std::ptrdiff_t

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

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

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

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

Possible implementation

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

Example

#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;
}

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See also