Ambiguous definition of operator() with multiple inheritance

Question

I compile this code with GCC (4.2.1 Apple build 5664)

#include <cstddef>

using std::size_t;

template <char I> struct index { };

struct a
{
    void operator()(size_t const &) { }
};

struct b
{
    template <char I>
    void operator()(index<I> const &) { }
};

struct c: public a, public b { };

int main (int argc, char const *argv[])
{
    c vc;
    vc(1);

    return 0;
}

and give me the following error:

main.cpp: In function ‘int main(int, const char**)’:
main.cpp:22: error: request for member ‘operator()’ is ambiguous
main.cpp:14: error: candidates are: template<char I> void b::operator()(const index<I>&)
main.cpp:9:  error:                 void a::operator()(const size_t&)

I don't understand the reason why this code is ambiguous; the two methods have different signatures.

Answer 1

Modify c this way:

struct c: public a, public b
{
    using a::operator();
    using b::operator();
};

C++ (prior to C++0x) is kind of awkward in inheriting functions: if you provide a function with the same name of a base class' function it hides base class ones.

It looks like also inheriting from two classes has the same problem.

// looking for the standard...