Yvan Seth's Hole in the Internet

I had a hard time Googling a solution to this particular woe. Alas the exact string "unresolved overloaded function type" doesn't give anything useful with "boost::bind" and "boost:mem_fn". This is the typical sort error message from g++:

foo.cpp:18: error: no matching function for call to 
    'bind(<unresolved overloaded function type>, 
        const boost::reference_wrapper<FooMaster>, boost::arg<1> (&)())'

I've generated this using a rather contrived example based on the use pattern that was causing my woe:

#include <iostream>
#include <boost/bind.hpp>

class FooMaster {
public:
    void foo(std::string &s) {
        std::cout << "foo(s=" << s << ")" << std::endl;
    }

    void foo(int x) {
        std::cout << "foo(x=" << x << ")" << std::endl;
    }
};

int main(int argc, char **argv) {
    FooMaster fm;
    boost::bind(&FooMaster::foo, boost::ref(fm), _1)(10);
    return 0;
}

The problem is obvious of course, it is exactly as the compiler is telling you. Mr compiler has no idea which "foo" you're talking about. "But Mr Compiler, it is obvious!" You say. "I'm not passing it a bloody string!" Sometimes you just have to understand that Mr Compiler isn't you and needs a little hand-holding.

The issue clearly boils down to FooMaster having two methods that match &FooMaster::foo. This signature, used to instantiate the bind template, doesn't take into account the arguments, so which foo are we taling about?! Alas, as obvious as the problem was I didn't find the solution so obvious. I just don't cut it as a C++ guy sometimes. I'd given up on the problem in fact, but then I stumbled on a solution while looking into some other problem I had. It was one of those "oh, duh!" moments really, I was looking at some code that assigned a member function pointer. The type specification for such a thing includes the argument types! So here's a modified main that uses one extra step in order to explain what we want to the compiler:

int main(int argc, char **argv) {
    FooMaster fm;
    // Create a variable of the desired function signature type and assign
    // using desired member function.  This will resolve the correct
    // function thanks to the signature including the argument types.
    void (FooMaster::*fn)(int) = &FooMaster::foo;
    // Now pass the function variable to the template method so that it knows
    // exactly which function we mean.
    boost::bind(fn, boost::ref(fm), _1)(10);
    return 0;
}

All you have to do is give Mr Compiler a little bit of a helping hand…

Joy!