How do traits classes work and what do they do?

Traits classes do not determine the type of the object. Instead, they provide additional information about a type, typically by defining typedefs or constants inside the trait.

Perhaps you’re expecting some kind of magic that makes type traits work. In that case, be disappointed – there is no magic. Type traits are manually defined for each type. For example, consider iterator_traits, which provides typedefs (e.g. value_type) for iterators.

Using them, you can write

iterator_traits<vector<int>::iterator>::value_type x;
iterator_traits<int*>::value_type y;
// `x` and `y` have type int.

But to make this work, there is actually an explicit definition somewhere in the <iterator> header, which reads something like this:

template <typename T>
struct iterator_traits<T*> {
    typedef T value_type;
    // …
};

This is a partial specialization of the iterator_traits type for types of the form T*, i.e. pointers of some generic type.

In the same vein, iterator_traits are specialized for other iterators, e.g. typename vector<T>::iterator.