Is Overriding virtual methods with template methods in derived class possible? - c++11

Question

I want to override virtual methods from base class with template methods in derived class; Just want to know if there is any smart way or work around to make this possible.
#include <iostream>
using namespace std;
struct A
{
virtual void AF(int i)
{
std::cout << "Function in A" << std::endl;
}
virtual void af(int i)
{
std::cout << "Function in A" << std::endl;
}
};
struct B
{
virtual void BF(int i)
{
std::cout << "Function in B" << std::endl;
}
virtual void bf(int i)
{
std::cout << "Function in B" << std::endl;
}
};
template<bool IS_A>
struct C : public A, public B
{
template<class I>
typename std::enable_if<std::is_same<int, I>::value && IS_A,void>::type AF(I i)
{
std::cout << "Function override from A in C" << std::endl;
}
template<class I>
typename std::enable_if<std::is_same<int, I>::value && !IS_A,void>::type BF(I i)
{
std::cout << "Function override from B in C" << std::endl;
}
template<class I>
void af(I i)
{
std::cout << "Function override from A in C" << std::endl;
}
template<class I>
void bf(I i)
{
std::cout << "Function override from B in C" << std::endl;
}
};
int main() {
int i(0);
{
A * a = new C<true>();
a->AF(i);
a->af(i);
}
{
B * b = new C<false>();
b->BF(i);
b->bf(i);
}
return 0;
}
Output is:
Function in A //Want this to be Function override from A in C
Function in A //Want this to be Function override from A in C
Function in B //Want this to be Function override from B in C
Function in B //Want this to be Function override from B in C
Thanks.

Answer 1

First, to "enable/disable" member functions according to IS_A, you can just define both true and false specializations of C:
template <bool IS_A>
struct C;
template <>
struct C<true> : A, B
{
// functions only defined for IS_A == true
};
template <>
struct C<false> : A, B
{
// functions only defined for IS_A == false
}
And to introduce the overriding by the template's, you can override the ones from A and B where you call the template ones:
void af(int i) override { af<int>(i); }
void bf(int i) override { bf<int>(i); }

Answer 2

You can use the CRTP idiom to do that:
#include <iostream>
#include<type_traits>
struct A
{
virtual void AF(int)
{
std::cout << "Function in A" << std::endl;
}
virtual void af(int)
{
std::cout << "Function in A" << std::endl;
}
};
struct B
{
virtual void BF(int)
{
std::cout << "Function in B" << std::endl;
}
virtual void bf(int)
{
std::cout << "Function in B" << std::endl;
}
};
template<typename D>
struct CRTP: public A, public B
{
template<typename I> void CAF(I i) { A::AF(i); }
template<typename I> void af(I i) { A::af(i); }
template<typename I> void CBF(I i) { B::BF(i); }
template<typename I> void cbf(I i) { B::bf(i); }
void AF(int i) override { static_cast<D*>(this)->CAF(i); }
void af(int i) override { static_cast<D*>(this)->caf(i); }
void BF(int i) override { static_cast<D*>(this)->CBF(i); }
void bf(int i) override { static_cast<D*>(this)->cbf(i); }
};
template<bool IS_A>
struct C : CRTP<C<IS_A>>
{
template<class I>
void CAF(I i)
{
std::cout << "Function override from A in C" << std::endl;
}
template<class I>
void CBF(I i)
{
std::cout << "Function override from B in C" << std::endl;
}
template<class I>
void caf(I i)
{
std::cout << "Function override from A in C" << std::endl;
}
template<class I>
void cbf(I i)
{
std::cout << "Function override from B in C" << std::endl;
}
};
int main()
{
int i(0);
{
A * a = new C<true>();
a->AF(i);
a->af(i);
}
{
B * b = new C<false>();
b->BF(i);
b->bf(i);
}
return 0;
}