Multiple generation of Obj with a generic class

Question

I'm having some trouble with generics and constructors.

I would like to have a Generic class that can handle (and create) multiple objects of the same class. Moreover, I have some code that I would like to use whatever the specific class actually is.

I thought Generics are a good solution to this, but I'm not quite sure.

type
  TMultiBlock<T: IBlock, constructor> = class(TObject)
    blocks: array of array of T;
    constructor Create(const owner: someClass, const n: integer);
  end;

constructor TMultiBlock<T>.Create(const owner: someClass, const n: integer);
var
  i: integer;
begin
  for i := 0 to n-1 do
    T.Create();
end;

The solution above works, but the Create() that is called is not the one of the class T that I give to the Generic.

I know that I can do the Create outside the TMultiBlock class, since I know class T there, as show below:

TM := TMultiBlock<TFinestra>.Create();
for i := 0 to n do
begin
  TM.blocks[i] := TFinestra.Create();
end;

Here the class TFinestre is one of the class that I want to use in the Generic. But the thing is that I want to do some common operations on the T element, and these operation will be common to whatever the T type is, so I would like to do them on the TMultiBlock.

IBlock is an interface implemented by each class of type T.

Hi, We need more information if you want meaningful answers. The parameter of your generic is constrained to `IBlock` but you haven't given us any definition of what `IBlock` is. Normal conventions would indicate that it's in Interface, but if it is this is a very unusual way to use it. Your code is invalid (the declaration of `TMultiBLOCK` has no `end`. Please clarify and help us to help you. — Rob Lambden, Jan 31 '23 at 11:50
you are rigth, i made some typo trying to explain the problem. the things is i'm not sure i'm able to explain the situation in a better way. I edit it, maybe now is more clear — matteofolli, Jan 31 '23 at 12:50
Did you check to the side of Spring4D ? or a service locator ? https://stackoverflow.com/questions/63885398/delphi-service-locator — Bosshoss, Jan 31 '23 at 13:06
Have you heard of the design patterns abstract factory or factory method? Not sure you need Generics here, maybe plain old polymorphism is enough!? — Delphi Coder, Jan 31 '23 at 13:54

Rob Lambden · Answer 1 · 2023-02-03T08:29:06.887

An Interface is, in OOP terms, a pure abstract definition. It can be implemented by lots of different classes. You can't create objects from an interface (unless the interface gives you a method to create an object), you can get an interface from an Object if that object supports it. As all interfaces are implemented by a class then they can do different things, but ultimately what they do is dependent on what the object is, so you are back to having to know the class.

If you want to create the objects you have to know the class of the object you are creating. The interface could provide a method for returning the class of the object which is implementing the interface, allowing you to create more of them, but then you may as well use that class.

If your different types do not have a common ancestor, then you can specify, as you have, that T must support an interface, but you still instantiate the TMutliBLOCK<T> with a class. As interfaces are always implemented by a class then T will always derive from TObject so it will always support Create. The problem is you can't call T.Create unless IBlock includes the definition of Create ...

This means that you can have a TMulitBLOCK and a TMultiBLOCK but each would be holding the objects you have declared for it. If they're not inheriting from a common class then you would not be able to restrict the type of T to that common ancestor.

You can check that the type you are using supports an interface in the constructor, and then restrict to TObject.

TMultiBLOCK<T: class> = class(TObject)
protected
  blocks:  TArray<T>;

public
  constructor Create(AOwner: pSomeObject; nSize: Integer);
end;

constructor TMultiBLOCK<T>.Create(AOwner: pSomeObject; nSize: Integer);
begin
  if(not Supports(T, IBlock)) then
    raise EInvalidCast.Create('Class '+T.ClassName+' must support IBlock')
  else
    ...
end;

To call the members of IBlock you will need to get an interface pointer for each object. Bear in mind that depending on the implementation in the different implementing classes then when the interface references go out of scope the object may delete itself. To prevent that happening you probably want to store the interface references alongside the objects when you create them so the reference count is held above 0.

If you can organise the code so that all members are derived from a common ancestor then you can restrict your TMultiBLOCK<T> to that common ancestor, rather than a common interface.

Multiple generation of Obj with a generic class

1 Answers1