std::size_t

Defined in header `<cstddef>`
Defined in header `<cstdio>`
Defined in header `<cstring>`
Defined in header `<ctime>`
Defined in header `<cstdlib>`		(since C++11)
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).

[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 (since C++14) On many platforms (an exception are 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.

[edit] Example

Run this code

#include <cstddef>
 
int main()
{
    const std::size_t N = 100;
    int* a = new int[N];
 
    for (std::size_t n = 0; n < N; ++n)
        a[n] = n;
 
    delete[] a;
}

[edit] 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)
C documentation for size_t

Language
Standard library headers
Concepts
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Technical Specifications

Type support (basic types, RTTI, type traits)
Dynamic memory management
Error handling
Program utilities
Variadic functions
Date and time
Function objects
initializer_list (C++11)
bitset
hash (C++11)
Relational operators
rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=
Pairs and tuples
pair
tuple (C++11)
piecewise_construct_t (C++11)
piecewise_construct (C++11)
integer_sequence (C++14)
Swap, forward and move
swap
exchange (C++14)
forward (C++11)
move (C++11)
move_if_noexcept (C++11)
declval (C++11)