TObjectList.Clear access violation

Question

I'm running into a very weird problem with a large application. I make heavy use of TObjectList storing a custom object on them. On large lists im experiencing weird crashes with "Access violation at address.. read of address.. " "privileged instruction" and others, when I use the CLEAR method. I've tracked this to happen exactly when attempting to delete the last item in the list. I've checked this by logging the contained objects deletion from their destroy proc, and also trying to deleting them on my own (for a := olist.count-1 downto 0 do.. debugmsg('deleting '+inttostr(a)).. olist.delete(a) ), both ways I get the access violation right when deleting the LAST remaining item in the list.

This doesnt happen always, cause I use clear in other areas, and also a few different (smaller) lists, but at a very specific point in my app this happens.

I've no clue what might be wrong, there's nothing trying to access the list during the clear, and the cointained objects do not have access to their parent objectlist, there has to be something screwing up in the TObjectList.delete/clear methods when it comes to clearing the last item.

Any suggestions? Using Delphi XE.

Answer 1

That sounds to me like you're freeing objects that have already been freed. To track this down, download the full version of FastMM, add FullDebugMode to the Conditional Defines line under Project Options->Delphi Compiler and the Map File option under Linking set to Detailed, and rebuild. (Build, not Compile.) Then copy the FullDebugMode DLL to the same folder as your EXE and run it. It'll watch your memory as you allocate and free and when you try to free the same object for a second time, it'll catch that and give you some very detailed debug data as to where the problem is coming from.

Answer 2

Are you sure the last object is valid and not already deleted? It could be in the list twice e.g. due to other bugs.