Is there any use for unique ptr with array

Question

std  unique ptr has support for arrays  for instance   std  unique ptr lt int   gt  p new int 10      but is it needed  probably it is more convenient to use std  vector or std  array   Do you find any use for that construct

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In a nutshell  it s by far the most memory-efficient   A std  string comes with a pointer  a length  and a  short-string-optimization  buffer   But my situation is I need to store a string that is almost always empty  in a structure that I have hundreds of thousands of   In C  I would just use char    and it would be null most of the time   Which works for C    too  except that a char   has no destructor  and doesn t know to delete itself   By contrast  a std  unique ptr lt char   gt  will delete itself when it goes out of scope   An empty std  string takes up 32 bytes  but an empty std  unique ptr lt char   gt  takes up 8 bytes  that is  exactly the size of its pointer   The biggest downside is  every time I want to know the length of the string  I have to call strlen on it

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Contrary to std  vector and std  array  std  unique ptr can own a NULL pointer  This comes in handy when working with C APIs that expect either an array or NULL   void legacy func const int  array or null    void some func             std  unique ptr lt int   gt  ptr      if  some condition            ptr reset new int 10               legacy func ptr get

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There are tradeoffs  and you pick the solution which matches what you want  Off the top of my head   Initial size   vector and unique ptr lt T   gt  allow the size to be specified at run-time array only allows the size to be specified at compile time   Resizing   array and unique ptr lt T   gt  do not allow resizing vector does   Storage   vector and unique ptr lt T   gt  store the data outside the object  typically on the heap  array stores the data directly in the object   Copying   array and vector allow copying unique ptr lt T   gt  does not allow copying   Swap move   vector and unique ptr lt T   gt  have O 1  time swap and move operations array has O n  time swap and move operations  where n is the number of elements in the array   Pointer reference iterator invalidation   array ensures pointers  references and iterators will never be invalidated while the object is live  even on swap   unique ptr lt T   gt  has no iterators  pointers and references are only invalidated by swap   while the object is live   After swapping  pointers point into to the array that you swapped with  so they re still  valid  in that sense   vector may invalidate pointers  references and iterators on any reallocation  and provides some guarantees that reallocation can only happen on certain operations     Compatibility with concepts and algorithms   array and vector are both Containers unique ptr lt T   gt  is not a Container   I do have to admit  this looks like an opportunity for some refactoring with policy-based design

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I have used unique ptr lt char   gt  to implement a preallocated memory pools used in a game engine  The idea is to provide preallocated memory pools used instead of dynamic allocations for returning collision requests results and other stuff like particle physics without having to allocate   free memory at each frame  It s pretty convenient for this kind of scenarios where you need memory pools to allocate objects with limited life time  typically one  2 or 3 frames  that do not require destruction logic  only memory deallocation

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I faced a case where I had to use std  unique ptr lt bool   gt   which was in the HDF5 library  A library for efficient binary data storage  used a lot in science   Some compilers  Visual Studio 2015 in my case  provide compression of std  vector lt bool gt   by using 8 bools in every byte   which is a catastrophe for something like HDF5  which doesn t care about that compression  With std  vector lt bool gt   HDF5 was eventually reading garbage because of that compression   Guess who was there for the rescue  in a case where std  vector didn t work  and I needed to allocate a dynamic array cleanly   -

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To answer people thinking you  have to  use vector instead of unique ptr I have a case in CUDA programming on GPU when you allocate memory in Device you must go for a pointer array  with cudaMalloc   Then  when retrieving this data in Host  you must go again for a pointer and unique ptr is fine to handle pointer easily  The extra cost of converting double  to vector lt double gt  is unnecessary and leads to a loss of perf

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You need your structure to contain just a pointer for binary-compatibility reasons  You need to interface with an API that returns memory allocated with new   Your firm or project has a general rule against using std  vector  for example  to prevent careless programmers from accidentally introducing copies You want to prevent careless programmers from accidentally introducing copies in this instance    There is a general rule that C   containers are to be preferred over rolling-your-own with pointers  It is a general rule  it has exceptions  There s more  these are just examples

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A common pattern can be found in some Windows Win32 API calls  in which the use of std  unique ptr lt T   gt  can come in handy  e g  when you don t exactly know how big an output buffer should be when calling some Win32 API  that will write some data inside that buffer       Buffer dynamically allocated by the caller  and filled by some Win32 API function      Allocation will be made inside the  while  loop below   std  unique ptr lt BYTE   gt  buffer      Buffer length  in bytes     Initialize with some initial length that you expect to succeed at the first API call  UINT32 bufferLength               LONG returnCode   ERROR INSUFFICIENT BUFFER  while  returnCode    ERROR INSUFFICIENT BUFFER           Allocate buffer of specified length     buffer reset  BYTE bufferLength                           Or  in C  14  could use make unique   instead  e g                buffer   std  make unique lt BYTE   gt  bufferLength                         Call some Win32 API                If the size of the buffer  stored in  bufferLength   is not big enough         the API will return ERROR INSUFFICIENT BUFFER  and the required size        in the  in  out  parameter  bufferLength          In that case  there will be another try in the next loop iteration         with the allocation of a bigger buffer                 Else  we ll exit the while loop body  and there will be either a failure        different from ERROR INSUFFICIENT BUFFER  or the call will be successful        and the required information will be available in the buffer             returnCode     SomeApiCall inParam1  inParam2  inParam3                                   amp bufferLength     size of output buffer                                buffer get        output buffer pointer                                 amp outParam1   amp outParam2      if  Failed returnCode            Handle failure  or throw exception  etc                All right     Do some processing with the returned information

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An std  vector can be copied around  while unique ptr lt int   gt  allows expressing unique ownership of the array  std  array  on the other hand  requires the size to be determined at compile-time  which may be impossible in some situations

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They may be the rightest answer possible when you only get to poke a single pointer through an existing API  think window message or threading-related callback parameters  that have some measure of lifetime after being  caught  on the other side of the hatch  but which is unrelated to the calling code   unique ptr lt byte   gt  data   get some data     threadpool- gt post work    void  param    do a thing unique ptr lt byte   gt   byte  param                             data release       We all want things to be nice for us  C   is for the other times

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If you need a dynamic array of objects that are not copy-constructible  then a smart pointer to an array is the way to go  For example  what if you need an array of atomics

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One reason you might use a unique ptr is if you don t want to pay the runtime cost of value-initializing the array   std  vector lt char gt  vec 1000000      allocates AND value-initializes 1000000 chars  std  unique ptr lt char   gt  p new char 1000000       allocates storage for 1000000 chars   The std  vector constructor and std  vector  resize   will value-initialize T - but new will not do that if T is a POD   See Value-Initialized Objects in C  11 and std  vector constructor  Note that vector  reserve is not an alternative here  Is accessing the raw pointer after std  vector  reserve safe   It s the same reason a C programmer might choose malloc over calloc

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Scott Meyers has this to say in Effective Modern C        The existence of std  unique ptr for arrays should be of only intellectual interest to you  because std  array     std  vector  std  string are virtually always better data structure choices than raw arrays  About the only situation I can conceive of when a std  unique ptr lt T   gt  would make sense would be when you re using a C-like API that returns a raw pointer to a heap array that you assume ownership of    I think that Charles Salvia s answer is relevant though  that std  unique ptr lt T   gt  is the only way to initialise an empty array whose size is not known at compile time  What would Scott Meyers have to say about this motivation for using std  unique ptr lt T   gt

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unique ptr lt char   gt  can be used where you want the performance of C and convenience of C    Consider you need to operate on millions  ok  billions if you don t trust yet  of strings  Storing each of them in a separate string or vector lt char gt  object would be a disaster for the memory  heap  management routines  Especially if you need to allocate and delete different strings many times   However  you can allocate a single buffer for storing that many strings  You wouldn t like char  buffer    char  malloc total size   for obvious reasons  if not obvious  search for  why use smart ptrs    You would rather like unique ptr lt char   gt  buffer new char total size     By analogy  the same performance amp convenience considerations apply to non-char data  consider millions of vectors matrices objects

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One additional reason to allow and use std  unique ptr lt T   gt   that hasn t been mentioned in the responses so far  it allows you to forward-declare the array element type   This is useful when you want to minimize the chained  include statements in  headers  to optimize build performance    For instance -     myclass h    class ALargeAndComplicatedClassWithLotsOfDependencies   class MyClass          private     std  unique ptr lt ALargeAndComplicatedClassWithLotsOfDependencies   gt  m InternalArray          myclass cpp     include  myclass h   include  ALargeAndComplicatedClassWithLotsOfDependencies h      MyClass implementation goes here   With the above code structure  anyone can  include  myclass h  and use MyClass  without having to include the internal implementation dependencies required by MyClass  m InternalArray   If m InternalArray was instead declared as a std  array lt ALargeAndComplicatedClassWithLotsOfDependencies gt   or a std  vector lt     gt   respectively - the result would be attempted usage of an incomplete type  which is a compile-time error

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Some people do not have the luxury of using std  vector  even with allocators  Some people need a dynamically sized array  so std  array is out  And some people get their arrays from other code that is known to return an array  and that code isn t going to be rewritten to return a vector or something   By allowing unique ptr lt T   gt   you service those needs   In short  you use unique ptr lt T   gt  when you need to  When the alternatives simply aren t going to work for you  It s a tool of last resort

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I can t disagree with the spirit of the accepted answer strongly enough   A tool of last resort   Far from it   The way I see it  one of the strongest features of C   compared to C and to some other similar languages is the ability to express constraints so that they can be checked at compile time and accidental misuse can be prevented  So when designing a structure  ask yourself what operations it should permit  All the other uses should be forbidden  and it s best if such restrictions can be implemented statically  at compile time  so that misuse results in a compilation failure   So when one needs an array  the answers to the following questions specify its behavior  1  Is its size a  dynamic at runtime  or b  static  but only known at runtime  or c  static and known at compile time  2  Can the array be allocated on the stack or not   And based on the answers  this is what I see as the best data structure for such an array          Dynamic         Runtime static             Static Stack std  vector      unique ptr lt T   gt           std  array Heap  std  vector      unique ptr lt T   gt      unique ptr lt std  array gt    Yep  I think unique ptr lt std  array gt  should also be considered  and neither is a tool of last resort  Just think what fits best with your algorithm   All of these are compatible with plain C APIs via the raw pointer to data array  vector data     array data     uniquePtr get      P  S  Apart from the above considerations  there s also one of ownership  std  array and std  vector have value semantics  have native support for copying and passing by value   while unique ptr lt T   gt  can only be moved  enforces single ownership   Either can be useful in different scenarios  On the contrary  plain static arrays  int N   and plain dynamic arrays  new int 10   offer neither and thus should be avoided if possible - which should be possible in the vast majority of cases  If that wasn t enough  plain dynamic arrays also offer no way to query their size - extra opportunity for memory corruptions and security holes

[c++] Is there any use for unique_ptr with array?

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