std::size_t is the unsigned integer type of the result of the following operators:

• sizeof

If a program attempts to form an oversized type (i.e., the number of bytes in itsobject representation exceeds the maximum value representable instd::size_t), the program is ill-formed.

[edit]Notes

std::size_t can store the maximum size of a theoretically possible object of any type (including array). 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 withstd::uintptr_t.

std::size_t is commonly used for array indexing and loop counting. Programs that use other types, such asunsignedint, for array indexing may fail on, e.g. 64-bit systems when the index exceedsUINT_MAX or if it relies on 32-bit modular arithmetic.

When indexing C++ containers, such asstd::string,std::vector, etc, the appropriate type is the nested typesize_type provided by such containers. It is usually defined as a synonym forstd::size_t.

It is unspecified whether the declaration ofstd::size_t is available in any other standard library header. An implementation may avoid introducing this name even when the standard requiresstd::size_t to be used.

[edit]Possible implementation

using size_t= decltype(sizeof0);

[edit]Example

#include <array>#include <cstddef>#include <iostream> int main(){std::array<std::size_t,10> a; // Example with C++23 std::size_t literalfor(auto i= 0uz; i!= a.size();++i)std::cout<<(a[i]= i)<<' ';std::cout<<'\n'; // Example of decrementing loopfor(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 99 8 7 6 5 4 3 2 1 0

[edit]Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

1. ↑The definition ofstd::size_t was exactly the same as the definition ofsize_t in C, which is “the result type ofsizeof”. There is no circular definition in C because the result type ofsizeof in C is an implementation-defined unsigned integer type.

[edit]References

[edit]See also

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