Question

consider the following :

#include <iostream>
#include <string>
using namespace std;


class A {
public:
   A(const char* sName) //conversion constructor
           : _sName(sName) {cout<<"(1)"<<endl;} ;
   A(const A& s) {cout<<"(2)"<<endl;} //copy constructor
   virtual ~A() {cout<<"(3)"<<endl;} //destructor
    void f1() {cout<<"(4)"<<endl; f2();} //Notice two commands!
    virtual void f2() =0;
  private:
    string _sName;
  };



  class B1: virtual public A {
  public:
     B1(const char* sAName, const char* sSName)
             : _sName1(sAName), A(sSName) {cout<<"(5)"<<endl;}
     B1(const B1& b1) : A(b1) {cout<<"(6)"<<endl;}
     ~B1() {cout<<"(7)"<<endl;}
     virtual void f1() {cout<<"(8)"<<endl;}
     virtual void f2() {cout<<"(9)"<<endl; f3();}
     virtual void f3() {cout<<"(10)"<<endl;}
  private:
     string _sName1;
  };



  class B2: virtual public A {
  public:
     B2(const char* sAName, const char* sSName)
             : _sName2(sAName), A(sSName) {cout<<"(11)"<<endl;}
     B2(const B2& b2) : A(b2) {cout<<"(12)"<<endl;}
     ~B2() {cout<<"(13)"<<endl;}
     virtual void f3() {f1(); cout<<"(14)"<<endl;}
  private:
      string _sName2;
  };

  class C: public B1, public B2 {
  public:
         C () : A(" this is A ") , B1(" this is " , " B1 ") , B2 (" this is " , " B2 ") {}
         C (const C& c) :  A(c) , B1(c) , B2(c) {}
         ~C() {cout<<"(15)"<<endl;}
         virtual void f1() {A::f1(); cout<<"(16)"<<endl;}
         void f3 () {cout<<"(17)"<<endl;}
  };


  int main() {
      /* some code */
      return 0;
  }

As you can see , I added in class C the implementation of C’s Ctor (constructor) . What is not clear to me is why do I need also the upcast from C to A , if B1 does that job for me in its Ctor ?
Meaning , if I wrote C’s Ctor as :

C () : A(" this is A ") , B1(" this is " , " B1 ") , B2 (" this is " , " B2 ") {}

why can’t I write :

C () : B1(" this is " , " B1 ") , B2 (" this is " , " B2 ") {}

thanks ,
Ronen

Answer 1

Constructor of virtual base is called before nonvirtual ansestors ctor’s. In your example B1 ctor can not invoke the virtual base constructor for C as B1 ctor itself will be called later.