Re: [boost] Deterimining interest in: Optional Polymorphic Value object with Small Object optimization

From: Corrado Zoccolo (czoccolo_at_[hidden])
Date: 2007-10-22 04:37:09

On 10/22/07, Mathias Gaunard <mathias.gaunard_at_[hidden]> wrote:
>
> Corrado Zoccolo wrote:
>
> > BTW, the idea of the hand made vtable born in the discussion with gpd.
> My
> > first implementation used just one function ptr to encapsulate all the
> > behaviour (with a signature to fit all the methods), and was
> substantially
> > less efficient than the current solution.
>
> Instead of a hand make vtable, why not this?
>
> struct impl_base
> {
>      virtual void something(void*) = 0;
> };
>
> template<typename T>
> struct impl
> {
>      void something(void* p)
>      {
>          // do something useful
>      }
> };
>
> template<typename T>
> class poly
> {
>         template<typename U>
>         poly(const U& u)
>         {
>                 new(&type_info) impl<U>;
>                 ...
>         }
>
>         typename boost::aligned_storage<
>                 sizeof(impl<T>),
>                 boost::alignment_of< impl<T> >
>         >::type type_info;
> };
>
> This simply assumes that all instances of impl have the same size and
> alignment.

Because the vtables in C++ are not smart enough. If I make an hand made
vtable, I can put not only function pointers, but also values inside it.
This means that for example, I have to make a call through a function
pointer to query if the object was allocated on heap or on stack, or if it
has a no-throw copy constructor (all things that I can put directly in my
hand made vtable).

I think a valuable addition to c++0x would be "virtual constexpr" methods,
i.e. virtual methods that are guaranteed to return compile time constants,
but different classes in the hierarchy can return different values. This
allows for a big optimization, i.e. the compiler can store the value of the
constant into the vtable, and save an unpredictable call through a pointer.

Corrado

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-- 
__________________________________________________________________________
dott. Corrado Zoccolo                          mailto:zoccolo_at_[hidden]
PhD - Department of Computer Science - University of Pisa, Italy
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