[Lazarus] Generics type as parameter

[Leonardo M. Ramé]

On 2012-05-13 13:38:36 +0200, Sven Barth wrote:
> On 13.05.2012 12:46, Leonardo M. Ramé wrote:
> >On 2012-05-13 10:53:42 +0200, Sven Barth wrote:
> >>On 13.05.2012 02:49, Leonardo M. Ramé wrote:
> >>>On 2012-05-12 13:07:18 -0300, Leonardo M. Ramé wrote:
> >>>>Thanks Sven and Michalis. Using the suggestion by Michalis, I got a
> >>>>solution mixing Generics and regular objects.
> >>>>
> >>>>What I'm pursuing is to replace my TCollection based ORM, with a
> >>>>Generics based one, this allow users to write less code, in a clearer
> >>>>way.
> >>>>
> >>>>The way the ORM works, is to create a TCollection instance, then execute
> >>>>an ORM's method to load/save data into/from it, example:
> >>>>
> >>>>var
> >>>>   lCustomers: TCustomers; // this is a TCollection descendant
> >>>>begin
> >>>>   lCustomers := TCustomers.Create;
> >>>>   FConnector.LoadData(lCustomers, []);
> >>>>   ... do something with lCustomers ...
> >>>>   lCustomers.Free;
> >>>>end;
> >>>>
> >>>>As you can see, FConnector.LoadData receives a TCollection as parameter,
> >>>>and using RTTI, it fills each TCustomer published property.
> >>>>
> >>>>With Michalis's solution, I can turn my ORM from TCollections to
> >>>>Generics with very little changes, see how small is the unit customer
> >>>>now, instead of a complete TCollection/TCollectionItem definition:
> >>>>
> >>>
> >>>Following the same subject, inside a method receiving an TFPSList, how
> >>>can I know the type of items it will contain, even if the list is
> >>>empty?.
> >>>
> >>>In a TCollection, I can use myCollection.ItemClass to know it, but in a
> >>>specialized Generic type, how can I know, in an abstract way the type of
> >>>a TFPGList<T>?. I mean, what type is T?.
> >>
> >>You can't. That's why the method with the "parent class" might be considered
> >>less powerful than the one with generic methods/procedures.
> >>
> >>Regards,
> >>Sven
> >>
> >
> >Hm. Another thing I'm stuck on, is specialized classes doesn't seem to
> >have a ClassName, am I righ?.
> >
> 
> Specialized classes are just normal classes. They DO have a class name, but
> it is not the one that you might expect.
> 
> Following example:
> 
> === example begin ===
> 
> type
>   TTest<T> = class
> 
>   end;
> 
>   TTest1 = specialize TTest<LongInt>;
>   TTest2 = specialize TTest<LongInt>;
> 
> begin
>   with TTest1.Create do
>     try
>       Writeln(ClassName);
>     finally
>       Free;
>     end;
>   with TTest2.Create do
>     try
>       Writeln(ClassName);
>     finally
>       Free;
>     end;
> end.
> 
> === example end ===
> 
> In both cases the same name "TTest$1$crc31B95292" will be written which is
> composed out of the name of the generic (which is "TTest$1", where the "$1"
> stands for "has one generic parameter") and the crc of the given type
> parameters. This name is an implementation detail though, so DO NOT RELY ON
> IT, because I WILL change it sooner or later (to be more like
> "TTest<System.LongInt>").
> 
> The fact that the name is in both cases the same is because the generic is
> only specialized once per unit, so that code like the following can find the
> correct specialization (this works in mode Delphi):
> 
> === example begin ===
> 
> var
>   t: TTest<LongInt>;
> begin
>   t := TTest<LongInt>.Create;
>   try
>     t.Value := TTest<LongInt>.SomeConstant;
>   finally
>     t.Free;
>   end;
> end.
> 
> === example end ===
> 
> Regards,
> Sven
> 

Ok, so, what happens if I declare the same specialization in different
units, then pass instances of each specialization to the same method,
its ClassName should be the same, no?.

for example:

----
unit1;
...
type
  TCustomers = specialize TFPGList<TCustomer>;
-----
----
unit2;
...
type
  TCustomers_from_branch1 = specialize TFPGList<TCustomer>;
-----

What I would like, is, when I ask for the instance's classname, it write
TCustomers and TCustomers_from_branch1, instead of TFPGList$1$....

Is this planned?.

-- 
Leonardo M. Ramé
http://leonardorame.blogspot.com