Re: [Boost-users] Flyweight: wrapping shared_ptr

Subject: Re: [Boost-users] Flyweight: wrapping shared_ptr
From: Akim Demaille (akim_at_[hidden])
Date: 2014-10-07 12:26:34

Le 7 oct. 2014 à 18:18, Joaquín Mª López Muñoz <joaquin_at_[hidden]> a écrit :

Hi Joaquín,

Still so fast, thanks :)

> El 07/10/2014 15:06, Akim Demaille escribió:
>> I have tried deriving from flyweight to add an operator->, but it
>> fails horribly.
> 
> This is how the derivation would be done (I've tried with your code and it works,
> effectively eliminating the need to do .get()-> rather than -> ):
> 
> struct Exp:boost::flyweight<Exp_>
> {
>   using super=boost::flyweight<Exp_>;
>   using super::super;
>   using element_type=super::value_type::element_type;
>   
>   const element_type& operator*() const{return *(this->get());}
>   const element_type* operator->()const{return &*(this->get());}
> };

That's about what I did, except that I felt free to use
decltype.

> bool operator==(const Exp& l, const Exp& r)
> {
>   return *l == *r;
> }
> 
> bool operator!=(const Exp& l, const Exp& r)
> {
>   return !(l == r);
> }

Actually I also had to solve this issue for == and !=, which
seemed fishy, and I was afraid I would have to address a larger
set of functions in the Real World Case.

> I think this approach is OK: elements must be stored through some type of pointer
> because they're polymorphic, and, variants banned, std::shared_ptr<Base> seems a
> sensible choice.

Agreed.

> An alternative would be to use some sort of cloning smart pointer
> to spare the reference count, but it's not clear to me this would be faster than
> shared_ptr:
> 
> * element exists: shared_ptr creation is slower than clone_ptr creation
> * element does not exist: shared_ptr creation and copy is probably faster than clone_ptr
> creation and cloning.

Yes, indeed.

> So, if in doubt profile and determine what's best for you.

Yes, I'll bench the result.

Is there anyway Flyweight could have supported the operator->
natively?  It seems that with some SFINAE under the hood,
it would work, don't you think?