T        if   mem           my malloc failed handler              or throw               return new  mem T std  forward args                Now though in fixing all the issues we identified so far we ve practically reinvented the default new operator  If you re going to use malloc and placement new then you might as well just use new to begin with

User · Answer

The new and delete operators can operate on classes and structures  whereas malloc and free only work with blocks of memory that need to be cast   Using new delete will help to improve your code as you will not need to cast allocated memory to the required data structure

User · Answer

If you work with data that doesn t need construction destruction and requires reallocations  e g   a large array of ints   then I believe malloc free is a good choice as it gives you realloc  which is way faster than new-memcpy-delete  it is on my Linux box  but I guess this may be platform dependent   If you work with C   objects that are not POD and require construction destruction  then you must use the new and delete operators   Anyway  I don t see why you shouldn t use both  provided that you free your malloced memory and delete objects allocated with new  if can take advantage of the speed boost  sometimes a significant one  if you re reallocing large arrays of POD  that realloc can give you   Unless you need it though  you should stick to new delete in C

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Dynamic allocation is only required when the life-time of the object should be different than the scope it gets created in  This holds as well for making the scope smaller as larger  and you have a specific reason where storing it by value doesn t work   For example    std  vector lt int gt   createVector       Bad  std  vector lt int gt  createVector        Good   auto v   new std  vector lt int gt        Bad  auto result   calculate   optional output      v    auto v   std  vector lt int gt        Good  auto result   calculate   optional output       amp v     From C  11 on  we have std  unique ptr for dealing with allocated memory  which contains the ownership of the allocated memory  std  shared ptr was created for when you have to share ownership   you ll need this less than you would expect in a good program   Creating an instance becomes really easy   auto instance   std  make unique lt Class gt    args        C  14 auto instance   std  make unique lt Class gt  new Class   args         C  11 auto instance   std  make unique lt Class   gt  42      C  14 auto instance   std  make unique lt Class   gt  new Class   42       C  11   C  17 also adds std  optional which can prevent you from requiring memory allocations  auto optInstance   std  optional lt Class gt     if  condition      optInstance   Class      As soon as  instance  goes out of scope  the memory gets cleaned up  Transferring ownership is also easy    auto vector   std  vector lt std  unique ptr lt Interface gt  gt      auto instance   std  make unique lt Class gt      vector push back std  move instance       std  move - gt  transfer  most of the time    So when do you still need new  Almost never from C  11 on  Most of the you use std  make unique until you get to a point where you hit an API that transfers ownership via raw pointers    auto instance   std  make unique lt Class gt      legacyFunction instance release        Ownership being transferred   auto instance   std  unique ptr lt Class gt  legacyFunction        Ownership being captured in unique ptr   In C  98 03  you have to do manual memory management  If you are in this case  try upgrading to a more recent version of the standard  If you are stuck    auto instance   new Class       Allocate memory  delete instance                 Deallocate  auto instances   new Class 42        Allocate memory  delete   instances                   Deallocate   Make sure that you track the ownership correctly to not have any memory leaks  Move semantics don t work yet either   So  when do we need malloc in C    The only valid reason would be to allocate memory and initialize it later via placement new    auto instanceBlob   std  malloc sizeof Class       Allocate memory  auto instance   new instanceBlob Class       Initialize via constructor  instance  Class       Destroy via destructor  std  free instanceBlob      Deallocate the memory   Even though  the above is valid  this can be done via a new-operator as well  std  vector is a good example for this   Finally  we still have the elephant in the room  C  If you have to work with a C-library where memory gets allocated in the C   code and freed in the C code  or the other way around   you are forced to use malloc free   If you are in this case  forget about virtual functions  member functions  classes     Only structs with PODs in it are allowed   Some exceptions to the rules    You are writing a standard library with advanced data structures where malloc is appropriate You have to allocate big amounts of memory  In memory copy of a 10GB file   You have tooling preventing you to use certain constructs You need to store an incomplete type

User · Answer

The new and delete operators can operate on classes and structures  whereas malloc and free only work with blocks of memory that need to be cast   Using new delete will help to improve your code as you will not need to cast allocated memory to the required data structure

User · Answer

Unless you are forced to use C  you should never use malloc   Always use new   If you need a big chunk of data just do something like   char  pBuffer   new char 1024     Be careful though this is not correct     This is incorrect - may delete only one element  may corrupt the heap  or worse    delete pBuffer    Instead you should do this when deleting an array of data     This deletes all items in the array delete   pBuffer    The new keyword is the C   way of doing it  and it will ensure that your type will have its constructor called   The new keyword is also more type-safe whereas malloc is not type-safe at all    The only way I could think that would be beneficial to use malloc would be if you needed to change the size of your buffer of data   The new keyword does not have an analogous way like realloc   The realloc function might be able to extend the size of a chunk of memory for you more efficiently    It is worth mentioning that you cannot mix new free and malloc delete   Note  Some answers in this question are invalid    int  p scalar   new int 5       Does not create 5 elements  but initializes to 5 int  p array    new int 5       Creates 5 elements

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Unless you are forced to use C  you should never use malloc   Always use new   If you need a big chunk of data just do something like   char  pBuffer   new char 1024     Be careful though this is not correct     This is incorrect - may delete only one element  may corrupt the heap  or worse    delete pBuffer    Instead you should do this when deleting an array of data     This deletes all items in the array delete   pBuffer    The new keyword is the C   way of doing it  and it will ensure that your type will have its constructor called   The new keyword is also more type-safe whereas malloc is not type-safe at all    The only way I could think that would be beneficial to use malloc would be if you needed to change the size of your buffer of data   The new keyword does not have an analogous way like realloc   The realloc function might be able to extend the size of a chunk of memory for you more efficiently    It is worth mentioning that you cannot mix new free and malloc delete   Note  Some answers in this question are invalid    int  p scalar   new int 5       Does not create 5 elements  but initializes to 5 int  p array    new int 5       Creates 5 elements

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From the C   FQA Lite       16 4  Why should I use new instead of   trustworthy old malloc         FAQ  new delete call the   constructor destructor  new is type   safe  malloc is not  new can be   overridden by a class       FQA  The virtues of new mentioned by   the FAQ are not virtues  because   constructors  destructors  and   operator overloading are garbage  see   what happens when you have no garbage   collection    and the type safety   issue is really tiny here  normally   you have to cast the void  returned by   malloc to the right pointer type to   assign it to a typed pointer variable    which may be annoying  but far from    unsafe         Oh  and using trustworthy old malloc   makes it possible to use the equally   trustworthy  amp  old realloc  Too bad we   don t have a shiny new operator renew or something       Still  new is not bad enough to   justify a deviation from the common   style used throughout a language  even   when the language is C    In   particular  classes with non-trivial   constructors will misbehave in fatal   ways if you simply malloc the objects    So why not use new throughout the   code  People rarely overload operator   new  so it probably won t get in your   way too much  And if they do overload   new  you can always ask them to stop    Sorry  I just couldn t resist

User · Answer

Use malloc and free only for allocating memory that is going to be managed by c-centric libraries and APIs  Use new and delete  and the    variants  for everything that you control

User · Answer

From the C   FQA Lite       16 4  Why should I use new instead of   trustworthy old malloc         FAQ  new delete call the   constructor destructor  new is type   safe  malloc is not  new can be   overridden by a class       FQA  The virtues of new mentioned by   the FAQ are not virtues  because   constructors  destructors  and   operator overloading are garbage  see   what happens when you have no garbage   collection    and the type safety   issue is really tiny here  normally   you have to cast the void  returned by   malloc to the right pointer type to   assign it to a typed pointer variable    which may be annoying  but far from    unsafe         Oh  and using trustworthy old malloc   makes it possible to use the equally   trustworthy  amp  old realloc  Too bad we   don t have a shiny new operator renew or something       Still  new is not bad enough to   justify a deviation from the common   style used throughout a language  even   when the language is C    In   particular  classes with non-trivial   constructors will misbehave in fatal   ways if you simply malloc the objects    So why not use new throughout the   code  People rarely overload operator   new  so it probably won t get in your   way too much  And if they do overload   new  you can always ask them to stop    Sorry  I just couldn t resist

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Unless you are forced to use C  you should never use malloc   Always use new   If you need a big chunk of data just do something like   char  pBuffer   new char 1024     Be careful though this is not correct     This is incorrect - may delete only one element  may corrupt the heap  or worse    delete pBuffer    Instead you should do this when deleting an array of data     This deletes all items in the array delete   pBuffer    The new keyword is the C   way of doing it  and it will ensure that your type will have its constructor called   The new keyword is also more type-safe whereas malloc is not type-safe at all    The only way I could think that would be beneficial to use malloc would be if you needed to change the size of your buffer of data   The new keyword does not have an analogous way like realloc   The realloc function might be able to extend the size of a chunk of memory for you more efficiently    It is worth mentioning that you cannot mix new free and malloc delete   Note  Some answers in this question are invalid    int  p scalar   new int 5       Does not create 5 elements  but initializes to 5 int  p array    new int 5       Creates 5 elements

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new will initialise the default values of the struct and correctly links the references in it to itself   E g   struct test s       int some strange name   1      int  amp easy   some strange name      So new struct test s will return an initialised structure with a working reference  while the malloc ed version has no default values and the intern references aren t initialised

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new vs malloc    1  new is an operator  while malloc   is a function   2  new calls constructors  while malloc   does not   3  new returns exact data type  while malloc   returns void     4  new never returns a NULL  will throw on failure  while malloc   returns NULL  5  Reallocation of memory not handled by new while malloc   can

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Dynamic allocation is only required when the life-time of the object should be different than the scope it gets created in  This holds as well for making the scope smaller as larger  and you have a specific reason where storing it by value doesn t work   For example    std  vector lt int gt   createVector       Bad  std  vector lt int gt  createVector        Good   auto v   new std  vector lt int gt        Bad  auto result   calculate   optional output      v    auto v   std  vector lt int gt        Good  auto result   calculate   optional output       amp v     From C  11 on  we have std  unique ptr for dealing with allocated memory  which contains the ownership of the allocated memory  std  shared ptr was created for when you have to share ownership   you ll need this less than you would expect in a good program   Creating an instance becomes really easy   auto instance   std  make unique lt Class gt    args        C  14 auto instance   std  make unique lt Class gt  new Class   args         C  11 auto instance   std  make unique lt Class   gt  42      C  14 auto instance   std  make unique lt Class   gt  new Class   42       C  11   C  17 also adds std  optional which can prevent you from requiring memory allocations  auto optInstance   std  optional lt Class gt     if  condition      optInstance   Class      As soon as  instance  goes out of scope  the memory gets cleaned up  Transferring ownership is also easy    auto vector   std  vector lt std  unique ptr lt Interface gt  gt      auto instance   std  make unique lt Class gt      vector push back std  move instance       std  move - gt  transfer  most of the time    So when do you still need new  Almost never from C  11 on  Most of the you use std  make unique until you get to a point where you hit an API that transfers ownership via raw pointers    auto instance   std  make unique lt Class gt      legacyFunction instance release        Ownership being transferred   auto instance   std  unique ptr lt Class gt  legacyFunction        Ownership being captured in unique ptr   In C  98 03  you have to do manual memory management  If you are in this case  try upgrading to a more recent version of the standard  If you are stuck    auto instance   new Class       Allocate memory  delete instance                 Deallocate  auto instances   new Class 42        Allocate memory  delete   instances                   Deallocate   Make sure that you track the ownership correctly to not have any memory leaks  Move semantics don t work yet either   So  when do we need malloc in C    The only valid reason would be to allocate memory and initialize it later via placement new    auto instanceBlob   std  malloc sizeof Class       Allocate memory  auto instance   new instanceBlob Class       Initialize via constructor  instance  Class       Destroy via destructor  std  free instanceBlob      Deallocate the memory   Even though  the above is valid  this can be done via a new-operator as well  std  vector is a good example for this   Finally  we still have the elephant in the room  C  If you have to work with a C-library where memory gets allocated in the C   code and freed in the C code  or the other way around   you are forced to use malloc free   If you are in this case  forget about virtual functions  member functions  classes     Only structs with PODs in it are allowed   Some exceptions to the rules    You are writing a standard library with advanced data structures where malloc is appropriate You have to allocate big amounts of memory  In memory copy of a 10GB file   You have tooling preventing you to use certain constructs You need to store an incomplete type

User · Answer

Use malloc and free only for allocating memory that is going to be managed by c-centric libraries and APIs  Use new and delete  and the    variants  for everything that you control

User · Answer

Unless you are forced to use C  you should never use malloc   Always use new   If you need a big chunk of data just do something like   char  pBuffer   new char 1024     Be careful though this is not correct     This is incorrect - may delete only one element  may corrupt the heap  or worse    delete pBuffer    Instead you should do this when deleting an array of data     This deletes all items in the array delete   pBuffer    The new keyword is the C   way of doing it  and it will ensure that your type will have its constructor called   The new keyword is also more type-safe whereas malloc is not type-safe at all    The only way I could think that would be beneficial to use malloc would be if you needed to change the size of your buffer of data   The new keyword does not have an analogous way like realloc   The realloc function might be able to extend the size of a chunk of memory for you more efficiently    It is worth mentioning that you cannot mix new free and malloc delete   Note  Some answers in this question are invalid    int  p scalar   new int 5       Does not create 5 elements  but initializes to 5 int  p array    new int 5       Creates 5 elements

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From the C   FQA Lite       16 4  Why should I use new instead of   trustworthy old malloc         FAQ  new delete call the   constructor destructor  new is type   safe  malloc is not  new can be   overridden by a class       FQA  The virtues of new mentioned by   the FAQ are not virtues  because   constructors  destructors  and   operator overloading are garbage  see   what happens when you have no garbage   collection    and the type safety   issue is really tiny here  normally   you have to cast the void  returned by   malloc to the right pointer type to   assign it to a typed pointer variable    which may be annoying  but far from    unsafe         Oh  and using trustworthy old malloc   makes it possible to use the equally   trustworthy  amp  old realloc  Too bad we   don t have a shiny new operator renew or something       Still  new is not bad enough to   justify a deviation from the common   style used throughout a language  even   when the language is C    In   particular  classes with non-trivial   constructors will misbehave in fatal   ways if you simply malloc the objects    So why not use new throughout the   code  People rarely overload operator   new  so it probably won t get in your   way too much  And if they do overload   new  you can always ask them to stop    Sorry  I just couldn t resist

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To answer your question  you should know the difference between malloc and new  The difference is simple   malloc allocates memory  while new allocates memory AND calls the constructor of the object you re allocating memory for   So  unless you re restricted to C  you should never use malloc  especially when dealing with C   objects  That would be a recipe for breaking your program   Also the difference between free and delete is quite the same  The difference is that delete will call the destructor of your object in addition to freeing memory

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Use malloc and free only for allocating memory that is going to be managed by c-centric libraries and APIs  Use new and delete  and the    variants  for everything that you control

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Always use new in C    If you need a block of untyped memory  you can use operator new directly   void  p   operator new size          operator delete p

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If you have C code you want to port over to C    you might leave any malloc   calls in it   For any new C   code  I d recommend using new instead

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If you are using C    try to use new delete instead of malloc calloc as they are operators  For malloc calloc  you need to include another header  Don t mix two different languages in the same code  Their work is similar in every manner  both allocates memory dynamically from heap segment in hash table

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There are a few things which new does that malloc doesn   t    new constructs the object by calling the constructor of that object new doesn   t require typecasting of allocated memory  It doesn   t require an amount of memory to be allocated  rather it requires a number of  objects to be constructed    So  if you use malloc  then you need to do above things explicitly  which is not always practical  Additionally  new can be overloaded but malloc can   t be

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There is one big difference between malloc and new  malloc allocates memory  This is fine for C  because in C  a lump of memory is an object   In C    if you re not dealing with POD types  which are similar to C types  you must call a constructor on a memory location to actually have an object there  Non-POD types are very common in C    as many C   features make an object automatically non-POD   new allocates memory and creates an object on that memory location  For non-POD types this means calling a constructor   If you do something like this   non pod type  p    non pod type   malloc sizeof  p     The pointer you obtain cannot be dereferenced because it does not point to an object  You d need to call a constructor on it before you can use it  and this is done using placement new    If  on the other hand  you do   non pod type  p   new non pod type      You get a pointer that is always valid  because new created an object   Even for POD types  there s a significant difference between the two   pod type  p    pod type   malloc sizeof  p   std  cout  lt  lt  p- gt foo    This piece of code would print an unspecified value  because the POD objects created by malloc are not initialised   With new  you could specify a constructor to call  and thus get a well defined value   pod type  p   new pod type    std  cout  lt  lt  p- gt foo     prints 0   If you really want it  you can use use new to obtain uninitialised POD objects  See this other answer for more information on that   Another difference is the behaviour upon failure  When it fails to allocate memory  malloc returns a null pointer  while new throws an exception   The former requires you to test every pointer returned before using it  while the later will always produce valid pointers   For these reasons  in C   code you should use new  and not malloc  But even then  you should not use new  in the open   because it acquires resources you need to release later on  When you use new you should pass its result immediately into a resource managing class   std  unique ptr lt T gt  p   std  unique ptr lt T gt  new T        this won t leak

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If you are using C    try to use new delete instead of malloc calloc as they are operators  For malloc calloc  you need to include another header  Don t mix two different languages in the same code  Their work is similar in every manner  both allocates memory dynamically from heap segment in hash table

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In the following scenario  we can t use new since it calls constructor    class  B    private      B  ptr      int x  public      B int n             cout lt  lt  B  ctr  lt  lt endl            ptr   new B     keep calling ctr  result is segmentation fault         ptr    B   malloc sizeof B            x   n          ptr- gt x   n   10             B                delete ptr          free ptr           cout lt  lt  B  dtr  lt  lt endl

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In the following scenario  we can t use new since it calls constructor    class  B    private      B  ptr      int x  public      B int n             cout lt  lt  B  ctr  lt  lt endl            ptr   new B     keep calling ctr  result is segmentation fault         ptr    B   malloc sizeof B            x   n          ptr- gt x   n   10             B                delete ptr          free ptr           cout lt  lt  B  dtr  lt  lt endl

[c++] In what cases do I use malloc and/or new?

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