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

Alan M. Carroll wrote:

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 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(...)". 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.

Unfortunately boost::bind is a bit simplistic and always has a fixed return type. Since when the member pointer is bound there is no way to determine whether it will be applied to a const or a non-const T, the default is the conservative R const& that can be used with both (but may cause failures upstream if the outer bind expects a R&.) I assumed that writing bind won't be that big of a deal in the relatively rare cases where it's needed.

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

It's an implementation artifact of mem_fn. The proper overload taking T& doesn't match (the argument is const) and another unrelated overload (used for derived types and smart pointers) is selected.