Generic primitives, generic records.

[Wodzu]

Hi, is it possible to create something like this:

Code: [Select]

type
  T3DPosition = record
    X: _T;  //instead of T any primitive type f.e. Single, Double, Integer...
    Y: _T;
    Z: _T;
  end;

var
  Pos1, Pos2: T3DPosition;

begin
  Pos1.X := Pos1.X + Pos2.X; //X is any single primitive
end;
Do you catch my drift?

[azagaros]

My suggestion is learn Object Orientated programming.  You can do this with OOP tricks of Free Pascal.  I don't remember the exact code.. I can do this in C++ easily, another OOP language, even with overloaded operators and Run time information.

[Wodzu]

This is not an answer for my question.

But thanks anyway, I've never laugh so much reading "technical" post. 
Especially the part with C++ (totaly irrevelant  ) and RTTI (even if FPC would publish it for records(!) that would be so ineffective).

So in other words, everything is easy but you don't remember the exact code

Are you a forum troll or something?

[Ñuño_Martínez]

Wodzu: I don't understand. Your code is a valid record and will compile without problems (if _T was previously defined ) so, what are you actually asking for?

[Batuhan]

use 'variant' instead of _T

http://www.freepascal.org/docs-html/ref/refsu18.html#x46-520003.7.1

[Leledumbo]

Perhaps he'd like to treat _T as a template. You can use generics for this, ex:

Code: [Select]

type
  generic T3DPosition<_T> = class
    X: _T;  //instead of T any primitive type f.e. Single, Double, Integer...
    Y: _T;
    Z: _T;
  end;
 
  TInteger3DPos = specialize T3DPosition<Integer>;

var
  Pos1, Pos2: TInteger3DPos;

begin
  Pos1:=TInteger3DPos.Create;
  Pos2:=TInteger3DPos.Create;
  ...
  Pos1.X := Pos1.X + Pos2.X; //X is any single primitive
  ...
end;
Currently generics have problem when the + operator is not overloaded before the generic class definition. So, if you're going to use non-primitive types declare the operator definition before the generic class.

[Wodzu]

Thank you guys for your answers:

@Ñuño_Martínez : I want to create a template for X,Y,Z and I want to store it in record. The template should be able to "support" any simple type: Integer, Double, Single, Byte and so on.

@Batuhan : Thanks but it will be to slow. I need to perform large number of calculations during runtime.

@Leledumbo : Close, but I don't want to use a class. It is to much overhead to keep tree simple variables.

Records are yet not supported? 

[Ñuño_Martínez]

Records are yet not supported? 

  You're kidding, aren't you?

Records are supported. They're a basic feature of Pascal language.

(...) Your code is a valid record and will compile without problems (if _T was previously defined ) (...)

As Leledumbo said it seems you're asking for generic classes* or may be interfaces.

(* Actually I think "generic classes" are a bad programming practice and shows poor design capabilities. Actually that's one reason I hate C++/C#/Java so much.)

[Wodzu]

Ñuño_Martínez sorry, I meant generic records like in my topic title.
I've also said that I don't want to use a generic classes. I want generic records

I do not think that they are bad programming pracitce and show poor design capabilities.

I will give you one example, and you tell me what is wrong with this example.

Code: [Select]

function AddNumbers(Number1, Number2: Integer):Double; overload;
function AddNumbers(Number1, Number2: Single): Doube; overload;
function AddNumbers(Number1, Number2: Double): Double; overload;
To add two numbers you need to have 3 overloaded methods.

But with generics you could do it like this:

Code: [Select]

function AddNumbers(Number1, Number2: _T): _T; overload;

(Please note that this is only an example in pseudocode. I don't know how to define generics in Pascal yet)

What is wrong with that?

[Ñuño_Martínez]

Ñuño_Martínez sorry, I meant generic records like in my topic title.

Ok then not, "generic records" aren't supported AFAIK.

What is wrong with that?

Hard to explain in plain English (I'm European ).

IMO "generics" opens a door to "anything". It's powerful but it's a common source of problems too. I have the same problem with operator overload. When I used C++ I had a lot of problems with all that stuff and when I read that FPC planned to support them I thought "Oh, no! Here we go again!".

I remember a nightmare-project (the last I used C++) where in some cases returned bad results. There were no errors in compilation nor in execution. Just a value changed with apparently no reason. We needed almost two days to discover that the problem was a combination of operator overloading and templates (as C++ calls "generics") in the deepest of the STL library. The compiler did a bad conversion somewhere before to call a bad assignation operator (overloaded) and finished with the creation of a bad implementation of a container.  We had to rewrite almost the whole thing including several of the STL classes and I (and a coworker) promised myself to never use templates nor operator overloading again in my life. Of course this wasn't the first time we have a similar problem but before this we were lucky and can fix it easily.

If we were do it in Delphi, we would use interfaces which solves the same problems in a more elegant way IMO. Of course nobody forces me to use "generics" at the moment and I can't forbid you to use them but I think you really don't need them at all.

[Wodzu]

I am sorry to hear that. But I think that in your case it was not error caused by compiler but by wrong usage of a tool (templates, operator overloading). Every tool might be used in a wrong way but that doesn't mean that the tool itself is wrong

For example. I can take a knife and cut the loaf of bread but accidently I can cut my finger too Does it mean that the knife is bad?

Anyway, I have shown you an example in which I think that generics might be useful. I would like to ask you to show me in my particular (taken from life) example what is bad in using genrics here

[raistware]

I can, it generates a very difficult to debug code. I also hate templates, generics or whatever you want to name it.

Pascal is a heavy typed programming language, and IMO think that generics is not a good thing to use. For simple types can work good, but abuse of templates or generics converts code to something unmantainable to me.

But it is just my opinion.

[raistware]

Also, you should ask this on FPC forums / lists, not in Lazarus one ;-)

[eny]

Pascal is a heavy typed programming language, and IMO think that generics is not a good thing to use

Generics are a very powerful mechanism to make programs even more typesafe!

[Wodzu]

For simple types can work good, but abuse of templates or generics converts code to something unmantainable to me.

So again we are talking about the wrong usage of a tool And I was talking about simple types...

Also, you should ask this on FPC forums / lists, not in Lazarus one ;-)

Why not? This is Forum->Programming->General so we are talking here about programming in Pascal or am I wrong?

Unfortunaltey FPC mailing list is very hard to read :|