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Object tree using smart pointers


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#1 Monkeynuts   Members   -  Reputation: 126

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Posted 23 January 2014 - 11:34 AM

I'm working on the data structure of objects in my 2D game engine, such as sprites, score displays etc.  The data structure is a tree, where each node owns its children. The root node could be a Level, for instance, which contains a BigBoss object, which in turn contains Sprites that make up the boss. (I will use the words node and object interchangeably in this post.)

 

I know that I want a tree structure, but I'm not sure about the implementation details, so I invite suggestions and comments. Please note that my focus at this stage is correctness, not speed. Also note that my tree structure supports an editor, where objects can be added and deleted  (and reparented) at any time.

Here are my thoughts so far:

 

* My Node baseclass stores its children as std::shared_ptr in a container (probably a std::vector). The node owns its children, but I'm using shared_ptr instead of unique_ptr to allow other nodes to reference it using weak_ptr.

 

* If a Node wants to keep a reference to another node, it stores it in a weak_ptr. For instance, a Snake node might want to keep track of a Food node, even if the food is not in the same subtree as the Snake. If the Food disappears, the Snake will notice that when it runs its updatePath routine, as foodWeakPtr.lock() will return null, and it will stop chasing it.

 

* Getters, such as objectWithName(const std::string& name) or trackedObject(), return (lists of) weak_ptr<Node>, pointing to item(s) in the shared_ptr collection.

 

Does this sound like a sound (smile.png) way to do it? I'd appreciate comments from people who have experience with this kind of thing. I'm new to the whole smart pointer thing.

 

Thanks in advance!



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#2 SeanMiddleditch   Crossbones+   -  Reputation: 10072

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Posted 23 January 2014 - 06:06 PM

Typically you'd use your own handle/id system instead of shared or weak ptrs.

You'd have a SpriteMgr that has strong/owning references to all sprites. Anyone using a sprite asks SpriteMgr to load it and return an ID/handle. Then when you need to access it, you can ask the SpriteMgr to convert the ID to a Sprite*. If the ID is invalid for some reason, the SpriteMgr can return a Sprite* to a "default" sprite (say a red square with the text "missing sprite" sprawled across it).

In general, I _strongly_ recommend avoiding shared_ptr (and weak_ptr). It is almost always the wrong conceptual model for what you want. I've worked with large engines using shared_ptr or things very similar and there are countless bugs and problems with how they work in complex cases.

#3 Monkeynuts   Members   -  Reputation: 126

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Posted 24 January 2014 - 02:22 AM

Thank you for your answer! I will seriously consider the approach you mention.

 

If you have time, could you elaborate a little bit on what kind of problems you encountered and what the thought processes were, and what considerations led you to adopt the manager approach? I thought I came up with a pretty clean solution, and I didn't see that its the wrong conceptual model - obviously my lack of experience in game development has led me astray. Any input to put me on the path to thinking like a game developer is welcome. :)
 



#4 Monkeynuts   Members   -  Reputation: 126

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Posted 24 January 2014 - 03:06 AM

This article and its siblings answers parts of my question:

 

http://www.gamedev.net/page/resources/_/technical/game-programming/managing-decoupling-part-4-the-id-lookup-table-r3062

 

Still, comments welcome.






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