On Mon, 2 Feb 2026, Martin Frb via fpc-devel wrote:
On 02/02/2026 20:48, Michael Van Canneyt via fpc-devel wrote:
Are you deliberately trying to shoot yourself in the foot here ?
(better your foot than my foot obviously...)
Well, the simplest would be
generic TGen<DATA>
no constraint, I can pass in whatever, and if it has the methods, then
good ... (or well...)
But, that feels more like risking my foot. I quite prefer if I can give
a constraint, and if the compiler will know earlier what the generic is
about.
I have a list. Well several lists. They share some functionality.
But I can't just pack it into a base class.
For once, at least in some parts of the code, the list will only be
known as "interface".
The lists have different item classes too. So now I thought I write a
generic that implements the shared bit.
Actually the shared bit is in another class, that accepts the list as
argument. A worker class, if you will.
generic SharedBit<_BASE, _LIST, _ITEM> = class(_BASE)
procedure ProcessItem(Itm: _ITEM); virtual; // called by DoFoo
procedure DoFoo(List: _LIST);
end;
procedure DoFoo(List: _LIST);
begin
ProcessItem(List.Entries[i]); // Entries must return the specific
interface
end
ProcessItem must have the subclass, or at least the more specific interface.
And yes, I could add to the interface
function GetObject: TObject
and then type cast that.
That feels like a bandaid.
Since I do have some code that has the list as interface, the simplest
is that all code that does SharedBit uses the interfaces.
(Well, yes, leave away the constraint, and I can specialize for either
class or interface)
I assume this should be:
property Bar: TBar read GetBar;
yes
property Bar: TSubBar read GetSubBar;
property overrides exist, but they must keep the same type, they can only
change the specifiers. So I don't think this will compile. If it does, it
is probably by accident.
It does actually compile.
And on classes, properties can be reintroduces with any changes possible
(wouldn't necessarily do but works)
Yes, I forgot, Delphi allows this. It's a redeclaration, not an override.
(I always thought that was a bad idea)
Well, you have your answer. It compiles and probably should compile,
at least in mode delphi.
Whether it is a good approach, there we probably differ in opinion :-)
Michael.
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