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 representable as
   std::ptrdiff_t, the result of subtracting two such pointers is undefined.
   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) - static_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