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std::realloc(3) 	       C++ Standard Libary		 std::realloc(3)

NAME
     std::realloc - std::realloc

Synopsis
	Defined in header <cstdlib>
	void* realloc( void* ptr, std::size_t new_size );

	Reallocates the given area of memory (implicitly creating objects in the
     destination
	area).	It  must  be previously allocated by std::malloc, std::calloc or
     std::realloc
	and not yet freed with std::free, otherwise, the results are undefined.

	The reallocation is done by either:

	a) expanding or contracting the existing area pointed to by ptr, if pos-
     sible. The
	contents of the area remain unchanged up to the lesser of  the	new  and
     old sizes. If
	the  area is expanded, the contents of the new part of the array are un-
     defined.
	b) allocating a new memory block of size new_size bytes, copying  memory
     area with
	size  equal the lesser of the new and the old sizes, and freeing the old
     block.

	If there is not enough memory, the old memory block  is  not  freed  and
     null pointer is
	returned.

	If  ptr is a null pointer, the behavior is the same as calling std::mal-
     loc(new_size).

	If new_size is	zero,  the  behavior  is  implementation  defined:  null
     pointer may be
	returned (in which case the old memory block may or may not be freed) or
     some
	non-null pointer may be returned that may not be used to access storage.
	Such usage is deprecated (via C DR 400).
	(since C++20)

	The following functions are required to be thread-safe:

	  * The library versions of operator new and operator delete
	  * User replacement versions of global operator new and operator
	    delete
	  *	      std::calloc,	     std::malloc,	    std::realloc
     (since C++11)
	    , std::aligned_alloc
	    (since C++17), std::free

	Calls to these functions that allocate or deallocate a particular unit
	of storage occur in a single total order, and each such deallocation
	call happens-before the next allocation (if any) in this order.

Parameters
	ptr	 - pointer to the memory area to be reallocated
	new_size - new size of the array

Return value
	On success, returns a pointer to the beginning of newly  allocated  mem-
     ory. To avoid a
	memory leak, the returned pointer must be deallocated with std::free or
	std::realloc.  The original pointer ptr is invalidated and any access to
     it is
	undefined behavior (even if reallocation was in-place).

	On failure, returns a null pointer. The  original  pointer  ptr  remains
     valid and may
	need to be deallocated with std::free.

Notes
	Because reallocation may involve bytewise copying (regardless of whether
     it expands
	or  contracts  the area), it is necessary (but not sufficient) for those
     objects to be
	of TriviallyCopyable type.

	Some non-standard libraries define a type trait "BitwiseMovable" or "Re-
     locatable",
	which describes a type that does not have:

	  * external references (e.g. nodes of a list or a tree that holds  ref-
     erence to
	    another element), and
	  *  internal  references  (e.g. member pointer which might hold the ad-
     dress of another
	    member).

	Objects of such type can be accessed after their storage is  reallocated
     even if
	their copy constructors are not trivial.

Example
     // Run this code

      #include <cassert>
      #include <cstdlib>
      #include <new>

      class MallocDynamicBuffer
      {
	  char* p;
      public:
	  explicit MallocDynamicBuffer(std::size_t initial = 0) : p(nullptr)
	  {
	      resize(initial);
	  }

	  ~MallocDynamicBuffer() { std::free(p); }

	  void resize(std::size_t newSize)
	  {
	      if (newSize == 0) // this check is not strictly needed,
	      {
		  std::free(p); // but zero-size realloc is deprecated in C
		  p = nullptr;
	      }
	      else
	      {
		  if (void* mem = std::realloc(p, newSize))
		      p = static_cast<char*>(mem);
		  else
		      throw std::bad_alloc();
	      }
	  }

	  char& operator[](size_t n) { return p[n]; }
	  char operator[](size_t n) const { return p[n]; }
      };

      int main()
      {
	  MallocDynamicBuffer buf1(1024);
	  buf1[5] = 'f';
	  buf1.resize(10); // shrink
	  assert(buf1[5] == 'f');
	  buf1.resize(1024); // grow
	  assert(buf1[5] == 'f');
      }

See also
	C documentation for
	realloc

http://cppreference.com 	   2024.06.10			 std::realloc(3)
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