🚀 go-pugleaf

Rob Stewart wrote:
> 
> Roman Lechtchinsky wrote:
> >
> > Hi,
> >
> > sorry for being curious but I'm only human... My question is: when declaring
> > templates, why isn't it required to declare what one expects of the
> > template's parameters? Let's create a template function:
> >
> > template<class T> bool equal( const T& t1, const T& t2 )
> > {
> >   return t1==t2;
> > }
> >
> > Now, personally I would prefer to write something like:
> >
> > template<class T { bool operator==( const T& t ) const; }> ...
> >[...]
> 
> I can't answer whether this has been considered (or rejected),

Yes, it has been. I forgot that D&E contains a ( rather short ) discussion on 
this topic. Probably I should have written: "Has there been an attempt to 
define this", a question still unanswered.

> but I can see a problem with your proposal.  What if a template
> required ten, fifteen, twenty mfs in its parameterizing type?
> What if those mfs have many parameters?  The resulting list of
> mfs and parameters would become quite ungainly.

Actually, when I define a template I usually describe what it expects from 
its parameters in a comment. This means I have to write this list anyway. Do 
other programmers do it?

 
> Perhaps you could declare a class with the exact interface you
> expect and provide that as a model for the parameterizing type?
> This still leaves issues like equivalence.  Are implicit
> conversion allowed in mfs of a parameterizing type in order to
> match the model?  At what point do you decide there is no match?

I'm currently trying to define how this can be handled. In general, I think 
the following would do: a function matches a function required by the 
template if
a) its return type can be implicitly converted to the required return type 
and
b) the types of its parameters can be implicitly converted to the types of 
the correspondenting required parameters.
I wonder if someone has already tried this and what the results are.

> 
> What about the template code?  What if it you misuse the
> parameterizing type relative to your model?  Is the model wrong
> or is your code wrong?
> 

I'm not sure what you mean here.

> There's probably a lot more I haven't thought of yet. 
Yes, definitely. However, there's probably a lot more that hasn't been 
thought of ( or that is being thought of a this very moment ;-) about 
templates as well.

Bye

Roman


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