How to forward constructor arguments for class template

Question

Consider a sort of default Bar:

struct Bar
{
};

and a specific case of a Bar, MyBar, which has a constructor that requires an int:

struct MyBar
{
    MyBar(int n){}
};

I then have a class that uses a Bar:

template<class ThisBar = Bar> struct Foo : ThisBar
{
    Foo(double){}; // Construct the 'Foo' from a 'double'
};

A snag though is that if I want a Foo<MyBar> I need to be able to construct the MyBar:

Foo<MyBar> f(1.0, 1);

where the int is used to construct the MyBar. My question is how do I write this constructor, so the below compiles in C++17?

int main()
{
    Foo<MyBar> f(1.0, 1);
}

Answer 1

You can create a templated constructor and forward the template arguments:

#include <utility>

struct Bar
{
};

struct MyBar
{
    MyBar(int n){}
};

template<class ThisBar = Bar> struct Foo : ThisBar
{
    template <typename... Ts>
    Foo(double, Ts&&... ts)
     : ThisBar(std::forward<Ts>(ts)...) 
    {};
};

int main()
{
    Foo<MyBar> f(1.0, 1);
}

https://godbolt.org/z/WjfosqYqn