[GiNaC-list] new tinfo method makes is_a less efficient

[Sheplyakov Alexei]

Hello!

First of all, it would be nice to see the actual code which proves $subject.

On Mon, Sep 10, 2007 at 04:26:49PM +0200, Diego Conti wrote:

> Unfortunately, the new tinfo mechanism in version 1.4.0 breaks my code, 
> and I am not sure how to fix that. Basically, what I need is an 
> efficient version of is_a (actually, is_a<ncmul>) that does not rely on 
> dynamic_cast.

> With the old tinfo method, I could simply redefine:
> 
> template<> inline bool is_a<ncmul>(const basic & obj)
> {
>    return (obj.tinfo()&TINFO_MASK)==TINFO_ncmul;
> }

I think such a redifinition might break some GiNaC classes...

> where TINFO_MASK is the constant 0x001fffffU, and the classes I derived 
> from ncmul had appropriate tinfo constants.

... but anyway, is it really faster? 

> With the new method, where the tinfo_key is a pointer, the above code 
> does not work. One obvious alternative is the following:
> 
> struct tinfo_static_t 
> <http://www.ginac.de/reference/structGiNaC_1_1tinfo__static__t.html>
> {
>    const tinfo_static_t* derives_from;   //NULL if derived from void, 
> otherwise points to tinfo_static member in superclass 
> <http://www.ginac.de/reference/structGiNaC_1_1tinfo__static__t.html>
> };

Ugh, now every object (even a symbol) carries information about relationship
between _all_ GiNaC classes. Not so nice.

> typedef const tinfo_static_t * tinfo_t;
> 
> and then
> 
> template <class T>
> inline bool is_a(const basic &obj)
> {
>    const tinfo_static_t* tinfo=obj.tinfo();
>    do
>    {
>        if (tinfo==&T::tinfo_static) return true;
>        tinfo = tinfo->derives_from 
> <http://www.ginac.de/reference/structGiNaC_1_1tinfo__static__t.html>;
>    } while (tinfo!=NULL);
>    return false;
> }
> 
> This appears reasonably efficient,

Yes, but I doubt it will be any better than dynamic_cast. It uses very similar
technique (see e.g. <cxxabi.h> from GNU libstdc++).

Best regards,
 Alexei.

-- 
All science is either physics or stamp collecting.

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