Question

I was reading Modern C++ design and not able to fully understand how template template parameter works.

For example as given in this article http://www.informit.com/articles/article.aspx?p=376878 we can create a Stack with template parameters of type and containers .

If we use just type and container as parameter to template Stack then it may create some issues like

template <typename T, class Cont>
class Stack;

template <typename> class List;
Stack<int, List<int> > aStack1; // OK
Stack<double, List<int> > aStack2; // legal, not OK           
Stack<std::string, Deque<char *> > aStack3; // error! 

In above code I can understand that aStack1 is fine,aStack2 and aStack3 are issues because if incompatible types between Stack element type and container element type.

As per article this can be resolved if we use template template parameter

template <typename T, template <typename> class Cont>
class Stack;

Stack<int,List> aStack1;
Stack<std::string,Deque> aStack2;

My doubt here is how can Deque knows that its element type is std::string or List element type is int???Is this done by template argument deduction?

Here we are creating a Deque with type T .
If we have defined stack as

template <typename T,template U, template <typename> class Cont>
    class Stack;

then how can we instantiate Stack

   Stack<std::string,int,Deque> // will this work????

Answer 1

My doubt here is how can Deque knows that its element type is std::string or List element type is int???Is this done by template argument deduction?

No, you explicitly instantiate the class template in the implementation of Stack which is likely to be implemented as:

template <typename T, template <typename> class Cont>
class Stack
{
  Cont<T> _container; //here you use `T` as template argument to `Cont`

  Cont<int> _ints; //you can do even this if you need a containter of int.
                   //though in this case you don't need this.

  public:
        void push(T const & data)
        {
           _container.push_back(data); 
        }
        //etc
};