Bounded Type Parameters?

[blondMammuth]

I want to do something I have done often in Java:

I want to define a generic class C with a type parameter <T> such that T implements a given Interface I. Or in order to be more general,  the implementation of C should take use of the fact that instances of  T offer a given function f.

How can I do that? My idea would have been:

type

I = interface
  function f: Boolean;
end;

generic C <T: I> = class
  function g(t:T):Boolean;
end;

implementation

function C.g(t:T);
begin
  g:=t.f;
end;

According to the language reference, this is syntactically not possible. Is there any other way to achieve this kind of genericity? Can I write generic code for a restricted variety of type parameters?

Thanks in advance!

[User137]

Code: [Select]

type
  TBooleanProc = function(): boolean of object;
  // Most likely that you want "of object" if your boolean method is within a class
 
  C = class
    function g(t: TBooleanProc): boolean;
  end;

  D = class(C)
    function boolTest(): boolean;
    procedure Run();
  end;

implementation

function C.g(t: TBooleanProc): boolean;
begin
  result:=t();
end;

function D.boolTest(): boolean;
begin
  result:=true;
end;

procedure D.Run();
var b: boolean;
begin
  b:=g(@boolTest);
end;
Not sure if it's exactly what you needed but should get it started.

[rausm]

It is also called Constrained Generics.
googling for freepascal generic constraints yielded http://lists.freepascal.org/fpc-announce/2012-December/000586.html  (Feature announcement: Generic type constraints) on the first place.

[Leledumbo]

Generics contraints have been implemented in FPC trunk.

[blondMammuth]

Thanks, great answers!

[blondMammuth]

Especially thanks for the link.

Yet another thought: I read about future ideas for type constraints like "object" or "array" and "enum" and so on, and that they might be introduced in future. My suggestion to that: why not provide special predefined interfaces for them? That would eliminate the need for new Syntax.

This may not be the right place for this kind of suggestions. Is there a better place to discuss these issues?

[Mike.Cornflake]

This may not be the right place for this kind of suggestions. Is there a better place to discuss these issues?

Sounds like you're after the fpc mail lists
http://lists.freepascal.org/mailman/listinfo

[blondMammuth]

Yes, thanks!

[Mike.Cornflake]

If you want to browse past archives, you can use
http://lists.freepascal.org/pipermail/fpc-pascal/

March 2025 was an interesting inclusion :-)

[blondMammuth]