> As you can (probably) see, I would like t_barnyard to be able to create
> any descendant of t_mammal. In the above example, I want it to create a
> brown pig if it doesn't find one, so I tried to pass the t_pig
> constructor, along with the brown parameter that said constructor will
> require. But I can't figure out the syntax. Perhaps this kind of thing
> is not supported?
Constructor procvars are indeed not supported but the way to achieve
what you want is to use class type variables (untested, but it should
illustrate the idea):
{$mode objfpc}
uses classes, sysutils;
type
t_mammal = class
public
constructor create (color : byte); virtual;
end;
t_pig = class (t_mammal)
// some fields, methods
end;
// the following fails:
t_mammal_class = class of t_mammal;
// a list of animals...
t_barnyard = class
public
function find_or_create_animal (color : byte;
mammal_creator : t_mammal_creator) : t_mammal;
end;
constructor t_mammal.create (color : byte);
begin
end;
function t_barnyard.find_or_create_animal (color : byte;
mammal_creator: t_mammal_creator) : t_mammal;
begin
// look for the animal within the list. if not found:
result := mammal_creator.create(color);
end;
const
brown = 18;
var
pig_pen : t_barnyard;
brown_pig : t_pig;
begin
pig_pen := t_barnyard.create;
brown_pig := t_pig (pig_pen.find_or_create_animal
(brown, t_pig));
end.
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