Question

In a template, I want to drill down the template parameter to the real non-templated type. So:

template <typename T>
struct MyTemplate
{
    // sadly there's no extract_Base
    typedef typename extract_base<T>::MyType WorkType;
};
struct X {};
template <typename T> struct Templ {};
//MyTemplate<Templ<X>>::WorkType is X;
//MyTemplate<X>::WorkType is X;

The only solution I see is to define real base type like std::vector<X>::value_type is X. But I am curious if there’s a way to do this without defining auxiliary types inside each destination template.

I saw something like http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2965.html but this is draft? and I don’t quite get it.

Yes I know there’s multiple inheritance, but even for simple case this would be nice.

UPDATE: Nawaz solution works for me very well, and it is easy to extend to specific cases, e.g.

template<template<typename, typename> class X, typename T1, typename T2>
struct extract_base <X<T1, T2>>   //specialization
{
    typedef T1 base;
};

I can even apply is_base_of or other filters to T1/T2 and so on. So it does work for X<T,U> – at least with g++ 4.6.7.

Answer 1

First of all, let’s call it value_type instead of base, because value_type seems to be more appropriate term to describe the type which you want to extract.

You can use this:

template<typename T>
struct extract_value_type //lets call it extract_value_type
{
    typedef T value_type;
};

template<template<typename> class X, typename T>
struct extract_value_type<X<T>>   //specialization
{
    typedef T value_type;
};

It should work as long as the template argument to extract_value_type is of the form of either T or X<T>. It will not work for X<T,U>, however. But then it is easy to implement it in C++11 using variadic template.

Use it as:

template <typename T>
struct MyTemplate
{
    typedef typename extract_value_type<T>::value_type value_type;
};

Online demo : http://ideone.com/mbyvj

---

Now in C++11, you can use variadic template to make extract_value_type work with class templates which take more than one template arguments, such as std::vector, std::set, std::list etc.

template<template<typename, typename ...> class X, typename T, typename ...Args>
struct extract_value_type<X<T, Args...>>   //specialization
{
    typedef T value_type;
};

Demo : http://ideone.com/SDEgq