Using Assigned() with a dynamic array.

[surjay]

A simple function example:

begin
if not Assigned(FArray[AIndex]) then
FArray[AIndex] := CreateItem(AIndex);

Result := FArray[AIndex];
end;


where FArray is a dynamic array of objects. It seems that Assigned() doesn't work well here. For example, when Length(FArray) = 1, Assigned(FArray[1]) = True, Assigned(FArray[198]) = True, and so on. Some indexes return true, some false.

I assume this just has to do with the nature of a dynamic array in the first place, so I'm wondering if there's a different way to use Assigned() in this case, or a different command. I know workarounds like checking something about that index's specific object, etc., but I'm mainly looking for a better understanding of Assigned() in this case and something directly related to it.

Thanks.

 

[Glenn9999]

http://www.delphibasics.co.uk/RTL.asp?Name=Assigned

Measurement is not management.

 

[whosrdaddy]

if you are working with dynamic arrays that contain objects, why not use a TObjectList instead?

/Daddy

-----------------------------------------------------
What You See Is What You Get
Never underestimate tha powah of tha google!

 

[fajac]

It is generally not a good idea to access array-elements that are behind the allocated size of the array. If, like in your example, your array has a lenght of 1, then you should never access elements with an index bigger than 0.
Compile your Project with the comoiler-option "Range checking" set. Next time you step over the Assigned()-statement, it will generate an exception, if the index is too big.

The Assigned()-function just receives a Pointer-Argument which is checked for nil. If Range-checking is disabled, accessing an element behind an object-array´s length means just reading 4 Bytes of memory from the heap, somewhere behind the array. These Bytes are interpreted as a pointer. This Pointer may or may not be nil, since the memory, it was read from, might be in use for storing anything else.