c++ - 虚继承中构造函数的调用顺序是怎样的？

Question

c++虚继承中构造函数的调用顺序是怎样的？

针对下面两种多重继承的情况；

(I) 对于下面的代码，没有虚继承；

class a
{
    public: 

        a()
        {
            cout<<"\t a";
        }

};

class b: public a
{
    public:
        b()
        {
            cout<<"\t b";
        }

};

class c: public b
{
    public:

        c()
        {
            cout<<"\t c";
        }

};

class d: public c
{
    public:

        d()
        {
            cout<<"\t d";
        }
};

class e: public c, public d
{
    public:

        e()
        {
            cout<<"\t e";
        }
};

class f: public b, public e
{
    public:

        f()
        {
            cout<<"\t f";
        }
};


int main()
{

    f aaa;

    return 0;
}

输出是:

     a       b       a       b       c       a       b       c       d       e       f

(II)类e虚继承:

class a
{
    public: 

        a()
        {
            cout<<"\t a";
        }

};

class b: public a
{
    public:
        b()
        {
            cout<<"\t b";
        }

};

class c: public b
{
    public:

        c()
        {
            cout<<"\t c";
        }

};

class d: public c
{
    public:

        d()
        {
            cout<<"\t d";
        }
};

class e: public c, public d
{
    public:

        e()
        {
            cout<<"\t e";
        }
};

class f: public b, public virtual e
{
    public:

        f()
        {
            cout<<"\t f";
        }
};


int main()
{

    f aaa;

    return 0;
}

输出是:

     a       b       c       a       b       c       d       e       a       b       f

谁能解释一下这两种情况下的输出是如何获得的？虚继承如何影响对象的构造？

Answer 1

首先初始化虚基类，否则直接基类按照基类声明从左到右的顺序初始化。

对于类f，class f: public b, public e，没有虚基类，直接基类b会是首先初始化，然后是e。 (从左到右的顺序)

对于class f: public b, public virtual e，首先会初始化虚基类e，然后是b。

参见 Initialization order :

1) If the constructor is for the most-derived class, virtual base classes are initialized in the order in which they appear in depth-first left-to-right traversal of the base class declarations (left-to-right refers to the appearance in base-specifier lists)

2) Then, direct base classes are initialized in left-to-right order as they appear in this class's base-specifier list

3) Then, non-static data members are initialized in order of declaration in the class definition.

4) Finally, the body of the constructor is executed