如何实现对通用接口有约束的通用类？

Question

I have an Interface

IMyInterface<T>

and a class

TMyClass<T>

and now I want that I can only pass classes to TMyClass as a type, that also implement IMyInterface.

First I tried

TMyClass<T:IMyInterface<T>>

but as expected, now the compiler wants me to give him a type which implements an interface with the type of class itself that implements the interface

Second try was

TMyClass<D,T:IMyInterface<D>> 

where I thought, D would then be the shared DataType, both TMyInterface and TMyClass would use.

So after declaring the class implementing the interface

TMyIntegerClass = class(TInterfacedObject,IMyInterface<Integer>)

The declaration

GMyClass:TMyClass<Integer,TMyIntegerClass>

failed with compiler error:

E2514 Type parameter 'D' must support interface 'IMyInterface<System.Integer>' 

Any pointers?

Answer 1

This compiles in Delphi 10.2.2 Tokyo:

type
  IMyInterface<T> = interface
  ['{F810B6BC-78F7-4026-BA83-70435150B758}']
  end;

  TMyClass<D; T: IMyInterface<D>> = class // note the semicolon!
  end;

  TMyIntegerClass = class(TInterfacedObject, IMyInterface<Integer>)
  end;

var
  GMyClass: TMyClass<Integer, TMyIntegerClass>;

If, in the declaration of the class , I use <D, T: TSomeType> (comma!) then both D and T are declared to be of the same type , like parameters of a function:

procedure Blah(D, T: TSomeType);

Parameters D and T are of the same type, ie TSomeType .

Now, if you pass an Integer for D , you get an error, similar to the one you got. The compiler expects two TSomeType parameters.

But if I use <D; T: TSomeType> <D; T: TSomeType> then D and T are separate types , ie D is of an unknown type and T is of type TSomeType . So now, D is not declared as TSomeType and there is no error if you pass Integer .

Oh, and this is documented too .