# std::size_t

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< cpp‎ | types

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).

### 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.

### Example

Run this code

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

    int main()
    {
        std::array<std::size_t,10> a;
        for (std::size_t i = 0; i != a.size(); ++i)
            a[i] = i;
        for (std::size_t i = a.size()-1; i < a.size(); --i)
            std::cout << a[i] << " ";
    }

Output:

    9 8 7 6 5 4 3 2 1 0

### 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)

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