< cpp‎ | types
Utilities library
Language support
Type support (basic types, RTTI)
Library feature-test macros (C++20)
Dynamic memory management
Program utilities
Coroutine support (C++20)
Variadic functions
Debugging support
Three-way comparison
General utilities
Date and time
Function objects
Formatting library (C++20)
Relational operators (deprecated in C++20)
Integer comparison functions
Swap and type operations
Common vocabulary types
Elementary string conversions

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)


[edit] 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.

The integer literal suffix for std::size_t is any combination of z or Z with u or U (i.e. zu, zU, Zu, ZU, uz, uZ, Uz, or UZ).

(since C++23)

[edit] Possible implementation

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

[edit] 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


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

[edit] References

  • C++23 standard (ISO/IEC 14882:2023):
  • 6.8.4 Compound types [basic.compound] (p: 79-80)
  • Sizeof [expr.sizeof] (p: 136)
  • Alignof [expr.alignof] (p: 136)
  • 17.2.4 Sizes, alignments, and offsets [support.types.layout] (p: 504-505)
  • C++20 standard (ISO/IEC 14882:2020):
  • 6.8.3 Compound types [basic.compound] (p: 75-76)
  • Sizeof [expr.sizeof] (p: 129-130)
  • 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)

[edit] See also

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