C ++ 11构造函数继承和没有参数的构造函数

Question

In this piece of code, why is A's constructor with no parameters not inherited? Is there a special rule that prevents inheriting constructors with no parameters?

struct A {
    A(void *) {}
    A() {}
};

class B : public A {
public:
    using A::A;
    B(int x) {}
};

void f() {
    B b(1);
    B b2(nullptr);
    B b3; // error
}

clang++ -std=c++11 gives this error, and g++ -std=c++11 gives a substantially similar error message:

td.cpp:15:7: error: no matching constructor for initialization of 'B'
    B b3; // error
      ^
td.cpp:9:5: note: candidate constructor not viable: requires single argument 'x', but no arguments
      were provided
    B(int x) {}
    ^
td.cpp:8:14: note: candidate constructor (inherited) not viable: requires 1 argument, but 0 were
      provided
    using A::A;
             ^
td.cpp:2:5: note: inherited from here
    A(void *) {}

Answer 1

The relevant information is in 12.9 [class.inhctor] paragraph 3 (the highlighting is added):

For each non-template constructor in the candidate set of inherited constructors other than a constructor having no parameters or a copy/move constructor having a single parameter , 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. [...]

That is, default constructor are not inherited unless they have a [defaulted] argument. If they have a default argument they are inherited but without the defaulted argument as is pointed out by a node on the same paragraph:

Note: Default arguments are not inherited. [...]

Basically, that says that default constructors are not inherited.

Answer 2

B中没有没有参数的构造函数，试试

B() : A(){}