Thread: Getting value_type of container using variadic templates

Given a type T, i need the type of the elements of T if T is an STL container, otherwise the result should just be the type T.

I'm no good with templates, but here is what i've tried using the hit-and-miss methodology:

Code:

template <typename T> struct element_type
{
   typedef T type;
};

template <template <typename...> class Container, typename T, typename Ts...> struct element_type<Container<T, Ts...>>
{
   typedef T type;
};

The idea is that element_type<int>::type would just be int, and element_type<std::vector<int>>::type would also be int. Unfortunately this code just gives me a bunch of template errors, any hints as to what the problem is?

I seem to have figured it out:

Code:

template <typename T> struct element_type
{
   typedef T type;
};

template <template <typename...> class Container, typename T, typename... Ts> struct element_type<Container<T, Ts...>>
{
   typedef T type;
};

I'd managed to confuse myself about where the ellipsis should be.

O_o

So, nullary container templates get no love?

(Hint: "SFINAE" over a check for `<???>::value_type' and other requirements of a container.)

By the by, this should be done as `is_container<???>' paired with `value_type<???>'.

With a couple of such primitives, you get `typename if_<is_container<T>, value_type<T>, T>::type'.

You can, of course, implement `element_type' by inheriting `if_<is_container<T>, value_type<T>, T>' if you go that route, and you don't have to keep doing the same core--conditionally checking for a template expansion--mechanic for each new meta-function that needs to know about "containerness".

Soma