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VB Forum / COM / May 2005Returning object pointers
I have an ActiveX dll which exposes a public class - let's call it MyClass. Is it permissible to pass back to a client application an object pointer created with set xxx = new Myclass within the DLL? For example, can I give MyClass a method like this: Public Function Clone() as MyClass dim Answer as MyClass set Answer = new MyClass ' Set various attributes set Clone = Answer End Function Then in a calling application to say: set oNewThing = OldThing.Clone The above seems to work but I am getting all sorts of unexplained failures in my application (or even in the IDE when I try to run it in debug mode). Typical symptoms are trying to access bad memory pointers so I was wondering if the above could be the source of problems. Keith Hi Keith: I do the same thing all the time with no problems. In fact, I bet the if you created new projects and did the same thing (simply pass a created instance/clone) between the projects it will work fine. That being the case, I would suspect the issue is with what the code inside the projects are doing. hth, Doug. > I have an ActiveX dll which exposes a public class - let's call it MyClass. > [quoted text clipped - 21 lines] > > Keith Hi Keith -- Yeah, "what Doug said." I use this routine, without issue, in a library of mine: Public Function Clone() As Matrix Dim mRet As Matrix ' Create new matrix object, copy this object's ' data into it, and return. Set mRet = New Matrix mRet.Copy Me Set Clone = mRet End Function It's that nefarious "something else", I'm afraid. Sorry... Karl  SignatureWorking Without a .NET? http://classicvb.org/petition > I have an ActiveX dll which exposes a public class - let's call it > MyClass. [quoted text clipped - 23 lines] > > Keith >>It's that nefarious "something else", I'm afraid. Darn it! I just don't know what I've done to this application apart from make it just a little bit more complicated. At first I suspected ListViews but I've taken out most of those. What can earth can I do in code to blow the IDE out of the water? Keith >>> It's that nefarious "something else", I'm afraid. > Darn it! I just don't know what I've done to this application apart > from make it just a little bit more complicated. At first I > suspected ListViews but I've taken out most of those. What can earth > can I do in code to blow the IDE out of the water? You're familiar with the F8 key? At times like this, I'm not sure there's anything you can do but track down the exact location of failure. Usually that will lend *some* hint. SignatureWorking Without a .NET? http://classicvb.org/petition >>You're familiar with the F8 key? At times like this, I'm not sure there's anything >>you can do but track down the exact location of failure. Usually that will lend >>*some* hint. The problem is that it seems to be a moving target. The act of stepping through code seems to move the point of failure. I have actually gone back to the much older technique of putting in trace points and tracing to file. It seems to break at bizarre times. For example, it can go through the trace point at the end of the subroutine but fail to get back to the calling code. Sometimes it doesn't break in the application at all. It's the IDE which breaks - usually just after terminating a debug session. I have finally given up. I have reverted to the last working version (thank heavens for Visual Source Safe!) which does not exhibit the problem. I shall now add my new feature piece by piece, watching for any signs of it playing up. Fortunately, the delays have enabled me to think of a way to achieve what I was trying to do in a slightly simpler way. Keith This sounds like a memory cleanup error. Usually, it has something to do with string pointers and API calls and is a manifestation of releasing memory allocated to a string that has already been released. Were you making any API calls that involved string variables in the code for your feature? If not, it could be a problem in one of the components you are using. HTH, Bryan ____________________________________________________________ New Vision Software "When the going gets weird," Bryan Stafford "the weird turn pro." alpine_don'tsendspam@mvps.org Hunter S. Thompson - Microsoft MVP-Visual Basic Fear and Loathing in LasVegas >>>You're familiar with the F8 key? At times like this, I'm not sure there's >anything [quoted text clipped - 19 lines] > >Keith >>Were you making any API calls that involved string variables in the >>code for your feature? It is entirely possible. Current status is I have scrapped the enhancements and am re-adding them a bit at a time using a slightly different design philosophy. I shall look out for this as an area to be wary of. Something good came out of the first attempt in that I can avoid a few blind alleys second time around. Keith >>This sounds like a memory cleanup error. Another possibility which occurred to me is that I had a number of instances where a variable of type Object could have contained a pointer to a number of different object types depending upon context. I did wonder whether VB could have got confused and called the destructor for the wrong object type when it finally did its cleanup. Keith |
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