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

When creating an object using

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

Vector only contains the first value, but not others. I've checked and saw that constructor is called 3 times, so I'm probably creating temp objects and filling their vectors with the rest of the arguments. How do I solve this problem?

Answer 1

First, your code doesn't compile for me.

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

I changed the outer one to U.

template <class U>
class X{
public:
    vector<U> v;
    template <class T>
    X(T n) {
        v.push_back(n);
    }
    template <class T, class... T2>
    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<int> 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 <class T, class... T2>
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<T> argument. This is what std::vector itself does.

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