<p>Op 19 sep. 2011 05:13 schreef "M.E. O'Neill" <<a href="mailto:oneill@cs.hmc.edu">oneill@cs.hmc.edu</a>> het volgende:<br>
><br>
> Howard Hinnant wrote:<br>
> >>> There isn't a lot there yet. I've tried to get vector working (just the primary, not vector<bool> so far), and I believe I have done so. Though I don't have tests for it yet (I've just been spot checking). At this point I'm simply exploring whether the basic design is viable or not.<br>
> >>><br>
> >>> A major goal of the design of this debug mode is to keep the ABI stable as debug mode is turned on/off.<br>
><br>
> Sounds good, but it'd be useful to have a more detailed design document somewhere so that people can see where you're going and how.<br>
><br>
> ... and Christopher Jefferson replied:<br>
> >> I have often seen interest on a "cheap" debugging mode, which would add checks which were possible without breaking complexity requirements, or "excessive" cost.<br>
><br>
> Without seeing Howard's design, I don't know how "excessive" the costs of his design might be, but in principle, you can do some checks like iterator invalidation very cheaply -- if you have the right design.<br>
><br>
> This may be obvious (in which case I apologize), it may even be what Howard has already done (in which case, oops, and, uh, cool), but in case it isn't, let me outline how:<br>
><br>
> - For every container, associate a 64-bit tag (a.k.a. version stamp).<br>
><br>
> - For every iterator, also associate a 64-bit tag.<br>
><br>
> - When you create a new container, just pick a random 64-bit value.<br>
><br>
> - When you create an iterator, copy the 64-bit tag from the associated container. This represents the container/version the iterator belongs to. Any access via the iterator checks that the tag of the iterator matches the tag of the container. If it doesn't, BANG!<br>
><br>
> - When iterators are invalidated, generate a new tag for container. (For speed, you could just increment it, but the important point is that the value is a new and different one)<br>
><br>
> This method is probabilistic -- there is a 1 in 18446744073709551616 chance that it won't catch an invalid access, but personally I like those odds. It also imposes very very little in runtime overhead -- you can invalidate N iterators in O(1) time.</p>
<p>Why not just use the cool c++11 random number generators, if performance would be an issue, time to optimise those ;-) I don't like the idea of probabilistic odds, especially in hugely iterated and containered program this would make debugging useless (especially for long-running server-like programs that would need to be debugged)</p>
<p>Ruben</p>
<p>><br>
> I've always stored the tag in the containers/iterators, but it should also work with a database-based scheme.<br>
><br>
> M.E.O.<br>
><br>
><br>
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