Invoke method on generic type?

Question

Why does the following generate an error in Delphi (XE)?

unit UTest;

interface


type

TTest = class
  public
  procedure Foo<T>(A: T);
end;

implementation

{ TTest }

procedure TTest.Foo<T>(A: T);
begin
  A.Add('hej');
end;

end.

I thought that generic types in Delphi was simply inserted into the generic function, so it would only error out if used with a type which don't have an Add(string) method.

Answer 1

Your code produces a compilation error because the compiler has no way of knowing that T has a method named Add that receives a single string parameter.

I thought that generic types in Delphi was simply inserted into the generic function, so it would only error out if used with a type which don't have an Add(string) method.

If you were using Smalltalk or C++ templates, then your assumption would be accurate. However, generics are not the same as templates. For generics you need to apply a constraint to the type parameter. The constraint needs to tell the compiler what properties T must have.

For example, you could constrain T to be derived from a class that has a suitable Add method. Or you could constrain T to implement an interface with a suitable Add method.

Documentation link for Delphi generic constraints: http://docwiki.embarcadero.com/RADStudio/en/Constraints_in_Generics

The generic constraints that can be applied are rather limited, which is something of a shame. For example, I'd love to be able to constrain a type to have certain mathematical operators. For example, I'd like to be able to constrain a type to have + and - operators, say. However, there are pros and cons to both generics and templates, and so I do accept that these limitations are the result of a justifiable design decision by the Delphi language designers.