C++ vector::size_type Explained

C++ is a language for library writing*, and allowing the author to be as general as possible is one of its key strengths. Rather than prescribing the standard containers to use any particular data type, the more general approach is to decree that each container expose a size_type member type. This allows for greater flexibility and genericity. For example, consider this generic code:

template <template <typename...> Container, typename T>
void doStuff(const Container<T> & c)
{
  typename Container<T>::size_type n = c.size();
  // ...
}

This code will work on any container template (that can be instantiated with a single argument), and we don't impose any unnecessary restrictions on the user of our code.

(In practice, most size types will resolve to std::size_t, which in turn is an unsigned type, usually unsigned int or unsigned long -- but why should we have to know that?)

_{*) I'm not sure what the corresponding statement for Java would be.}

size_type is guaranteed to be large enough for the largest supported vector size, vector::max_size(). int is not: on many common platforms, int has 32 bits, while max_size() is considerably larger than 2³¹.

If you know the size is (and will always be) a small number like 20, then you can get away with using int or any other integer type instead of size_type. If you were to change the program, for example to read the size from the input, then it would go horribly wrong if that value were larger than INT_MAX; while using size_type, it would continue working for any value up to max_size(), which you can easily test for.