[Lazarus] Generics type as parameter

[Sven Barth]

On 13.05.2012 17:10, Leonardo M. Ramé wrote:
> 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?.

Yes, though currently a TFPGList<LongInt> declared in different units 
won't be compatible to each other (it's planned to change this).

>
> 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?.
>

No, this won't be possible. As I said the class name that is used is 
based on the "specialize ..." part. The type name on the left is only an 
alias to that specialization type.

Regards,
Sven