NAME

std::size_t - std::size_t

Synopsis

Defined in header <cstddef>
Defined in header <cstdio>
Defined in header <cstdlib>
Defined in header <cstring>
Defined in header <ctime>
Defined in header <cuchar> (since C++17)
Defined in header <cwchar>
typedef /*implementation-defined*/ 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).

The bit width of std::size_t is not less than 16. (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.
(since C++14) 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.

The integer literal suffix for std::size_t is uz (or UZ). (since C++23)

Example

// Run this code

#include <cstddef>
#include <iostream>
#include <array>

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] << ' ';

// 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
}

Output:

0 1 2 3 4 5 6 7 8 9
9 8 7 6 5 4 3 2 1 0

References

* C++20 standard (ISO/IEC 14882:2020):

* 6.8.3 Compound types [basic.compound] (p: 75-76)

* 7.6.2.5 Sizeof [expr.sizeof] (p: 129-130)

* 7.6.2.6 Alignof [expr.alignof] (p: 130)

* 17.2.4 Sizes, alignments, and offsets [support.types.layout] (p:
507-508)

* C++17 standard (ISO/IEC 14882:2017):

* 6.9.2 Compound types [basic.compound] (p: 81-82)

* 8.3.3 Sizeof [expr.sizeof] (p: 121-122)

* 8.3.6 Alignof [expr.alignof] (p: 129)

* 21.2.4 Sizes, alignments, and offsets [support.types.layout] (p: 479)

* C++14 standard (ISO/IEC 14882:2014):

* 3.9.2 Compound types [basic.compound] (p: 73-74)

* 5.3.3 Sizeof [expr.sizeof] (p: 109-110)

* 5.3.6 Alignof [expr.alignof] (p: 116)

* 18.2 Types [support.types] (p: 443-444)

* C++11 standard (ISO/IEC 14882:2011):

* 5.3.3 Sizeof [expr.sizeof] (p: 111)

* 5.3.6 Alignof [expr.alignof] (p: 116)

* 18.2 Types [support.types] (p: 454-455)

* C++03 standard (ISO/IEC 14882:2003):

* 5.3.3 Sizeof [expr.sizeof] (p: 79)

* C++98 standard (ISO/IEC 14882:1998):

* 5.3.3 Sizeof [expr.sizeof] (p: 77)

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.