c++ - 多路径继承和

Question

在下面的代码中，Multi Path Inheritance 是通过使用 Virtual Class 解决的构造函数是如何工作的？构造函数不能是继承的、虚拟的或静态的。

/*Multi Path Inheritance*/

class A{

public:
    int a;
    A(){
        a=50;
    }
};


class B:virtual public A{

public:
    /*B(){
        a = 40;
    }*/

};

class C:virtual public A{

public:
    /*C(){
        a = 30;
    }*/

};

class E:virtual public A{

public:
    E(){
        a = 40;
    }

};

class D : public B, public C, public E{

public:
    D(){
        cout<<"The value of a is : "<<a<<endl;  
    }

};

int main(int argc, char *argv[]){

    D d;
    return 0;
}

Answer 1

基于 12.6.2/10 标准的配额，所以构造函数体将按以下顺序调用:A->B->C->D，所以 a 的最终值为 40。

In a non-delegating constructor, initialization proceeds in thefollowing order:

 — First, and only for the constructor of the most
   derived class (1.8), virtual base classes are initialized in the order
   they appear on a depth-first left-to-right traversal of the directed
   acyclic graph of base classes, where “left-to-right” is the order of
   appearance of the base classes in the derived class
   base-specifier-list. 

 — Then, direct base classes are initialized in
   declaration order as they appear in the base-specifier-list
   (regardless of the order of the mem-initializers).