Inheriting constructor and providing new overload: No arguments base constructor seems to not participate in overload resolution 继承构造函数并提供新的重载：无参数基构造函数似乎不参与重载决议

Question

测试显示异常行为（c++14、g++ 4.9.1、clang 3.5.5）：

总结一下：

如果B没有提供它可以使用的其他构造函数A::A()
如果 B 提供它不能使用的其他构造函数 A::A()，但它使用 A::A(whatever arguments)，这是意外行为（至少对我而言）。

设置 1：

struct A {
  A() {};
  A(int) {}; // with or without this overload the result are the same
};

struct B : A {
  using A::A;
};

B b0{}; // OK

设置 2：

struct A {
  A() {}; // with a default constructor instead (empty class A)
          // the results are the same
};

struct B : A {
  using A::A;
  B(int){}
};

B b0{}; // no matching constructor
B b1{24}; // OK

设置 3：

struct A {
  A() {};
  A(int) {};
};

struct B : A {
  using A::A;
  B(int, int){}
};

B b0{}; // no matching constructor
B b1{24}; // OK
B b2{24, 42}; // OK

为什么会这样，怎么会这样"fixed"。

Answer 1

我不能告诉你这样做的理由，但我至少可以告诉你这是标准规定的：

[C++11: 12.9/3]: 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 class where the using-declaration appears. [..]

由于默认 B() 调用默认 A()，您可以 "fix" 像这样：

struct B : A
{
   B() = default;

   using A::A;
   B(int, int){}
};

(live demo)

以下 wording from the original proposal (n2540) 表明此修复程序的简便性和对称性或多或少是决定背后的驱动因素，但我仍然觉得有些不尽如人意。好吧。

Copy and default constructors are not forwarded, deferring to the existing rules for implicitly declaring copy/default constructors.

Inheriting constructor and providing new overload: No arguments base constructor seems to not participate in overload resolution 继承构造函数并提供新的重载：无参数基构造函数似乎不参与重载决议

Inheriting constructor and providing new overload: no arguments base constructor seems to not participate in overload resolution

c++

overloading

inheriting-constructors

c++14

(live demo)