On 19.12.2010 13:03, Honza wrote:
2010/12/19 Sven Barth<pascaldra...@googlemail.com>:
"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 type parameter in the inheritance part clause, i.e. class(T) part
of the generic class declaration has nothing to do with the
placeholder formal parameter. The placeholder formal arg list are
given inside the sharp brackets<> as a kind of macro parameters. And
the docs explicitly states that a placeholder identifier may not be
used elsewhere in the generic declaration except for identifying the
place where the "macro" expansion should substitute the
instantiation/specialization parameter. So using it in the inheritance
clause is invalid. IMO the docs are clear on this and my experiments
seems to confirm this behaviour. I have excersised generics a lot to
get heLib compiled and working. Still the latest changes in the
compiler broke the published code as I realized very recently and not
yet uploaded the remedy which sits on my local disk.
While I DO agree with you (after some thinking about the consequences)
that a base class should not be allowed to be specified by a template
parameter (and this is the way it already is), I don't agree with you
that the documentation states this as clearly as you propose it.
The documentation says that every occurrence of a template parameter
will be replaced by a type identifier (e.g. Integer, String, TObject).
Now the documentation of a normal class declaration (
http://www.freepascal.org/docs-html/ref/refse31.html#x67-770006.1 )
states that the heritage clause contains a "class type identifier" which
is just a special case of a type identifier.
So when one reads the documentation of generics one CAN (!) come to the
(wrong) conclusion that you can also put a template parameter into the
heritage clause of a class.
Here the documentation of generics should state more clearly that
"class(T)" is not allowed, neither in the main class nor in sub classes.
I'm still thinking how David's idea could be achieved in another way which
is supported by the compiler...
I've not yet got time to look at his goal at all, so I don't know. I
just spotted the invalid constructs presented.
@David: Maybe you can restructure your class hierarchy to something like
this (you'll need to be a bit creative here ^^):
generic gt_box<_num> = class
type
t_point = specialize gt_point<_num>;
var
f_position: t_point;
f_width, f_height : _num;
end;
or this
generic gt_box<_t_point, _num> = class
f_position: _t_point;
f_width, f_height: _num;
end;
I don't really see another possibility.
Regards,
Sven
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