1

if i run this code

#include<iostream>
using namespace std;
class Final;

class MakeFinal{
public:
friend class Final;
MakeFinal(){cout<<"makefinal\n";}
};

class Final: public virtual MakeFinal{
public:
 Final(){cout<<"Final\n";}
};

 class Derived:public Final{
public:
  Derived(){cout<<"Derived\n";}
};


int main(){
//Final f;
Derived d;
return 0;
}

Output is :

makefinal Final Derived

But if i make MakeFinal() constructor private , compiler shows error message. What is this different constructor call hierarchy based on ?

mohit
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  • If a base class constructor is private then nothing other than a friend class (or the class itself) can make one, including via inheritance. – Jonathan Potter Aug 22 '13 at 11:43
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    Horrible formatting aside, why are you using virtual inheritance in the first place in this code? Do you understand what problems virtual inheritance helps solve? – thokra Aug 22 '13 at 11:53

2 Answers2

1

Refer to:

C++ FAQs - virtual inheritance constructors http://www.parashift.com/c++-faq/virtual-inheritance-ctors.html

Because of the fact that "Initialization list of most-derived-class's ctor directly invokes the virtual base class's ctor. ", your most derived needs to invoke the constructor of the virtual base directly. Therefore, for what you want to do you'd need to make the most derived class a friend too...

Furthermore, it seems that you don't understand virtual inheritance correctly. Refer to this FAQ to understand the purpose and proper use of virtual inheritance.

Werner Erasmus
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1

If your class A have private constructor, you cannot create object a of this class like that (see):

A a;

When an object b of class B that derives from A is created, base class constructor must also be called. If it is private, it cannot be called and the derived object cannot be created.

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