How do i declare a function which takes a variable of a yet unidentified type as parameter

Question

the question is as the title reads. O get the error: Type identifier expected, which (i think) comes from the function declaration being read before the type declaration.

type
  TForm1 = class(TForm)
    //other declarations
    function ItemThere(x: TItem): boolean; // here it says TItem is unknown
  end;

type
  TItem = class
    Name : String;
    Description : String;
    constructor Create;
  end;

Also i would like let you know that i am a rather unexperienced programmer. How can i fix this? Should i just move the declaration of TItem above TForm1? Thank you for your help.

Answer 1

Delphi's compiler needs to know about the type before it's used. There are two ways to accomplish that with your code.

1. Move the declaration above the place it's first used:

   type
     TItem = class
       Name : String;
       Description : String;
       constructor Create;
     end;
   
     TForm1 = class(TForm)
       //other declarations
       function ItemThere(x: TItem): boolean; 
     end;
   

2. Use what is known as a forward declaration, which basically just tells the compiler that you're using a class which you'll define later (within the same type declaration section):

   type
     TItem = class;              // forward declaration
   
     TForm1 = class(TForm)
       //other declarations
       function ItemThere(x: TItem): boolean; 
     end;
   
     TItem = class               // Now define the class itself
       Name : String;
       Description : String;
       constructor Create;
     end;