Question

I have a struct defined as follows:

struct A : public B, public C
{
    A(const B& b) : B(b), C()
    {}

    template<typename... Args>
    A(Args&&... args) : B(), C(std::forward<Args>(args)...)
    {}
};

int main()
{
   B b;

   A sample1(b);
   A sample2(3); // For example, B has a B(int) constructor.
}

And this doesn’t work fine, because, A(b) tries use the second constructor (the non-constant reference is the preferred option, and the first constructor is a constant reference), but B hasn’t any B(A&).

And moreover, I want to add a move constructor for B:

struct A : public B, public C
{
    A(const B& b) : B(b), C()
    {}

    A(B&& b) : B(std::move(b)), C()
    {}

    template<typename... Args>
    A(Args&&... args) : B(), C(std::forward<Args>(args)...)
    {}
};

Now, the last step is to fusion the first two constructors:

struct A : public B, public C
{
   template<typename fw_B>
   A(fw_B&& b) : B(std::forward<fw_B>(b)), C()
   {}

   template<typename... Args>
   A(Args&&... args) : B(), C(std::forward<Args>(args)...)
   {}
 };

Question: if the first version causes collision, the last version (my final purpose) its clear that it doesn’t work also. How could I achieve this goal?

Answer 1

A possible solution would be use std::enable_if with std::is_convertible to only include the first constructor if the type of argument b is convertible to B:

template <
  class fw_B,
  class = typename std::enable_if<std::is_convertible<fw_B, B>::value, T>::type>
A(fw_B&& b)

For example:

#include <iostream>
#include <type_traits>

struct B
{
    B() {}
    B(int) {}
};

struct C {};

struct A : B, C
{
    template <
      class T,
      class = typename std::enable_if<std::is_convertible<T, B>::value, T>::type> 
    A(T&& t) { std::cout << "A(T&&)\n"; }

    template <class... TArgs>
    A(TArgs&&... targs) { std::cout << "A(TArgs&&)\n"; }
};

int main()
{
    B b;

    A a1(b);
    A a2(4);
    A a3("hello");

    return 0;
}

Output:

A(T&&)
A(T&&)
A(TArgs&&)

See demo at http://ideone.com/xJEjic .