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Asked: 2026-05-13T11:50:25+00:00

I need a way to write a generic procedure to act upon an object

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I need a way to write a generic procedure to act upon an object type or any of its descendants.

My first attempt was to declare

procedure TotalDestroy(var obj:TMyObject);

but when using it with a descendant object

type TMyNewerObject = class(TMyObject);
var someNewerObject: TMyNewerObject;

TotalDestroy(someNewerObject);

I get the infamous error “types of formal and actual parameters must be identical”

So, while strugling to find a solution, I looked at the source code of Delphi system FreeAndNil procedure. And I found this awesome declaration, along with this astonishing comment

{ FreeAndNil frees the given TObject instance and 
  sets the variable reference to nil.  
  Be careful to only pass TObjects to this routine. }

procedure FreeAndNil(var Obj);

It avoids the type checking error, but it uses no safety net.

My question is … is there any safe way to check the type of an untyped var parameter?

or in other words, can you improve this Delphi source code so that the warning would not be needed? 

procedure FreeAndNil(var Obj);
var
  Temp: TObject;
begin
  Temp := TObject(Obj);
  Pointer(Obj) := nil;
  Temp.Free;
end;
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1 Answer

  1. Editorial Team

    I’ve written about this before, using an example very similar to Lasse’s:

    Unless you’re writing an assignment statement to change the value of the input parameter itself, and not just one of its properties, you shouldn’t pass a parameter by reference in the first place.

    If you are writing an assignment statement to change the parameter’s value, then the compiler message really is true, and you should heed it.

    One reason for needing to by-pass the error is when you’re writing a function like TApplication.CreateForm. Its job is to change the input parameter’s value, and the type of the new value varies and cannot be determined at compile time. If you’re writing such a function, then your only option with Delphi is to use an untyped var parameter, and then there is extra burden on both the caller and the receiver to make sure everything goes right. The caller needs to make sure it passes a variable that is capable of holding values of whatever type the function will put in it, and the function needs to make sure it stores a value of a type compatible with what the caller requested.

    In the case of CreateForm, the caller passes in a class-reference literal and a variable of that class type. The function instantiates the class and stores the reference in the variable.

    I don’t think very highly of either CreateForm or FreeAndNil, largely because of the way their untyped parameters sacrifice type safety in return for comparatively little extra convenience. You haven’t shown the implementation of your TotalDestroy function, but I suspect its var parameter will ultimately provide the same low utility as in those other two functions. See my articles on both:

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