< cpp‎ | types
Utilities library
Language support
Type support (basic types, RTTI)
Library feature-test macros (C++20)
Dynamic memory management
Program utilities
Coroutine support (C++20)
Variadic functions
Debugging support
Three-way comparison
General utilities
Date and time
Function objects
Formatting library (C++20)
Relational operators (deprecated in C++20)
Integer comparison functions
Swap and type operations
Common vocabulary types
Elementary string conversions

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)


[edit] 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.

[edit] Possible implementation

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

[edit] 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;


10 9 8 7 6 5 4 3 2 1

[edit] See also

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