为什么虚拟基类必须由最派生的类构造?

Why must virtual base classes be constructed by the most derived class?(为什么虚拟基类必须由最派生的类构造?)

问题描述

以下代码无法编译:

class A {
public:
    A(int) {}
};

class B: virtual public A {
public:
    B(): A(0) {}
};

// most derived class
class C: public B {
public:
    C() {} // wrong!!!
};

如果我在C的构造函数初始化列表中调用A的构造函数，即:

If I call A's constructor in C's constructor initialization list, that is:

// most derived class
class C: public B {
public:
    C(): A(0) {} // OK!!!
};

确实有效.

显然，这是因为虚拟基类必须始终由大多数派生类构建.

Apparently, the reason is because virtual base classes must always be constructed by the most derived classes.

我不明白这个限制背后的原因.

I don't understand the reason behind this limitation.

推荐答案

因为它避免了这个:

class A {
public:
    A(int) {}
};

class B0: virtual public A {
public:
    B0(): A(0) {}
};

class B1: virtual public A {
public:
    B1(): A(1) {}
};

class C: public B0, public B1 {
public:
    C() {} // How is A constructed? A(0) from B0 or A(1) from B1?
};

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