[bind] use with shared_ptr
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))); The fact that I can work around this is obviously a good thing. But the work around is significantly less clear, especially when attempting to convince a development team that boost and bind are Good Things(tm) and they should be adopted. Is it horribly difficult to make the simpler composition work (i.e. was it tried and impossible or simply overlooked)? Thanks, Neal
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.)
participants (2)
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Neal Coombes -
Peter Dimov