Al Boldi wrote:
Bram Kuijvenhoven wrote:
function TAncestor.ToStringSafe:string;
begin
if self=nil then Exit('<nil>');
Result:=ToString; // now it's safe to call the virtual method
end;
Are you sure it's really safe to call?
I think so: function ToStringSafe:string; is not virtual. You can check it
yourself by running the code.
BTW, is there a way to make Assigned(obj) really check for a valid pointer?
Not really.
- When destroying an object, the pointer is invalid, but still points to a memory area with about the same data as before; how would you see the difference
- You could try and find a way to ask the memory manager whether there is a
valid block at pointer^
- You could register each object in the constructor and unregister it in the
destructor and then look up 'pointer' in the register (e.g. a hash table or avl
tree) each time, but that would
a) be inefficient, and
b) come down in fact to an inefficient partial implementation of a garbage
collector mechanism
Instead, you should just make sure the method is always called on a valid
object pointer be the design of you software. If you can't do that, don't use
Pascal or C.
Note that a try .. catch .. end block can be of help. When executing a piece of
code that is not critical (like a GUI callback) you can use it to catch errors
(and handle them!), such that your application does not simply terminate on
such errors. But don't use empty except on ... handlers! That is really bad
design.
Bram
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