In this question I see following:
for (vector<int>::size_type ix = 0; ix ! = ivec.size(); ++ix) {
ivec[ix] = 0;
}
I understand that why int
is not used here, but why not just use size_t
?
Under what circumstances I should use vector<int>::size_type
instead of size_t
?
The primary time to use size_type
is in a template. Although std::vector<T>::size_type
is usually size_t
, some_other_container<T>::size_type
might be some other type instead1. One of the few things a user is allowed to add to the std
namespace is a specialization of an existing template for some user defined type. Therefore, std::vector<T>::size_type
for some oddball T
could actually be some type other than size_t
, even though the base template defined in the standard library probably always uses size_t
.
Therefore, if you want to use the correct type for a specific container inside a template that works with that container, you want to use container::size_type
instead of just assuming size_t
.
Note, however, that generic code should rarely work directly with a container. Instead, it should typically work with iterators, so instead of container<T>::size_type
, it would typically use something like std::iterator_traits<WhateverIterator>::difference_type
instead.
T
, vector<T>::size_type
might be a different type as well--one of the few things you're allowed to put into the std
namespace is a specialization of an existing class for a user-defined type, so for some T
, vector<T>
could use a completely different container than for most other types. This is typical for vector<bool>
, but possible for other types as well.