C ++从具有相同虚拟函数名称的多个基类继承 [英] C++ inherit from multiple base classes with the same virtual function name

问题描述

我试过这段代码：

class A
{
    virtual void foo() = 0;
};

class B
{
    virtual void foo() = 0;
};

class C : public A, public B
{
    //virtual void A::foo(){}
    //virtual void B::foo(){}

    virtual void A::foo();
    virtual void B::foo();
};

void C::A::foo(){}
void C::B::foo(){}

int main()
{
    C c;
    return 0;
}

使用注释部分时可以，但是当我尝试写定义以外的类声明，编译器报告错误。
我使用的是MSVC11编译器，有没有人知道如何写这个？
我需要将代码移动到cpp文件中。

It is OK when using the commented part, but when I try to write the definitions outside the class declaration, the compiler reports errors. I am using the MSVC11 compiler, does anyone know how to write this? I need to move the code into the cpp file.

谢谢~~

推荐答案

函数根据名称和参数类型覆盖基类的虚函数（见下文）。因此，您的类 C 有两个虚函数 foo c $ c> A 和 B 。但是函数 void C :: foo（）覆盖两个：

A function overrides a virtual function of a base class based on the name and parameter types (see below). Therefore, your class C has two virtual functions foo, one inherited from each A and B. But a function void C::foo() overrides both:

.virtual] / 2

[class.virtual]/2

如果虚拟成员函数 vf class Base 和在 Derived 中直接或间接从 Base ，具有相同名称，参数类型列表，cv-qualification和ref-qualifier的成员函数 vf 同样）如果 Base :: vf 被声明，则 Derived :: vf 也是虚拟的是如此声明的），并且它覆盖 Base :: vf 。

If a virtual member function vf is declared in a class Base and in a class Derived, derived directly or indirectly from Base, a member function vf with the same name, parameter-type-list, cv-qualification, and ref-qualifier (or absence of same) as Base::vf is declared, then Derived::vf is also virtual (whether or not it is so declared) and it overrides Base::vf.

正如我在评论中所说，[dcl.meaning] / 1禁止在（成员）函数的声明中使用 qualified-id ：

As I already stated in the comments, [dcl.meaning]/1 forbids the use of a qualified-id in the declaration of a (member) function:

当 declarator-id 被限定时，声明将引用前面声明的类或命名空间的成员

When the declarator-id is qualified, the declaration shall refer to a previously declared member of the class or namespace to which the qualifier refers [...]"

因此任何 virtual void X :: foo（）; 作为 C 中的声明是非法的。

Therefore any virtual void X::foo(); is illegal as a declaration inside C.

class C : public A, public B
{
    virtual void foo();
};

$ b < ，它将覆盖 A :: foo 和 B :: foo 。没有办法为 A :: foo 和 B :: foo 进行不同的重写通过引入另一层继承：

Is the only way AFAIK to override foo, and it will override both A::foo and B::foo. There is no way to have to different overrides for A::foo and B::foo with different behaviour other than by introducing another layer of inheritance:

#include <iostream>

struct A
{
    virtual void foo() = 0;
};

struct B
{
    virtual void foo() = 0;
};

struct CA : A
{
    virtual void foo() { std::cout << "A" << std::endl; }
};

struct CB : B
{
    virtual void foo() { std::cout << "B" << std::endl; }
};

struct C : CA, CB {};

int main() {
    C c;
    //c.foo();  // ambiguous

    A& a = c;
    a.foo();

    B& b = c;
    b.foo();
}

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