std:: size_t

std::size_t is the unsigned integer type of the result of the sizeof operator as well as the sizeof... operator and the alignof operator (since C++11) .

Notes

std::size_t can store the maximum size of a theoretically possible object of any type (including array). A type whose size cannot be represented by std::size_t is ill-formed. On many platforms (an exception is systems with segmented addressing) std::size_t can safely store the value of any non-member pointer, in which case it is synonymous with std::uintptr_t .

std::size_t is commonly used for array indexing and loop counting. Programs that use other types, such as unsigned int , for array indexing may fail on, e.g. 64-bit systems when the index exceeds UINT_MAX or if it relies on 32-bit modular arithmetic.

When indexing C++ containers, such as std::string , std::vector , etc, the appropriate type is the member typedef size_type provided by such containers. It is usually defined as a synonym for std::size_t .

It is unspecified whether the declaration of std::size_t is available in any other standard library header. An implementation may avoid introducing this name even when the standard requires std::size_t to be used.

Possible implementation

using size_t = decltype(sizeof(0)); // valid since C++11

Example

#include <array>
#include <cstddef>
#include <iostream>
 
int main()
{
    std::array<std::size_t, 10> a;
 
    // Example with C++23 size_t literal
    for (auto i = 0uz; i != a.size(); ++i)
        std::cout << (a[i] = i) << ' ';
    std::cout << '\n';
 
    // Example of decrementing loop
    for (std::size_t i = a.size(); i--;)
        std::cout << a[i] << ' ';
    std::cout << '\n';
 
    // Note the naive decrementing loop:
    //  for (std::size_t i = a.size() - 1; i >= 0; --i) ...
    // is an infinite loop, because unsigned numbers are always non-negative
}

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References

See also