Compiler issue with pure virtual class and inheritance in C++ - c++

Question

There is a compiler issue that I am facing. The code that I want to compile is as follows
#include <iostream>
class IA
{
public:
virtual void f1() = 0;
};
class A
{
public:
virtual void f1() { std::cout << "in A::f1" <<std::endl; }
};
class IB : public IA
{
public:
virtual void f3() = 0;
};
class B : public A, public IB
{
public:
// virtual void f1() { std::cout << "in B::f1" <<std::endl; }
virtual void f3() { std::cout << "in f3" <<std::endl; }
};
int main()
{
IB* b = new B();
b->f1();
b->f3();
std::cout << "Hello, world!\n";
}
Here, If I uncomment B::f1 things workout fine. The current code gives the following compiler error.
source_file.cpp: In function ‘int main()’:
source_file.cpp:32:20: error: cannot allocate an object of abstract type ‘B’
IB* b = new B();
^
source_file.cpp:23:7: note: because the following virtual functions are pure within ‘B’:
class B : public A, public IB
^
source_file.cpp:8:18: note: virtual void IA::f1()
virtual void f1() = 0;
The intent here is that the interface, IA, would be defined in the public headers of the library. The implementation class, A, would be done in the source code. All other classes would make use of this base class(A) to implement the publicly exposed abstract interface. Is this possible?

Answer 1

You should change
class A
to
class A : public IA
But it's not enough, as now you get multiple instances of IA in B. So you have to make the inheritance virtual:
class A : virtual public IA {...};
class IB : virtual public IA {...};
UPDATE But is there a real reason for IB to be inherited from IA? If no, drop it, and you won't need virtual inheritance:
class A : public IA {...};
class IB {...};
int main()
{
IB* b = new B();
// b->f1(); - You can't do it now, but do you really need to?
b->f3();
return 0;
}