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POMaintaining a std::set<boost::shared_ptr>
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  1. POMaintaining a std::set<boost::shared_ptr>
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    copied!<p>I'm writing a game and an accompanying engine in C++. The engine relies heavily on automation using a simple embedded scripting language. Scripts can create object classes, define event listeners on them, and produce instances from them. At present, an instance must be bound to a script-global identifier in order to preserve its existence. The obvious result of this is that there can be no anonymous objects, which will be by far the most common.</p> <p>At present, instances are managed using a <code>std::set&lt;Instance*, spatial_sort&gt;</code>, where <code>spatial_sort</code> is a functor that sorts instances by position, for rendering and collision detection. Instances are removed and re-inserted each frame using their current position as a hint, under the assumption that they're not likely to move a whole lot in a fiftieth of a second. If a <code>dead</code> flag is set in the instance, it is erased from the set. The <code>Instance</code> constructors and destructor invoke <code>insert(this)</code> and <code>erase(this)</code>, respectively.</p> <p>In order to allow anonymous instances, I want to change the set to a <code>std::set&lt;boost::shared_ptr&lt;Instance&gt;, spatial_sort&gt;</code>, which would allow <code>Instance</code> to share ownership of instances and preserve their existence until they destroy themselves. Unfortunately, because the calls to <code>insert()</code> need to be placed in the constructor, <code>shared_from_this()</code> won't work for obtaining a <code>shared_ptr</code> to the <code>Instance</code>. It doesn't matter at all that <code>Instance</code> happens to already inherit from <code>boost::enable_shared_from_this&lt;&gt;</code> via its base class.</p> <p>Can anyone recommend a suitable workaround?</p> <p><strong>Edit:</strong></p> <p>I did what I should have been doing in the first place, and split the behaviour of the <code>Instance</code> class into two classes: <code>Instance</code> and <code>Reference</code>. The expression <code>new SomeClass</code> in a script then returns a <code>Reference</code> to a new <code>Instance</code>. The <code>Instance</code> objects themselves are never managed using a <code>shared_ptr</code>, so they are responsible for committing suicide in response to a suitable event, e.g., end of animation, end of level, etc.</p> <p>Thanks for the help! Refactoring is as good a solution as any if it Just Works.</p>
 

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