虚拟基类的创建顺序 [英] Virtual base classes order of creation

问题描述

我有以下问题:

struct A1 {
    A1() { std::cout << "A1, "; }
};

struct A2 {
    A2() { std::cout << "A2, "; }
};

struct AA1 : virtual A1,  A2 {
    AA1() { std::cout << "AA1, "; }
};

struct AA2 : A1, virtual A2 {
    AA2(){ std::cout << "AA2, "; }
};

struct B : AA1, virtual AA2 {
    B() { std::cout << "B "; }
};

int main() {
    B b;
}

运行此代码时，答案是:

A1 A2 A1 AA2 A2 AA1 B

我想了解第一个创建的A1在哪里.

我知道在非虚拟类之前先调用虚拟类的规则，但是首先出现A1的问题困扰着我.

解决方案

第一个A1来自B的(非虚拟)基础AA1的(虚拟)基础的初始化.. >

B的所有虚拟基数首先被初始化，并且依次是A1，A2和AA2. (AA2的初始化产生输出A1 AA2.)然后是直接基，只有一个直接基，AA1(其初始化打印A2 AA1)，最后是类本身，打印.所有虚拟基础都首先出现，然后只有其余的非虚拟基础.

I have the following problem:

struct A1 {
    A1() { std::cout << "A1, "; }
};

struct A2 {
    A2() { std::cout << "A2, "; }
};

struct AA1 : virtual A1,  A2 {
    AA1() { std::cout << "AA1, "; }
};

struct AA2 : A1, virtual A2 {
    AA2(){ std::cout << "AA2, "; }
};

struct B : AA1, virtual AA2 {
    B() { std::cout << "B "; }
};

int main() {
    B b;
}

When you run this code, the answer is:

A1 A2 A1 AA2 A2 AA1 B

I want to understand where is the first A1 created.

I know the rule that the virtual classes are called before non - virtual classes but that first A1 is the problem that is bothering me.

解决方案

The first A1 results from the initialization of the (virtual) base of the (non-virtual) base AA1 of B.

All the virtual bases of B are initialized first, and they are, in order, A1, A2 and AA2. (The initialization of AA2 results in the output A1 AA2.) Then come the direct bases, of which there is only one, AA1 (whose initialization prints A2 AA1), and finally the class itself, printing B. All the virtual bases come first, and then only the remaining non-virtual ones.

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