Writing variadic template constructor

Question

Recently I asked this question but now I would like to expand it. I wrote the following class:

template 
class X{
public:
    vector v;
          


        

Answer 1

First, your code doesn't compile for me.

main.cpp:7:15: error: declaration of ‘class T’
     template 
               ^
main.cpp:3:11: error:  shadows template parm ‘class T’
 template 
           ^

I changed the outer one to U.

template 
class X{
public:
    vector v;
    template 
    X(T n) {
        v.push_back(n);
    }
    template 
    X(T n, T2... rest) {
        v.push_back(n);
        X(rest...);
    }
};

You're correct that this causes the issue you gave in the question details...

X obj(1, 2, 3);  // obj.v containts only 1

This is because the statement X(rest...) at the end of your constructor doesn't recursively call the constructor to continue initializing the same object; it creates a new X object and then throws it away. Once a constructor's body begins to execute, it's no longer possible to invoke another constructor on the same object. Delegation must occur in the ctor-initializer. So for example, you could do this:

template 
X(T n, T2... rest): X(rest...) {
    v.insert(v.begin(), n);
}

That sucks though, because inserting at the beginning of a vector isn't efficient.

Better to take a std::initializer_list argument. This is what std::vector itself does.

X(std::initializer_list il): v(il) {}
// ...
X obj {1, 2, 3};