template instantiation logic ?

[Marc ESPIE]

On Sun, Feb 08, 1998 at 06:10:11PM -0200, Alexandre Oliva wrote:
> Marc ESPIE writes:
> 
> > class Gaussian_random_distribution: private Gaussian_random_generator, 
> > 	public Random_distribution
> 
> > The strange thing is that, in order to get egcs to compile my code,
> > instantiating the template as
> > template Vector<double>& Vector<double>::operator=(Random_distribution &);
> > is not enough !
> 
> If your code looks like:
> 
> Vector<double>& v;
> Gaussian_random_distribution& d;
> v = d;

> overload resolution selects
> Vector<double>::operator=<Gaussian_random_distribution>(Gaussian_random_distribution&),
> because it is an exact match, no matter the appropriate assignment
> operator for the base class is defined.

> just define an explicit specialization for
> Guassian_random_distribution that converts the argument to the base
> type and calls the appropriate template specialization, or cast the
> object of type Gaussian_random_distribution to Random_distribution&
> before when you assign it to a Vector<double>, so that the static type
> causes the overload resolution mechanism to select the template
> specialization you intended it to.

Hum...  I thought about that, but I'm not convinced. I know the classic trick
where you avoid building myriads of instances of list<A *> by playing with
types and just converting everyone to list<void *>, but I believe there is
a big difference there...  There is no inheritance relation between A * and
void *, whereas there is a clean inheritance relation between 
Gaussian_random_distribution and Random_distribution. Moreover, the template
instance only uses the Random_distribution::operator()  method, 
which is pure virtual.

Hence the compiler (IMHO) has everything it needs to know that inheritance
WILL work and should be able to optimize the call to use 
Random_distribution directly: the end result will be the same, and
everything will be `as if' the appropriate specialization had been called.
Looks like a useful optimization to me.

-- 
	Marc Espie