On Tuesday 05 August 2003 14:02, Pigeon wrote: > On Tue, Aug 05, 2003 at 03:40:42AM +0200, Sebastian Kapfer wrote: > > On Mon, 04 Aug 2003 05:00:12 +0200, MJM wrote: > > > // change the way it is accessed to prove a point int * p_b = (int *) > > > p_a; > > > // p_a and p_b point to the same block of dyn. allocated memory; > > > > Do they? Watch out for inheritance, user-defined casting operators and > > other funny stuff. C++ adds a few new meanings to the () casting syntax. > > In general, your assumption is _wrong_.
Sorry for getting back so late. By assumption are you referring to the pointers pointing to the same thing, or that deleting with p_b will correctly return what was allocated to p_a? I am convinced that deleting with p_b is undefined. I believe it can be managed for safety of the system but the practice leads to code that is confusing to read. I think from reading other comments that the safest approach is to delete with the same pointer type that was used in the allocation. > > > > > // return the memory to free storage using the second pointer delete > > > p_b; > > > > > > I think the free store will be maintained properly because there is a > > > control block attached to the allocated block of storage that defines > > > the length of the allocated memory. > > > > No. You have to delete the original pointer (with the original type). > > Everything else is undefined behaviour, i.e. it could work, it could leak > > memory (completely or partly), it could crash, or even print "42". It > > might even work sometimes, and fail mysteriously at other times. > > Interesting. I figured that the actual allocation/deallocation would > end up being handled by a call to malloc() / free(), and free() only > cares about having the correct value for the (void *) pointer. > > Would it not be the case, though, in the barebones example given, that > MJM's '_wrong_ assumption' would be correct, and 'it' would work, even > if in a more general case it might not? I think this can be true in some implementations. > > Or would it have been better for C++ to have been named D to stop people > wanting to think like this? I don't follow the logic there :-) I don't "want" to think like this. The reason I asked this question is because I've been doing some rapid protyping in a pressure situation. I coded a version of the example above inadvertently. In calmer moments I realized what I had done. I was worried about memory leaks. I read about alloc and free in the old K&R C book and found how there was a hidden header on each allocated block. That block could well contain enough information to effect a proper freeing of the block regardless of how it was returned. I don't think that the programmer can or should rely on such a contrivance since it is not documented officially. I have since modified the code to use a template where differant types are handled in an explicit manner. -- Mike Mueller -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of "unsubscribe". Trouble? Contact [EMAIL PROTECTED]
