On 19.12.2010 08:34, Honza wrote:
2010/12/19 David Emerson<dle...@angelbase.com>:
type
generic gt_box<_t_point,_num> = class (_t_point) // FAILS :-(
f_width, f_height : _num;
end;
I think it should fail according to the docs, see:
http://www.freepascal.org/docs-html/ref/refse42.html
"There is a single placeholder _T. It will be substituted by a type
identifier when the generic class is specialized. The identifier _T
*may not be used for anything else than a placehoder*. "
According to the documentation I'd say that it should succeed, because
in "class(BaseClass)" "BaseClass" is a type identifier. But of course
that has its own set of problems... see the second last paragraph of
this mail.
The bold part is IMO violated by the declaration. Anyway, it could be
perhaps (not tested) written as:
type
TBoxProxy = class(_t_point);
generic gt_box<_t_point, _num> = class(TBoxProxy)
f_width, f_height : _num;
end;
No, this won't work, because "_t_point" won't be defined when
"TBoxProxy" is parsed.
Another strange point is, that the declaration of gt_box doesn't use
the formal specialization paramater `_t_point` at all (in the posted
code), so the same the other way around should also work:
It IS used, because David wants to influence the class the generic class
gt_box inherits from when specializing the class.
E.g.:
type
TIntPoint = class
x, y: Integer;
end;
TFloatPoint = class
x, y: Single;
end;
generic gt_box<_t_point, _num> = class(_t_point)
width, height: _num;
end;
TFloatBox = specialize gt_box<TFloatPoint, Single>;
TIntBox = specialize gt_box<TIntPoint, Integer>;
type
generic gt_box<_num> = class<_t_point>
f_width, f_height : _num;
end;
This won't compile because of the "<...>" around "_t_point". Also it's
not what Daniel intends.
A 3rd note is that your code can't compile as _t_point is not declared
when gt_box is declared, but the declaration wants to inherit from
_t_point, so IMO also for this the code is rightfully rejected the
compiler.
The question is whether this should be rejected if "_t_point" is a
template parameter... on the other hand this would violate compile time
checks of the generic class...
I'm still thinking how David's idea could be achieved in another way
which is supported by the compiler...
Regards,
Sven
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