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mike3

What is the best way to implement this "weird" kind of graph in C++ for a game?

28 posts in this topic

[quote name='Antheus' timestamp='1328973736' post='4911974']
::iterator types are for iteration. While some may be held on to, it causes aforementioned problems.

In most newer languages, iterators are explicitly checked for modification and will cause failures when trying to use them as references.

Iterators are not identity types and cannot be used for such purpose, even though in some corner cases, depending on implementation, they can temporarily take such role.

Instead of insisting on such approach, use a different approach. Establish strong identity (NodeID, StringID, EdgeID). Keep track of those separately as well as their connections. Then build structures on top of that.


There really are no difficulties here. It's just a basic list/tree structure, which iterators make impossible to implement if any of them can be mutated, something that iterators, as a concept, cannot handle. Only general solution to this problem is garbage collection, which doesn't solve cross-linking issues.
[/quote]

However, I've noticed in some implementations like "Boost" that there are "descriptors" in addition to iterators, but at least in the case of Boost, with some forms of the graph these too can be invalidated, and so we get the same problems. Would I be right in guessing that these problems are saying that messing around with invalidating references is the wrong way to go, and there's simply no way to use invalidating references in this way without making code a mess? As it seems unavoidable, and so I'm really strongly thinking of using your ID-tag approach. Especially considering I was already going in that direction with giving strings ID codes by which one could reference and access them. The only downside I can see is the added time complexity of the accesses, but considering that this is not performance-critical code that is not actually a problem.
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NEW: Well, now I've settled on the approach of using a graph that can refer to nodes by ID codes instead of iterators. This removes problems 1, 2, and 3 in my original post, but I'm still not sure what to do about problem 4. Is it okay for the "string" class to be "unsafe to copy" due to how it creates/deletes iterators in the graph (see the OP)?
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[quote name='Antheus' timestamp='1328973736' post='4911974']
::iterator types are for iteration. While some may be held on to, it causes aforementioned problems.

In most newer languages, iterators are explicitly checked for modification and will cause failures when trying to use them as references.

Iterators are not identity types and cannot be used for such purpose, even though in some corner cases, depending on implementation, they can temporarily take such role.
[/quote]

This is how iterators work in the real world: their lifespan is limited to some activity that iterates through members of a data structure, and in this small scope one can take care not to invalidate them with inappropriate operations.
If you insist on storing iterators instead of plain pointers or names, you are abusing the standard library and looking for trouble.
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[quote name='LorenzoGatti' timestamp='1329816798' post='4915128']
[quote name='Antheus' timestamp='1328973736' post='4911974']
::iterator types are for iteration. While some may be held on to, it causes aforementioned problems.

In most newer languages, iterators are explicitly checked for modification and will cause failures when trying to use them as references.

Iterators are not identity types and cannot be used for such purpose, even though in some corner cases, depending on implementation, they can temporarily take such role.
[/quote]

This is how iterators work in the real world: their lifespan is limited to some activity that iterates through members of a data structure, and in this small scope one can take care not to invalidate them with inappropriate operations.
If you insist on storing iterators instead of plain pointers or names, you are abusing the standard library and looking for trouble.
[/quote]

Actually, I had programmed my own graph implementation and featured iterators. So I guess I was abusing the [i]concept[/i] of iterator, then. But there's no problem now -- I've since replaced that graph with a new version that uses ID codes to give stable references, like Antheus suggested. Granted, in theory the use of std::map gives a performance hit, but this code is not used in anything performance-critical so that is no problem.
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