多个（菱形）继承编译没有“虚拟”，但不带 [英] Multiple (diamond) inheritance compiles without "virtual", but doesn't with

问题描述

给定以下代码（无虚拟继承）：

Given the following code (without virtual inheritance) :

class A
{
public:
    virtual void f() = 0;
};

class B : public A
{
 public:
    virtual void f() {}
};

class C : public A
{
 public:
    virtual void f() {}
};

class D : public B, public C
{

/* some code */
};


int main()
{
    D d;
    return 0;
}

代码编译。

另一方面，这里：

class A
{
public:
    virtual void f() = 0;
};

class B : virtual public A
{
    virtual void f() {}
};

class C : virtual public A
{
    virtual void f() {}
};

class D : public B, public C
{
    /* some code */
};


int main()
{
    D d;
    return 0;
}

编译器显示编译错误：

no unique final overrider for 'virtual void A::f()' in 'D' . 

为什么在第二个代码中有所不同？

Why is it different in the second code ?

推荐答案

您的第一个场景层次结构对应于：

Your first scenario hierarchy corresponds to:

    F()   F()
     A     A
     |     |
 F() B     C F()
      \   /
        D 

其中D不是抽象的，因为在类型D的对象中有两个A子对象：一个由B通过B的格子而具体化，另一个通过C的格子具体化。

Where D is not abstract, because there are two A subobjects in an object of type D: One that is made concrete by B through the lattice of B, and another that is made concrete through the lattice of C.

除非你试图在D的对象上调用函数F（），否则不会有任何歧义。

Unless you try to invoke the function F() on object of D there will not be any ambiguity.

对应于：

       F()  
        A
      /   \
 F() B     C F()
      \   /
        D  

在这种情况下，对象D具有单个Base类A子对象，并且它必须重写并提供该子对象中纯虚函数的实现。

In this scenario, the object D has a single Base class A sub object, and it must override and provide implementation of the pure virtual function in that subobject.

Herb Sutter在Guru Of The Week（GOTW）中的文章是多重继承的好读法：

Herb Sutter's articles in Guru Of The Week(GOTW) are a nice read for Multiple Inheritance:

1. 多重继承第一部分


2. 多重继承第二部分


3. 多重继承第三部分

1. Multiple Inheritance Part I
2. Multiple Inheritance Part II
3. Multiple Inheritance Part III

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