虚基类和多重继承下构造函数的顺序

1.没有虚基类:

#include <iostream>#include <cstdio>using namespace std;class A{public:    A()    {        cout << "A" << endl;    };};class B : public A{public:    B()    {        cout << "B" << endl;    };};class C : public A{public:    C()    {        cout << "C" << endl;    };};class D : public B, C{public:    D()    {        cout << "D" << endl;    };};int main(){    B b; //输出: A B 先构造基类,再构造子类    D d; //输出: A B A C D    return 0;}

2.有虚基类:

#include <iostream>#include <cstdio>using namespace std;class A{public:    A()    {        cout << "A" << endl;    };};class VB : virtual public A{public:    VB()    {        cout << "VB" << endl;    };};class VC : virtual public A{public:    VC()    {        cout << "VC" << endl;    };};class VD : public VB, VC{public:    VD()    {        cout << "VD" << endl;    };};int main(){    VD vd; //输出: A VB VC VD    //虚基类的subobject总是在最深的地方最先被构造    return 0;}

3.存在虚基类的多继承:

#include <iostream>using namespace std;class A{public:    A(const string& s)    {        cout << "A " << s << endl;    }    virtual ~A() {}};class B{public:    B(const string& s)    {        cout << "B " << s << endl;    }    virtual ~B() {}};class C{public:    C(const string& s)    {        cout << "C " << s << endl;    }    virtual ~C() {}};class D{public:    D(const string& s)    {        cout << "D " << s << endl;    }    virtual ~D() {}};class E : public A, virtual public B, virtual public C{public:    E(const string& s) : A("from E"), B("from E"), C("from E")    {        cout << "E " << s << endl;    }};class F : virtual public B, virtual public C, public D{public:    F(const string& s) : B("from F"), C("from F"), D("from F")    {        cout << "F " << s << endl;    }};class G : public E, public F{public:    G(const string& s) : B("from G"), C("from G"), E("from G"),  F("from G")    {        cout << "G " << s << endl;    }};int main(){    //虚基类的构造函数最先执行    G g("from main");}

输出： 

引用

B from G 
C from G 
A from E 
E from G 
D from F 
F from G 
G from main

总结：虚基类构造函数最先被执行，然后按照从左到右，从上到下的顺序依次执行每一个构造函数。