C++ multiple inheritance with base classes deriving from the same class - c++

Question

I have stumbled on a problem while trying to re-use code from different classes. I post it here in hope that some of you might be able to help me.
I have a set of classes (B,C) deriving from the same class (A) which forces the implementation of some methods (foo, run). Class B implements these method, and both B and C provide other methods:
#include<iostream>
template<class I, class O>
class A {
public:
A() {}
virtual ~A() {}
virtual void foo() const = 0; // force implementation of this function
virtual void run() const = 0; // force implementation of this function
};
template<class I, class O>
class B : public A<I,O> {
public:
B() {}
virtual ~B() {}
virtual void foo() const { // implementation for the Base class
std::cout << "B's implementation of foo" << std::endl;
}
virtual void run() const { // implementation for the Base class
std::cout << "B's implementation of run" << std::endl;
}
virtual void foobar() const { // some other function provided by this class
std::cout << "B's implementation of foobar" << std::endl;
}
};
template<class I, class O, class M>
class C : public A<I,O> {
public:
C() {}
virtual ~C() {}
virtual void bar(M m) const { // some other function provided by this class
std::cout << "C's implementation of bar with: " << m << std::endl;
}
};
Now, what I am trying to do is inherit from both B and C so that I can have the extra methods (foobar, bar), but also not have to implement the method from class A (foo) because it is already defined in B:
template<class I, class O>
class D : public B<I,O>, public C<I,O,int> {
public:
D() {}
void run() const {
this->bar(123);
this->foo();
this->foobar();
}
};
But for some reason the compiler gives me this error:
test.cpp: In function ‘int main(int, char**)’:
test.cpp:68:35: error: cannot allocate an object of abstract type ‘D<float, double>’
A<float, double> *d = new D<float, double>(); // what I need to do
test.cpp:48:11: note: because the following virtual functions are pure within ‘D<float, double>’:
class D : public B<I,O>, public C<I,O,int> {
^
test.cpp:9:22: note: void A<I, O>::foo() const [with I = float; O = double]
virtual void foo() const = 0; // force implementation of this function
This is the code I use to run it:
int main(int argc, char **argv)
{
A<float, double> *b = new B<float, double>();
b->foo(); // prints "B's implementation of foo"
b->run(); // prints "B's implementation of run"
//A<float, double> *c = new C<float, double, int>(); // obviously fails because C does not implement any of A's functions
//A<float, double> *d = new D<float, double>; // line 68: what I need to do
//d->run(); // ***throws the abstract class error
return 0;
}
I want to use the 'run' function of an object of class D from a pointer to a A. As all the functions are virtual I expect to execute implementation of each function defined in the lowest inheritance point, meaning that 'B::run' will be discarded. As 'D::run' uses functions from both B and C I need to inherit from both classes.
I hope I have described it enough and not confused anybody.
Thanks for the help!

Answer 1

If you change B and C to virtually inherit from the A template class, they will share a single base instance when combined by D and this error will go away:
template<class I, class O>
class B : virtual public A<I,O> {
// ...
template<class I, class O, class M>
class C : virtual public A<I,O> {
However, this pattern (known as the diamond inheritance (anti-)pattern) can be very difficult to reason about and I would strongly suggest avoiding it if possible. You are likely to run into even more obscure problems later.
Here is a sample of this technique working, but showing some results that may not be expected at first glance:
class A {
public:
virtual void foo() = 0;
};
class B : virtual public A {
public:
virtual void foo() override;
};
void B::foo()
{
std::cout << "B::foo()" << std::endl;
}
class C : virtual public A { };
class D : public B, public C { };
int main() {
D d;
C & c = d;
c.foo();
return 0;
}
Note that even though you are calling C::foo(), which is pure virtual, since there is only one A instance the inherited pure virtual function resolves to B::foo() though the shared A vtable. This is a somewhat surprising side-effect -- that you can wind up invoking methods implemented on a cousin type.

Answer 2

The answer by #cdhowie gives you a solution.
To understand the problem the compiler is complaining about, take a set of simpler classes:
struct A
{
virtual void foo() = 0;
};
struct B : A
{
virtual void foo() {}
}
struct C : A
{
void bar() {}
}
struct D : B, C
{
};
The class hierarchy of D is:
A A
| |
B C
\ /
D
With this inheritance structure, D has two virtual tables, one corresponding to the B inheritance hierarchy and one corresponding to C inheritance hierarchy. The difference being that in the B hierarchy, there is an implementation of A::foo() while there isn't one in the C hierarchy.
Let's say you were allowed to construct an object of type D.
D d;
C* cp = &d;
Now cp points to the C hierarchy of D, and uses a virtual table in which foo is not implemented. That will be a run time error that the compiler is helping you avoid at compile time.

Answer 3

I know this is a late answer but since you are deriving from a pure virtual function for class C, you have to implement it, then in those functions you call the base class:
virtual void foo() const { // for class C
B::foo();
}