template template parameter

#include <stdexcept>
#include <iostream>
#include <memory>
#include <vector>
#include <string>
#include <deque>

template <typename T, template <typename ELEM, typename = std::allocator<ELEM>> class CONT = std::deque>
class Stack 
{
private:
	CONT<T> elems;         // elements

public:
	void push(T const&);   // push element
	void pop();            // pop element
	T top() const;         // return top element
	bool empty() const
	{   // return whether the stack is empty
		return elems.empty();
	}

	// assign stack of elements of type T2
	template<typename T2, template <typename ELEM2, typename = std::allocator<ELEM2> > class CONT2>
	Stack<T, CONT>& operator= (Stack<T2, CONT2> const&);
};

template <typename T, template <typename, typename> class CONT>
void Stack<T, CONT>::push(T const& elem)
{
	elems.push_back(elem);    // append copy of passed elem
}

template<typename T, template <typename, typename> class CONT>
void Stack<T, CONT>::pop()
{
	if (elems.empty()) 
	{
		throw std::out_of_range("Stack<>::pop(): empty stack");
	}
	elems.pop_back();         // remove last element
}

template <typename T, template <typename, typename> class CONT>
T Stack<T, CONT>::top() const
{
	if (elems.empty())
	{
		throw std::out_of_range("Stack<>::top(): empty stack");
	}
	return elems.back();      // return copy of last element
}

template <typename T, template <typename, typename> class CONT>
template <typename T2, template <typename, typename> class CONT2>
Stack<T, CONT>&
Stack<T, CONT>::operator= (Stack<T2, CONT2> const& op2)
{
	if ((void*)this == (void*)&op2) {    // assignment to itself?
		return *this;
	}

	Stack<T2, CONT2> tmp(op2);        // create a copy of the assigned stack

	elems.clear();                   // remove existing elements
	while (!tmp.empty())
	{           // copy all elements
		elems.push_front(tmp.top());
		tmp.pop();
	}
	return *this;
}

int main()
{
	try {
		Stack<int>   intStack;       // stack of ints
		Stack<float> floatStack;     // stack of floats

		// manipulate int stack
		intStack.push(42);
		intStack.push(7);

		// manipulate float stack
		floatStack.push(7.7);

		// assign stacks of different type
		floatStack = intStack;

		// print float stack
		std::cout << floatStack.top() << std::endl;
		floatStack.pop();
		std::cout << floatStack.top() << std::endl;
		floatStack.pop();
		std::cout << floatStack.top() << std::endl;
		floatStack.pop();
	}
	catch (std::exception const& ex) {
		std::cerr << "Exception: " << ex.what() << std::endl;
	}

	// stack for ints using a vector as an internal container
	Stack<int, std::vector> vStack;
	//...
	vStack.push(42);
	vStack.push(7);
	std::cout << vStack.top() << std::endl;
	vStack.pop();
}