Question

Consider the following use of template template parameters…

#include <iostream>  template <typename X> class A {     X _t; public:     A(X t)         :_t(t)     {     }     X GetValue()     {         return _t;     } };  template <typename T, template <typename T> class C > class B {     C<T> _c; public:     B(T t)         :_c(t)     {     }     T GetValue()     {         return _c.GetValue();     } };  using namespace std;  int main() {     B<int, A> b(10);     cout<<b.GetValue();     return 0; } 

Is there a way by which the template parameter T can be removed? For example is there a way to make the following work?

//Does not compile template <template <typename T> class C > class B {     C _c; public:     B(T t)         :_c(t)     {     }     T GetValue()     {         return _c.GetValue();     } };  int main() {     B< A<int> > b(10);     cout<<b.GetValue();     return 0; } 

Answer 1

I assume you’re after X, as well as A, in your code.

The usual pattern is to have

template<typename C> struct B {    C c; }; 

and then, inside classes eligible for substitution:

template<typename X> class A {    typedef X type_name;    X t; }; 

Then you can access the template parameter using C::type_name.