on Mon Jun 18 2012, "Hite, Christopher" <Christopher.Hite-AT-partner.commerzbank.com> wrote:
Dave Abrahams wrote:
I completely agree with using functors for simple interfaces.
By "functors" do you mean runtime-polymorphic function objects?
Not sure they're polymorphic (as in fusion polymorphic - taking different parameters). Is there a shorter term "[const?] callable (with a particular signature) and copyable."? http://www.boost.org/doc/libs/1_49_0/doc/html/boost_asio/reference/ReadHandl...
"Deferred Callable Object" http://www.boost.org/doc/libs/1_49_0/libs/fusion/doc/html/fusion/functional/...
Sorry, I don't follow any of this. When you say "functors," what do you mean? That word has a formal meaning in computer science that is probably not what you mean, and an informal meaning in C++ (just another word for "function object") that doesn't /seem/ to be what you mean.
Consider this big monster object with this interface. It's not movable/copyable (maybe it has its own threads). I think you'd find something like this acceptable.
struct monster : noncopyable{ typedef signal<void ()>::slot_type slot;
connection subscribeA(slot); connection subscribeB(slot); connection subscribeC(slot); ... };
If it's not movable/copyable it has already failed the test. The idea is to construct a world of values, and this is not one.
What "test"?
I mean, it's not relevant to the argument I'm making about the feasibility of coding entirely with type-erased value semantics in lieu of traditional OOP polymorphism.
You mean for any.
No
Well you could use any<...,_self&> with it. I'm going to assume your "idea" is meant generally for all/most C++.
All non-legacy.
Let's just say you've got to write a little monster like the one above.
I don't need to.
It's got threads, io_service, mutexes, signal2, containers, sockets, etc inside it that don't like being moved/copied and don't need to be. What would you do Dave?
I wouldn't write a monster in the first place.
Would you prefer to put all of monster's guts in a shared_ptr-ed pimple so monster can be copied?
This is an honest question. I may be a couple steps behind on best practices of C++. I'd probably give the user of monster an object he can't move/copy. Is that generally bad? Do you think all objects be copy/movable?
I think it's 99.9% reasonable to make all objects movable, and maybe 75% reasonable to make all objects copyable. You can do either or both of these, the difference is a matter of degree: just how uncompromising do you have to be to build a world that makes sense?
Didn't we say this was about new code? If you've got legacy stuff like this you do what you have to do.
This scenario could happen to you in the future. How would you write monster? At a later point how would you achieve polymorphism of two impls of monster?
I wouldn't do it.
Would you do something like this?
typedef function< connection (slot)> S; typedef tuple<S,S,S> deleted_monster;
template<T> deleted_monster make_ deleted_monster (T&); // uses names ABC
I think the use case for value based interface for a fat interface doesn't come up much.
I don't understand what point you're trying to make.
I'm trying to figure out if you prefer extremely functionally oriented code.
I often do.
Why expose member functions at all? We could just give the user a collection of free type-deleted functors.
Because that, by itself, wouldn't achieve runtime polymorphism. You'd still need to bind the "type-deleted functors" (by which I presume you mean e.g. std::function) to the object somehow.
For any copyable object I could take all the member functions, bind them with the object, type-delete and return them in a tuple or a fusion::map with name tags.
The user can then take any subset of those functions to be an interface.
I think Haskall programmers think like this: http://www.haskell.org/haskellwiki/OOP_vs_type_classes#Packing_data_.26_func... Seeing as how C++11 has gotten functional with lambda expressions and perfect function forwarding, maybe I need to go check out Haskell.
I recommend it, for mind expansion value if nothing else. -- Dave Abrahams BoostPro Computing http://www.boostpro.com