std::size_t

 

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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.

DR	Applied to	Behavior as published	Correct behavior
CWG 1122	C++98	the definition of size_t was in the C standard (<stddef.h>)	it is implementation-defined

See also

ptrdiff_t	signed integer type returned when subtracting two pointers (typedef)
offsetof	byte offset from the beginning of a standard-layout type to specified member (function macro)
integer literals	binary, (since C++14) decimal, octal, or hexadecimal numbers of integer type