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.
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] << " ";
|| signed integer type returned when subtracting two pointers |
|| byte offset from the beginning of a standard-layout type to specified member |