Multiple Inheritance

[lostromos]

Hello all,

Something i thought was simple has bothered my sleep last night. Imagine two classes A,B and a third C that inherits from the other two.

class C : public A, public B

There is an attribute in class A and in class B called n. It's private in A and protected in B. I thought that class C would have only acces to the B::n as the A::n is private. But whan i use the variable n inside class C, the complier finds the variable n ambiguous! Any ideas?

Thanks in advance, have a nice day

George

[Valmont]

Yes, this is the case:

#include <iostream>
#include <string>

//-
using std::string;
using std::endl;
using std::cout;
using std::cin;
//-

class A
{
public:
  A::A() : theString("A") {}
protected:
  string theString;  
};

class B
{
public:
  B::B() : theString("B") {}
private:
  string theString;  
};

class C : public A , public B
{
public:
  void print()
  {
    cout<<theString<<endl;
  }
 
};

int main()
{
  C theC;
  theC.print();
   
  cin.get();
  return 0;
}

And will error indeed.

Let's see how this looks in memory:

CLASS A string theString;
CLASS B string theString;
CLASS C void print

As you can see, internally the system doesn't care about access specifiers. Public, private, all the same memory wise. But it does know what member belongs to what class. And here lies our solution: We need to tell the system which one we mean.
Use the scope resolution operator "::"

class C : public A , public B
{
public:
  void print()
  {
    cout<<A::theString<<endl;
  } 
};

Or formally:
Access specifiers do not play a role in name lookup resolution.

That was another good question.

[lostromos]

Valmont,

Thanks a lot for your thorough answer and for all the explanation. I should guess that what you said is also valid for the private functions of the mother classes A and B.

Best,

George