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

NAME
     std::ptrdiff_t - std::ptrdiff_t

Synopsis
	Defined in header <cstddef>
	typedef /*implementation-defined*/ ptrdiff_t;

	std::ptrdiff_t	is  the signed integer type of the result of subtracting
     two pointers.

	The bit width of std::ptrdiff_t is not less than 17. (since C++11)

Notes
	std::ptrdiff_t is used for pointer arithmetic  and  array  indexing,  if
     negative values
	are  possible.	Programs that use other types, such as int, may fail on,
     e.g. 64-bit
	systems when the index exceeds INT_MAX or if it relies on 32-bit modular
     arithmetic.

	When working with the C++ container library, the  proper  type	for  the
     difference
	between  iterators is the member typedef difference_type, which is often
     synonymous
	with std::ptrdiff_t.

	Only pointers to elements of the same array (including the  pointer  one
     past the end
	of the array) may be subtracted from each other.

	If  an	array  is  so large (greater than PTRDIFF_MAX elements, but less
     than SIZE_MAX
	bytes), that the difference between  two  pointers  may  not  be  repre-
     sentable as
	std::ptrdiff_t,  the  result  of  subtracting two such pointers is unde-
     fined.

	For char arrays shorter than PTRDIFF_MAX,  std::ptrdiff_t  acts  as  the
     signed
	counterpart  of  std::size_t:  it can store the size of the array of any
     type and is,
	on most platforms, synonymous with std::intptr_t.

	It is unspecified whether the declaration of std::ptrdiff_t is available
     in any
	other standard library header. An implementation may  avoid  introducing
     this name
	even when the standard requires std::ptrdiff_t to be used.

Possible implementation
	// valid since C++11
	using	 ptrdiff_t    =   decltype(static_cast<int*>(nullptr)	-   sta-
     tic_cast<int*>(nullptr));

Example
     // Run this code

      #include <cstddef>
      #include <iostream>

      int main()
      {
	  const std::size_t N = 10;
	  int* a = new int[N];
	  int* end = a + N;
	  for (std::ptrdiff_t i = N; i > 0; --i)
	      std::cout << (*(end - i) = i) << ' ';
	  std::cout << '\n';
	  delete[] a;
      }

Output:
      10 9 8 7 6 5 4 3 2 1

See also
	size_t	 unsigned integer type returned by the sizeof operator
		 (typedef)
		 byte offset from the beginning of  a  standard-layout	type  to
     specified
	offsetof member
		 (function macro)
	C documentation for
	ptrdiff_t

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