size_t vs int in C++ and/or C

Question

Why is it that in C++ containers, it returns a size_type rather than an int? If we're creating our own structures, should we also be encouraged to use size_type?

Answer 1

In general, size_t should be used whenever you are measuring the size of something. It is really strange that size_t is only required to represent between 0 and SIZE_MAX bytes and SIZE_MAX is only required to be 65,535...

The other interesting constraints from the C++ and C Standards are:

• the return type of sizeof() is size_t and it is an unsigned integer
• operator new() takes the number of bytes to allocate as a size_t parameter
• size_t is defined in <cstddef>
• SIZE_MAX is defined in <limits.h> in C99 but not mentioned in C++98?!
• size_t is not included in the list of fundamental integer types so I have always assumed that size_t is a type alias for one of the fundamental types: char, short int, int, and long int.

If you are counting bytes, then you should definitely be using size_t. If you are counting the number of elements, then you should probably use size_t since this seems to be what C++ has been using. In any case, you don't want to use int - at the very least use unsigned long or unsigned long long if you are using TR1. Or... even better... typedef whatever you end up using to size_type or just include <cstddef> and use std::size_t.