[fpc-pascal] How to inline CompareFunc to Sort method in generic abstract class

[Sven Barth]

Am 14.11.2022 um 19:26 schrieb Vojtěch Čihák via fpc-pascal:
> Hi,
>   
> I wrote a generic abstract class - a list based on dynamic array (i.e. array of T;) and this class can be specialized elsewhere with any type (records or classes).
> Part of the class is sorting. There are more ways how to deliver *compare function* to sorting method. I can pass it as a parameter or I can define it as: function Compare(A, B: T): Integer; virtual; abstract;. But this way the function cannot be inlined.
>   
> Question: Is there a way how to *inline* compare function to sorting method in this general purpose generic abstract class?

Currently there is not. But once FPC supports reusing types in other 
specializations inside the generic parameter list you /should/ be able 
to do the following:

=== code begin ===

program tinline;

{$mode objfpc}

type
   generic TComparer<T> = class
     class function Compare(aLeft, aRight: T): Integer;
   end;

   // this kind of constraint that uses T does not work yet
   generic TList<T; Comparer: specialize TComparer<T>> = class
     procedure Sort;
   end;

   TLongIntComparer = class(specialize TComparer<LongInt>)
     class function Compare(aLeft, aRight: T): Integer; inline;
   end;

class function TComparer.Compare(aLeft, aRight: T): Integer;
begin
   Result := 0;
end;

procedure TList.Sort;
begin
   { sort algorithm that calls Comparer.Sort which /should/ be inlined }
end;

class function TLongInt.Compare(aLeft, aRight: T): Integer;
begin
   if aLeft < aRight then
     Result := -1
   else if aLeft > aRight then
     Result := 1
   else
     Result := 0;
end;

var
   l: specialize TList<LongInt>;
begin
   l : =specialize TList<LongInt, TLongIntComparer>.Create;
   l.Sort;
end.

=== code end ===

I can't test this currently whether it would really work due to the 
missing functionality, but it's at least possible.

Regards,
Sven