OGRE3D SceneManager如何真正找到* any * SceneNode？

Question

TL;DR;

How can the SceneManager actually find any SceneNode regardless of where it happens to be in the graph when:

1. The SceneManager::createSceneNode(...) method explicitly claims that the nodes created are not part of the graph?¹, and
2. SceneNode s can independently create their own children without the SceneManager 's knowledge?²

¹ _{The SM does not automatically turn the scene nodes it creats into children of other nodes (eg root); you have to manually call addChild on a node for that}

² _{The client can simply write sceneManager->getRootSceneNode()->createChildSceneNode("Child"); , and the SM wouldn't know about the new child's existence}

---

Background

I was going over the source code in OGRE3D and came across the following piece of documentation on the SceneManager class (>> << emphasis added):

/** Retrieves a named SceneNode from the scene graph.
    @remarks
        If you chose to name a SceneNode as you created it, or if you
        happened to make a note of the generated name, you can look it
        up >>wherever it is in the scene graph<< using this method.
        @note Throws an exception if the named instance does not exist
*/
virtual SceneNode* getSceneNode(const String& name) const;

When you look at the implementation, you see:

SceneNode* SceneManager::getSceneNode(const String& name) const
{
    SceneNodeList::const_iterator i = mSceneNodes.find(name);

    if (i == mSceneNodes.end())
    {
        OGRE_EXCEPT(Exception::ERR_ITEM_NOT_FOUND, "SceneNode '" + name + "' not found.",
            "SceneManager::getSceneNode");
    }

    return i->second;
}

So far, so good. We can see that the SM searches for your requested SceneNode in its own SceneNodeList named mSceneNodes . The part I'm trying to figure out is that the documentation claims it can find a node "wherever it is in the scene graph". New SceneNode s are only added into the mSceneNodes list when using SceneManager::createSceneNode(...) . The documentation for the SM's createSceneNode method says (>> << emphasis added):

/** Creates an instance of a SceneNode with a given name.
    @remarks
        Note that this >>does not add the SceneNode to the scene hierarchy<<.
        This method is for convenience, since it allows an instance to
        be created for which the SceneManager is responsible for
        allocating and releasing memory, which is convenient in complex
        scenes.
    @par
        To include the returned SceneNode in the scene, use the addChild
        method of the SceneNode which is to be it's parent.
    @par
        Note that this method takes a name parameter, which makes the node easier to
    retrieve directly again later.
*/
virtual SceneNode* createSceneNode(const String& name);

At the same time, if you look at the SceneNode class, it has its own createChild(const String& name, ...) method, which clearly does not add its own children into the SceneManager 's list, as shown below:

SceneNode* SceneNode::createChildSceneNode(const Vector3& inTranslate,
    const Quaternion& inRotate)
{
    return static_cast<SceneNode*>(this->createChild(inTranslate, inRotate));
}
//-----------------------------------------------------------------------
SceneNode* SceneNode::createChildSceneNode(const String& name, const Vector3& inTranslate,
    const Quaternion& inRotate)
{
    return static_cast<SceneNode*>(this->createChild(name, inTranslate, inRotate));
}

This means that if the client program says node.createChildSceneNode(...); , the SceneManager would not be aware of the existence of the new child node, AFAIK, so it'd never be able to find it.

I've been studying the source code for a while now and I've not found an answer to this question. I looked at the BspSceneManager and the BspSceneNode just to see if I could spot something else, but came up empty.

---

For the sake of completeness/reference, the most recent commit currently available in the master branch is:

commit 3b13abbdcce146b2813a6cc3bedf16d1d6084340
Author: mkultra333 <unknown>
Date:   Sun May 8 19:31:39 2016 +0800

Answer 1

It's no wonder this part confuses you because it's part of over-OOP from more than a decade ago. Some like it, some hate it.

The answer however, is quite simple if once you know what to look for: The code for Node::createChild is the following:

Node* newNode = createChildImpl( sceneType );
//...
return newNode;

It actually delegates creation to createChildImpl (the "implementer"). This function is a pure virtual function, thus SceneNode must overload.

When we go to SceneNode::createChildImpl , we get:

Node* SceneNode::createChildImpl(const String& name)
{
    return mCreator->_createSceneNode( name );
}

mCreator is a SceneManager pointer variable. So there you go: The SceneManager does get informed when a SceneNode is created via createChildSceneNode .

Note however an implementation (eg BspSceneNode in BspSceneManager ) may overload createChildImpl and not inform the SceneManager ; or they may.

Placing a breakpoint inside SceneManager::_createSceneNode and checking out the callstack will save you a lot of headaches though.