What is the order of constructor call in virtual inheritance?

问题

What is the order of constructor call in virtual inheritance in c++?

For the following two cases of multiple inheritance;

(I) for the following code, without virtual inheritance;

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;
}

The output is:

     a       b       a       b       c       a       b       c       d       e       f

(II)With virtual inheritance of class e:

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;
}

The output is:

     a       b       c       a       b       c       d       e       a       b       f

Can someone explain how the output is obtained in both cases? How does virtual inheritance affect the construction of objects?

回答1:

The virtual base class will be initialized firstly, otherwise, the direct base classes will be initialized in left-to-right order of the base class declaration.

For class f, class f: public b, public e, there's no virtual base class, the direct base class b will be initialized at first, then e. (left-to-right order)

For class f: public b, public virtual e, the virtual base class e will be initialized at first, then b.

See 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

来源：https://stackoverflow.com/questions/37235368/what-is-the-order-of-constructor-call-in-virtual-inheritance