< cpp‎ | types
Type support
Basic types
Fundamental types
Fixed width integer types (C++11)
Numeric limits
C numeric limits interface
Runtime type information
Type traits
Type categories
Type properties
(C++11)(deprecated in C++17)
Type trait constants
Supported operations
Relationships and property queries
Type modifications
Type transformations
(C++11)(deprecated in C++17)(C++17)
Defined in header <cstddef>
Defined in header <cstdio>
Defined in header <cstdlib>
Defined in header <cstring>
Defined in header <ctime>
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).


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.


#include <cstddef>
#include <iostream>
int main()
    const std::size_t N = 10;
    int* a = new int[N];
    for (std::size_t n = 0; n < N; ++n)
        a[n] = n;
    for (std::size_t n = N; n-- > 0;) // Reverse cycles are tricky for unsigned types.
        std::cout << a[n] << " ";
    delete[] a;


9 8 7 6 5 4 3 2 1 0

See also

signed integer type returned when subtracting two pointers
byte offset from the beginning of a standard-layout type to specified member
(function macro)
C documentation for size_t