How do I implement a generic class that has an constraint for a generic interface?

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?

What about using a semicolon instead of a comma: TMyClass<D; T: IMyInterface<D>>, analogue to parameters of a function? — Rudy Velthuis
– Rudy Velthuis, Commented Feb 17, 2018 at 16:13
That makes no difference. Syntax at this point still okay, but same compiler error in the declaration of the variable. — Art1st
– Art1st, Commented Feb 17, 2018 at 16:16
oh yes, it most definitely makes a difference. See my answer. — Rudy Velthuis
– Rudy Velthuis, Commented Feb 17, 2018 at 16:24

Rudy Velthuis · Accepted Answer · 2018-02-17 18:39:56Z

4

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, i.e. 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> then D and T are separate types, i.e. 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.

edited Feb 17, 2018 at 18:39

answered Feb 17, 2018 at 16:24

Rudy Velthuis

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1 Comment

Art1st Over a year ago

Yep, sorry it actually works, tried it on wrong class... set to accepted answer :)

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