Provide definition of constructor template outside a class template

Question

Consider the following class template with a constructor template:

template <class T>
class C{
    public:
    T val_;
    std::string str_;
    
    template <typename S>
    C(const S& str);
    // C(const S& str) : str_(str) {}; // Valid definition of constructor within class body
    
    void print(){std::cout << str_;}
};

I'm, however, trying to define the constructor outside of the class, but I can't seem to figure out a way to account for the type S.

template <typename T, typename S> // incorrect as too many parameters
C<T>::C(const S& str) : str_(str) {};

Alternatively, I tried

template <typename T>
C<T>::C(const std::string& str) : str_(str) {};

which also does not work (and probably defeats the purpose of S)

1. How can I correctly define this constructor outside the class.
2. Does such a pattern (where class template parameters differ from constructor template parameters) serve any practical purpose? An example would be very helpful.

Answer 1

The correct syntax is:

template <class T>
template <class S>
C<T>::C(const S& str)
{
    // ...
}

Regarding your second question:

One example of using this pattern (of templating a ctor) is the way to achieve type-erasure in C++ using templates.
This article has some examples: What is type erasure - Arthur O'Dwyer (search for "templated constructor").
Or this 10 minutes clip that demonstrates it: Type Erasure (YouTube).