8.6 Type compatibility

Whenever a generic class is specialized, this results in a new, distinct type. These types are assignment compatible if the same template types are used.

Take the following generic definition:

{$mode objfpc}  
unit ua;  
 
interface  
 
type  
  Generic TMyClass<T> = Class(TObject)  
    Procedure DoSomething(A : T; B : INteger);  
  end;  
 
Implementation  
 
Procedure TMyClass.DoSomething(A : T; B : Integer);  
 
begin  
  // Some code.  
end;  
 
end.

And the following specializations:

{$mode objfpc}  
unit ub;  
 
interface  
 
uses ua;  
 
Type  
  TB = Specialize TMyClass<string>;  
 
implementation  
 
end.

the following specializations is identical, but appears in a different unit:

{$mode objfpc}  
unit uc;  
 
interface  
 
uses ua;  
 
Type  
  TB = Specialize TMyClass<string>;  
 
implementation  
 
end.

The following will then compile:

{$mode objfpc}  
unit ud;  
 
interface  
 
uses ua,ub,uc;  
 
Var  
  B : ub.TB;  
  C : uc.TB;  
 
implementation  
 
begin  
  B:=C;  
end.

The types ub.TB and uc.TB are assignment compatible. It does not matter that the types are defined in different units. They could be defined in the same unit as well:

{$mode objfpc}  
unit ue;  
 
interface  
 
uses ua;  
 
Type  
  TB = Specialize TMyClass<string>;  
  TC = Specialize TMyClass<string>;  
 
 
Var  
  B : TB;  
  C : TC;  
 
implementation  
 
begin  
  B:=C;  
end.

Each specialization of a generic class with the same types as parameters is a new, distinct type, but these types are assignment compatible if the template types used to specialize them are equal.

If the specialization is with a different template type, the types are still distinct, but no longer assignment compatible. i. e. the following will not compile:

{$mode objfpc}  
unit uf;  
 
interface  
 
uses ua;  
 
Type  
  TB = Specialize TMyClass<string>;  
  TC = Specialize TMyClass<integer>;  
 
 
Var  
  B : TB;  
  C : TC;  
 
implementation  
 
begin  
  B:=C;  
end.

When compiling, an error will result:

Error: Incompatible types: got "TMyClass<System.LongInt>"  
                           expected "TMyClass<System.ShortString>"