Question

Is this valid template construct in C++ templates?

template < template <typename T2> class T> 
void foo() {

}

Answer 1

Yes. It is valid.

You can call this function with any class template which takes exactly one template parameter. For example,

template<typename T> 
struct A
{
   //...
};

foo< A >(); //ok

Note that you don’t have to provide the template argument for A class template, which means, the following would result in compilation error:

foo< A<int> >(); //error

Also, in your code T2 is optional, and in fact, you cannot use it in the function, so better remove it to make the definition simpler:

template < template <typename> class T> 
void foo() {

    T<int> x; //this is how T can be instantiated; provide template argument!
}

Demo : http://ideone.com/8jlI5