[Boost-users] Const problem with bind of data member

From: Alan M. Carroll (amc_at_[hidden])
Date: 2007-01-16 18:36:43

I am having a problem with Boost.Bind (1.33.1). Consider the following simplified code:

struct Bob {
        void func() { }
};
void bob_test() {
        typedef std::map<int, Bob> BobMap;
        BobMap bob_map;
        std::for_each(bob_map.begin(), bob_map.end(),
            bind(&Bob::func, bind(&BobMap::value_type::second, _1)));
}

This fails to compile under DevStudio 8. The problem is the outer bound functor expects (Bob* const) but is passed (Bob const*).

This seems a straight forward use -- call the "func" method on every Bob instance in the map. I can get this to compile if I change the inner bind to "bind<Bob&>(...)". Shouldn't the default return type of the binding of (R T::*) be (R&)? I looked through the bind and mem_fn documentation and didn't see any mention of that. The .* operator applied to a non-const T and a (R T::*) returns (R&), so it is implied that mem_fn of a data member does as well.

Also, why does the error message refer to pointers and not references? Is that an artifact of compiler argument matching failure?

Exact message:
1>inc\boost/bind/mem_fn_template.hpp(31) : error C2440: 'argument' : cannot convert from 'const Bob *' to 'Bob *const '
1>        inc\boost/bind/mem_fn_template.hpp(56) : see reference to function template instantiation 'R boost::_mfi::mf0<R,T>::call<const U>(U &,const T *) const' being compiled
1>        with
1>        [
1>            R=void,
1>            T=Bob,
1>            U=Bob
1>        ]