为什么编译成功？

Question

这段代码编译通过的原因是什么：

#include <iostream>
using namespace std;
class being {
public:
  void running(char c) {
        cout << "No one know ";
    }
};
class human :public being {
public:
    using being::running;
    void running(char y) {
        cout << "I am a human";
    }
};

int main() {
    human o;
    o.running('A');
    return 0;
}


the output : "I am a human"

我的意思是（我预计会有错误（人类的重新定义函数 class ））像这样：此代码编译：

#include <iostream>
using namespace std;
class being {
public:
    int v;
};
struct human :public being {
public:
    
    double v;

};

int main() {
    human o;
    o.v = 55.2;
    return 0;
}

但是当我添加（使用 being::v ）

#include <iostream>
using namespace std;
class being {
public:
    int v;
};
struct human :public being {
public:
    using being::v;

    double v;

};

int main() {
    human o;
    o.v = 55.2;
    return 0;
}

出现错误：error C2086: 'int being::v': redefinition

为什么这个错误没有出现在第一个代码中？

Answer 1

这是 using declaration 的预期行为。

If the derived class already has a member with the same name, parameter list, and qualifications, the derived class member hides or overrides (doesn't conflict with) the member that is introduced from the base class.

所以human::running(char)隐藏being::running(char)而不是冲突

编辑

第二个代码片段格式错误。对于数据成员，根据标准，[namespace.udecl]/10:

If a declaration named by a using-declaration that inhabits the target scope of another declaration potentially conflicts with it ([basic.scope.scope]), and either is reachable from the other, the program is ill-formed. If two declarations named by using-declarations that inhabit the same scope potentially conflict, either is reachable from the other, and they do not both declare functions or function templates, the program is ill-formed.

[Example 6:
...

namespace B {
  int i;
  ...
}

void func() {
  int i;
  using B::i;                           // error: conflicts
  ...
...

所以你不能 using being::v 与 human::v 冲突。但是对于成员函数，[namespace.udecl]/11:

The set of declarations named by a using-declarator that inhabits a class C does not include member functions and member function templates of a base class that correspond to (and thus would conflict with) a declaration of a function or function template in C.

[Example 7:
struct B {
  virtual void f(int);
  virtual void f(char);
  void g(int);
  void h(int);
};

struct D : B {
  using B::f;
  void f(int);      // OK: D::f(int) overrides B::f(int);

  using B::g;
  void g(char);     // OK

  using B::h;
  void h(int);      // OK: D::h(int) hides B::h(int)
};
...

为什么编译成功？

Why is this compiling successfully?

c++

derived-class

using-declaration

redefinition

language-lawyer