Derived class implementing multiple interfaces with a common function signature - c++

Question

I'm getting a compile error when I try to compile my code.
The error is this:
multi.cc: In function ‘int main()’:
multi.cc:35: error: cannot declare variable ‘mdc’ to be of abstract type ‘MostDerivedClass’
multi.cc:27: note: because the following virtual functions are pure within ‘MostDerivedClass’:
multi.cc:13: note: virtual int Interface2::common_func()
multi.cc:36: error: request for member ‘common_func’ is ambiguous
multi.cc:13: error: candidates are: virtual int Interface2::common_func()
multi.cc:21: error: virtual int InterimClass::common_func()
And here is my code:
class Interface1 {
public:
virtual int common_func() = 0;
virtual ~Interface1() {};
};
class Interface2 {
public:
virtual int common_func() = 0;
virtual int new_func() = 0;
virtual ~Interface2() {};
};
class InterimClass : public Interface1 {
public:
virtual int common_func() {
return 10;
}
};
class MostDerivedClass : public InterimClass, public Interface2 {
public:
virtual int new_func() {
return 20;
}
};
int main() {
MostDerivedClass mdc;
int x = mdc.common_func();
cout << "The value = " << x << endl;
Interface2 &subset_of_funcs = dynamic_cast<Interface2 &>(mdc);
x = subset_of_funcs.common_func();
}
My questions:
How do I tell the compiler that common_func() is already implemented by the InterimClass which is a base class of MostDerivedClass?
Is there another way to fix my problem? What I would really like to do is to be able to also call common_func from Interface2. I'm working with some legacy code with a huge amount of methods in Interface1. In my new code, I only want to call a small set of these Interface1 functions, plus a few that I need to add.

Answer 1

You need to define a common_func() anyway in MostDerivedClass to satisfy your inheritance from Interface2
you can try something like
virtual int common_func() {
return InterimClass::common_func();
}
This is most useful if you cannot change the first Interface1
If you want a real inheritance relationship between your classes you need to follow Lol4t0 advice. Extract a superclass from Interface1, and make Interface2 subclass of this newly created class. Example :
class RootInterface{
public :
virtual int common_func() = 0;
virtual ~RootInterface(){}
};
class Interface1 : public virtual RootInterface{
public:
virtual ~Interface1() {};
};
class Interface2 : public virtual RootInterface{
public:
virtual int new_func() = 0;
virtual ~Interface2() {};
};
class InterimClass : public Interface1 {
public:
virtual int common_func() {
return 10;
}
};
class MostDerivedClass : public InterimClass, public Interface2 {
public:
virtual int new_func() {
return 20;
}
};

Answer 2

Add an override in MostDerivedClass, and from it call InterimClass::common_func().

Answer 3

First of all, I don't really understand the sense of your code.
You need to know that only Interface1::common_func is implemented.
Why don't you make Interface2 inherit from Interface1? I guess you want for both common_func methods to be equal.
Example code (uses polymorphism):
class Interface1
{
public:
virtual int common_func() = 0;
virtual ~Interface1() {};
};
class Interface2 : public Interface1 {
public:
virtual int common_func() = 0;
virtual int new_func() = 0;
virtual ~Interface2() {};
};
class InterimClass : public Interface2 {
public:
virtual int common_func() {
return 10;
}
};
class MostDerivedClass : public InterimClass {
public:
virtual int new_func() {
return 20;
}
};
int test_func()
{
Interface1 * i1 = new MostDerivedClass;
int x = i1->common_func();
cout << "The value = " << x << endl;
Interface2 * i2 = new MostDerivedClass;
x = i2->common_func();
return 0;
}

Answer 4

Let the second interface be derived from the first interface, remove the declaration of virtual int common_func() = 0; from the second interface, & use the keyword virtual to guide the compiler to the implementation.
class Interface1 {
public:
virtual int common_func() = 0;
virtual ~Interface1() {};
};
class BaseClass : public virtual Interface1 {
public:
virtual int common_func() {
return 10;
}
};
class Interface2 : public virtual Interface1{
public:
virtual int new_func() = 0;
virtual ~Interface2() {};
};
class DerivedClass : public virtual BaseClass, public virtual Interface2 {
public:
virtual int new_func() {
return 20;
}
};