Inherit and overload default constructor

Question

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;
}

That is correct code. See http://stackoverflow.com/questions/9979194/what-is-constructor-inheritance. — R Sahu, Apr 06 '15 at 18:57
@RSahu It would be correct without the other constructor in `B`. — David G, Apr 06 '15 at 19:07
@0x499602D2, that would be silly -- to not allow `B` to add more constructors just because it inherits constructors from `A`. — R Sahu, Apr 06 '15 at 19:09
@RSahu `B(int)` is suppressing the creation of a default constructor. He can either remove it or create one himself. — David G, Apr 06 '15 at 19:12
@0x499602D2, shouldn't `B` inherit the default constructor from `A` after `using A::A;`? — R Sahu, Apr 06 '15 at 19:13
@RSahu [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**. — David G, Apr 06 '15 at 19:16
@0x499602D2, Thanks for digging that up. That explains the compiler problem. That would be a perfect answer to the question. — R Sahu, Apr 06 '15 at 19:21

score 10 · Accepted Answer · edited Sep 07 '16 at 22:05

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;

Inherit and overload default constructor

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