Why can't the D class object access the disp function in class A? (Diamond Problem) (Code inside the post)

Question

#include<iostream>
using namespace std;
class A
{
public:
void disp()
{
cout << "Class A";
}
};

class B : public A
{
};

class C : public A
{
};

class D : public B, public C
{
};

int main()
{
D obj;
obj.A::disp();
return 0;
}

Answer 1

first sorry for my english ....

line : obj.A::disp () gives compile time error.
it is becouse compile goes to confuse for what path disp() is inherited .....either by the
1. class A -> classB -> classD or
2 classA-> classC-> classD
......
and if draw the arrows to the flow of inheritance of classes its seems like diamond or rhombus. ...
that's by this problem is known as diamond problem

Answer 2

#include<iostream>
using namespace std;
class A
{
public:
void disp()
{
cout << "Class A";
}
};

class B :virtual public A
{
};

class C :virtual public A
{
};

class D : public B, public C
{
};

int main()
{
D obj;
obj.A::disp();
return 0;
}

notice that class B and class C virtually inherit A (additional virtual right before public)

this now able to run on the app compiler

this wikipedia page explains the situation with a nice example:
https://en.m.wikipedia.org/wiki/Virtual_inheritance

Answer 3

their is an ambiguity
compile get confuse which path has to follow
1. D-> B ->A::dis ()
2. D-> C-> A::dis ()

Answer 4

@james .......during the compilation procces, compiler finds more then one path for the same .... so it is ambitious for the compiler ..... and gives compile time error.

Answer 5

I just tried running your code in Visual Studio and it compiled and executed fine. Is it perhaps a problem with the compiler the website uses?

Answer 6

I use the CodeBlocks IDE. And it shows the same error. This problem is written in Robert Lafore's book on C++ but unfortunately, it doesn't provide it's solution. But still, thanks for the reply and your time. :) Hope someone here gives a lil explanation regarding it.

Answer 7

@Rajkumar, but I do write obj.A::disp( ) i.e., explicitly pointing out the object A here, shouldn't the compiler know what I'm trying to do here? And don't worry, I understood each word of yours perfectly. :)

Answer 8

Even though you use .A, your object is still of class D, making it have the ambiguous path that others have mentioned.

Answer 9

Thanks a lot everyone. All of your answers were really helpful.