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

[Diego Conti]

Hi,
I'm a mathematician and I find GiNaC very useful for doing calculations. 
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;
}

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

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>
};
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, but it will not work because, for 
instance, add::return_type_tinfo 
<http://www.ginac.de/reference/classGiNaC_1_1add.html#b2>() can return 
this, which can be converted to a pointer to void but not to a pointer 
to tinfo_static_t. I really do not understand the meaning of this 
choice. Can someone explain? What would I need to change in order to 
make the above code work?

Thanks in advance
Diego Conti