Inherit and overload default constructor

问题

I've been searching for this and I'm amazed I haven't found anything. Why can't I inherit a base class constructor using using declaration and add an overload in the derived class? I'm using Visual C++ 2013, the base class constructor is ignored when default-constructing b:

error C2512: 'B' : no appropriate default constructor available

I've dealt with this by re-defining the constructors, but I don't like that. This is just a minimal example, it wouldn't bother me if I had only one base class constructor.

struct A
{
    A() : a(10) {}

    int a;
};

struct B : A
{
    using A::A;

    explicit B(int a) { this->a = a; }
};

int main()
{
    B b;
}

回答1:

The problem is that default-constructors are not inherited. From [class.inhctor]/p3:

For each non-template constructor in the candidate set of inherited constructors [..], a constructor is implicitly declared with the same constructor characteristics unless there is a user-declared constructor with the same signature in the complete class where the using-declaration appears or the constructor would be a default, copy, or move constructor for that class.

You also have a user-declared constructor that suppresses the creation of an implicit default-constructor. Just add a defaulted one to make it work:

B() = default;

来源：https://stackoverflow.com/questions/38685216/constructor-of-the-base-is-not-made-available-by-the-using-declaration