C++: request for member is ambiguous - c++

Question

#include <iostream>
using namespace std;
class c1{
public:
void f(){std::cout<<"In f1\n";}
};
class c2{
public:
void f(){std::cout<<"In f2\n";}
};
template <typename T>
class c:public c1, c2 {
T* pT;
public:
T* operator->() { return pT; }
};
int main()
{
c<c1>* cMain;
cMain->f();
return 0;
}
Error:
g++ cache.cc
cache.cc: In function ‘int main()’:
cache.cc:22:8: error: request for member ‘f’ is ambiguous
cMain->f();
^
cache.cc:9:8: note: candidates are: void c2::f()
void f(){std::cout<<"In f2\n";}
^
cache.cc:5:8: note: void c1::f()
void f(){std::cout<<"In f1\n";}
why is it not accessing the function f of c1 as specified in the template? is my template usage wrong? How can i make it point to the required class using template.?

Answer 1

The problem is that you're using pointers. You aren't calling c::operator-> but just dereferencing cMain which is uninitialized and leads to UB. Removing the pointers (for the most part) fixes the problem:
#include <iostream>
class c1{
public:
void f(){std::cout<<"In f1\n";}
};
class c2{
public:
void f(){std::cout<<"In f2\n";}
};
template <typename T>
class c { // No longer derives from c1 and c2
T t; // No longer a pointer
public:
T* operator->() { return &t; }
};
int main()
{
c<c1> cMain; // No longer a pointer
cMain->f(); // Calls c::operator->() now
return 0;
}
see it work
Of course you don't need c to derive from c1 and c2 (or anything) for this to work.

Answer 2

If you don't particularly need the pointer semantics for something else, you can add a function to c that redirects to the proper base class function:
template <typename T>
class c:public c1, c2 {
public:
void f() { T::f(); }
};

Answer 3

Your calling code is equivalent to
c<c1>* cMain;
(*cMain).f(); // equivalent to: c<c1> cMain; cMain.f();
Value *cMain has type of c<c1>. Obviously, now a call to f() is ambiguous, because both c1 and c2 has f() function and c<c1> inherits them both. Templates are not to blame (you can make class c non-template and get the same result).
If you want to call -> operator you should say (*cMain)->f(); but don't forget to initialize the pointer before.