From cppreference.com
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
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
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
elements, but less than
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
, std::ptrdiff_t acts as the signed counterpart of
: it can store the size of the array of any type and is, on most platforms, synonymous with
.
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++11usingptrdiff_t=decltype(static_cast<int*>(nullptr)-static_cast<int*>(nullptr));Example
Run this code
#include<cstddef>#include<iostream>intmain(){conststd::size_tN=10;int*a=newint[N];int*end=a+N;for(std::ptrdiff_ti=N;i>0;--i)std::cout<<(*(end-i)=i)<<' ';std::cout<<'\n';delete[]a;}Output:
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