Question

template<typename T> class A // template parameterization
{
   private:
      T t;
   A(const T& v) : t(v) {}
};

class B
{
    template<typename T>
    B(const T& v)
    {
        std::cout << v << std::endl;
    }
};

// usage of A and B
A<int> a;
B      b(10);

Question> In what circumstances, we have to provide template parameters in order to define a class variable.

For example,

If the class contains a template member variable or ???

Thank you

Answer 1

You have to provide template parameters to create an instance if the class is a class template. In your example, class A is a class template, and class B isn’t.

Class template:

template <typename T> class A {};

Not a class template:

class B { 
  // code may include function template, etc.
  // but the class itself is not a class template
};

In your example, class B has a template constructor, and the argument can be used by the compiler to determine which specialization to make. So in this case, it generates a constructor equivalent to

B(const int&);

because the literal 10 is an int. Constructors are not like functions, so this can only work if the compiler can figure out what T is. See this related question for more details.