Using base class constructor when it is protected

Question

I would like to use the base class constructor in a subclass, the base class constructor is protected so it cannot be instantiated without a subclass.

class A {
protected:
    A() {}
};

class B : public A {
public:
    B() : A() {}
};

but I cannot use a using directive like this without the compiler complaining that B::B() is protected, even though the using is in the public block.

class B {
public:
    using A::A;
};

Where is this behavior specified?

EDIT

I attempted to simplify the example, and did not check to see it compiles, apparently with a trivial constructor it works, but this code does not compile

class A {
protected:
    A(int x) {}
};

class B : public A{
public:
    using A::A;
};

int main() {
  B b(4);
}

with error

main.cpp: In function 'int main()':
main.cpp:12:8: error: 'A::A(int)' is protected within this context
   B b(4);
        ^
main.cpp:3:5: note: declared protected here
     A(int x) {}
     ^

exit status 1

I think you forgot to make `B` inherit from `A` in your example. It I make `B` derive from `A`, the examples seem to work fine. — François Andrieux, Sep 19 '17 at 17:13
The code does not support your question, please create a [mcve] — Richard Critten, Sep 19 '17 at 17:13
Obviously you meant to inherit from A, but did you mean to do it publicly, privately or protectedly? And [what is your error exactly](http://coliru.stacked-crooked.com/a/33b66b6b7d39ecf3)? — Yakk - Adam Nevraumont, Sep 19 '17 at 17:14
You forgot to inherit in the second `class B`. Your code works fine with inheritance ([demo](https://ideone.com/qp9BAg)). — Sergey Kalinichenko, Sep 19 '17 at 17:16
do you really want to know where this is specified ( in the c++ standard ) or how to fix it? — 463035818_is_not_an_ai, Sep 19 '17 at 17:16
I cannot [reproduce](http://coliru.stacked-crooked.com/a/7fdd280c8a9ab56d) what you're claiming. — user0042, Sep 19 '17 at 17:18
Apologies, I updated the question to the correct code example — shane, Sep 19 '17 at 17:20
The answer is "because the standard says so" - see https://stackoverflow.com/a/24015931/241631 — Praetorian, Sep 19 '17 at 17:23
The answer to your question can be found at https://timsong-cpp.github.io/cppwp/n3337/class.inhctor#4. — R Sahu, Sep 19 '17 at 17:30

Using base class constructor when it is protected

0 Answers0