ptrdiff_t

ptrdiff_t is a special signed integer type defined in the standard library of C and C++ languages. It is a type of pointer subtraction result. The behavior of the type is similar to size_t: on a 32-bit system, the size of ptrdiff_t will be 32 bits, on a 64-bit system – 64 bits.

Also, when working with standard library containers, the result of subtracting two iterators has the difference_type type of the container being used, which, depending on the standard library, is often equal to ptrdiff_t.

The ptrdiff_t type is often used for address arithmetic and array indexing if negative values are possible. Programs that use regular integer types for this purpose may encounter undefined behaviour. For example, if the index value exceeds INT_MAX.

For arrays smaller than PTRDIFF_MAX, ptrdiff_t behaves like an analog of size_t: it can store the size of an array of any type and is synonymous with intptr_t on most platforms. However, if the array is large enough (larger than PTRDIFF_MAX, but smaller than SIZE_MAX) and the difference of its pointers cannot be represented as ptrdiff_t, the result of subtracting such pointers is undefined.

In C, the ptrdiff_t type is declared in the <stddef.h> header file. In C++, its declaration is located in and is placed in the global namespace and in std. Standard header files of the C language for backward compatibility can also be included in C++ programs.