Re: STL, Ada, C++ (Was Re: The Next Microsoft?)

[Brian Rogoff <bpr@shell5.ba.best.com>]

On 8 May 2000, Hyman Rosen wrote:
> Robert Dewar <robert_dewar@my-deja.com> writes:
> > I am not at all convinced that automatic instantiation is
> > a help to the reader and maintainer, on the contrary, it
> > seems very awkward to have uses of templates not be visible
> > from this point of view.
> 
> I consider automatic instantiation of templates to be the compile-time
> polymorphism equivalent of run-time dispatching on class-wide types.

Correct, and type theorists refer to this as "parametric polymorphism". 
Ada doesn't really have parametric polymorphism, it's generics are really 
monomorphic. 

> When you make a dispatching call, you do not know what function you
> are calling. Similarly, you should be able to call a generic function
> without explicitly stating which one you are calling.

This is one good way to look at it, since dispatching is sometimes a 
negative with respect to readability and maintainability. It is also 
similar to overloading; if you have several overloaded functions in scope 
you don't explicitly say which you are calling in Ada or C++. In languages 
without overloading you have to use qualified names. 

As much as I like Ada, this is one thing I really prefer about the C++
design. This discussion might be more interesting with specific examples,
so I'll put some together in an Ada-like language with automatic
instantiation. Shen and Cormack's paper from Tri-Ada 91 is a good place to 
start for anyone interested.

-- Brian