When is it safe to Dispose a nil pointer?

January 23, 2023

In the following example:

program DisposeProblem;

{$APPTYPE CONSOLE}

{$R *.res}

uses
  System.SysUtils;

type
  Person = record
    name: string;
    age: Integer;
  end;

var
  p: ^Person;

begin
  p := nil;
  Dispose(nil); // OK
  Dispose(p); // AV
end.

Why is the first Dispose() call OK, while the second causes an access violation error? When I step through the code, the first Dispose() calls System._FreeMem(), while the second calls System._Dispose(), but I don’t understand why this is the case. I would like to understand this behavior so I can know when it is safe to call Dispose() on a nil pointer.

>Solution :

I would like to understand this behavior so I can know when it is safe to call Dispose() on a nil pointer.

It is never OK to call Dispose() on a nil pointer variable. The RTL will unconditionally try to finalize the object that is being pointed at. Passing in a pointer variable that is nil leads to undefined behavior and will likely crash.

FreeMem() allows a nil pointer as input. Dispose() on a pointer variable does not allow nil, and never has.