[fpc-devel] Generics Basics

[Christian Iversen]

On Tuesday 08 November 2005 21:33, L505 wrote:
> >Hello,
> >
> >I am trying to understand what exactly generics are. I read the wiki
> >page, but there are lot's of code examples and very few explanations.
> >Can someone explain it to me in a (relatively) simple way?
> >
> >What problem is it trying to solve?
> >
> >And how do generics relate to interfaces?
>
> I had the same problem for a few years. It took me a few months/years to
> first diagnose "what the fudge" templates/generics actually were, and what
> problem they exactly solved. All the information about generics is pretty
> non-real world and non-case study on the internet. It is also hyped by
> programmers who again seem to show no real world or case studies. But I can
> see how they can be useful in theory, nevertheless.
>
> I'm not going to give you a technical explanation on what they are, because
> someone else can do that (and I don't have experience with generics, so I'm
> not qualified to do so).
>
> Here is a vague non-technical explanation:
> Basically generics helps you save some Typing and syntax in your units. You
> can address several things at once with the same Type. Supposedly, this
> encourages code reuse. You can address and create things via a shortcut
> syntax sort of way.

The Very Big Advantage (Tm), is that you get syntax checking, while still 
using a type diversely. That's impossible to do (at compile-time) without 
generics.

Probably the best example of this is something like TList:

Without generics:

TOrange = class ... end;
TApple  = class ... end;

var
  T: TList;
begin
  T := TList.Create();
  T.Add(TApple.Create());
  T.Add(TOrange.Create());
  // Whoops, what was that? Now our program might crash.
end;

Of course, we could create a specialized TAppleList that only accepts Apples, 
but that's boring and hard work. A much cleaner way is generics:

var
  T: TList<TApple>;
begin
  T := TList<TApple>.Create();
  T.Add(TApple.Create());
  T.Add(TOrange.Create());
  // This wont compile! The problem is prevented at compile-time!
end;

I hope that answers your question as to why it's a good idea :-)

-- 
Regards,
Christian Iversen