Question

I have a type like Type<Param>. How I can retrieve the Param in c++11?

May be something like that:

// I know it's not correct but it conveys the idea very well
template
<
   template <class Param> class Type 
>
struct GetParam
{
   typedef Param Result;
};

// e.g.
typedef GetParam<std::vector<double>>::Result X; // must return double
typedef GetParam<std::list<double>>::Result X; // double
typedef GetParam<std::vector<std::list<double>>::Result X; // std::list<double>

Answer 1

template<class Type>
struct GetParam;

template<template<typename ...> class Tmp, typename T1, typename ...Types>
struct GetParam<Tmp<T1, Types...>> {
  typedef T1 type;
};

However the class template specialization you pass to GetParam can only have type template arguments. So you cannot pass std::array for example. It is not possible to do a fully generic GetParam, because you would have to enlist every possible template parameter list variety, and there are basically infinitely many.