Re: [Boost-users] [bind] use with shared_ptr

From: Peter Dimov (pdimov_at_[hidden])
Date: 2005-09-21 13:16:29

Neal Coombes wrote:
> Bind is very convenient for use with shared_ptr:
>
> struct C { void func() {} };
> shared_ptr<C> c(new C);
> bind(&C::func, c)();  // calls c->func()
>
> However I've found a snag when trying to do function composition:
>
> class A { shared_ptr<C> func() { return shared_ptr<C>(new C); } };
> A a;
> bind(&C::func, bind(&A::func, a)); // doesn't work, instead must:
> bind(&C::func, bind(&get_pointer<B>, bind(&A::func, a)));

This is actually a problem in mem_fn. Bind is doing the right thing, but the 
function object that is implicitly created by mem_fn(&C::func) doesn't 
handle rvalues of type shared_ptr<C>.

If you patch mem_fn_template.hpp below the lines

    template<class U> R operator()(U & u) const
    {
        BOOST_MEM_FN_RETURN call(u, &u);
    }

and insert

    template<class U> R operator()(U const & u) const
    {
        BOOST_MEM_FN_RETURN call(u, &u);
    }

it should work.

The reason that this overload is not already present is historical: when 
mem_fn was written many compilers had problems with the additional overload. 
Now most compilers are in a much better shape, but nobody has reported this 
as a problem, so it never got fixed.

I'll fix it in CVS when I have time (to write the additional mem_fn tests 
based on your example.)