Very goofy pointer issue with cameras

[DrAnonymous]

I setup a development environment so that I could use irrLicht without changing any of its source. I want to be able to drop in any new updates by just overwriting the old source.

I wrote a new camera class, which supports zooming with the scroll wheel.

Since I didn't want to touch the original irrlicht source, I just manually did the same call the scenemanager.addCameraSceneNodeMaya(...)

The scenemanager code is -

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if(!parent)
   parent = this;

ICameraSceneNode* node = new CCameraMayaSceneNode(parent, this, id, rotate...);
node->drop();
setActiveCamera(node);
return node;

In my main.cpp I did the following -

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smgr = Device->getSceneManager();
ICameraSceneNode* node = new CCameraSimpleNode((ISceneNode*)smgr, smgr, id);
node->drop();
setActiveCamera(node);

I discovered 2 problems, if I do the drop I get a segmentation fault. Why?

The other one is if I print the parent pointer of the MayaCamera, it doesn't match the pointer to the scene manager. If you put a printf in the CSceneManager class and print the parent, its not NULL and its not the this pointer either. Since the method addCameraSceneNodeMaya defaults the parent pointer to 0, where is this other value coming from?

The only way I can get my camera to work is do this -

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ISceneNode* node = smgr->addCameraSceneNodeMaya();
node = new CCameraSimpleNode(node->getParent(), smgr, id);
node->drop();
setActiveCamera(node);

Can someone please explain whats going on?

Cheers,
Dr. A>

[hearsedriver]

Hi DrA,

there are some cases of multiple inheritance, where static C-casts can cause problems, and maybe you just discovered one. Try new CCameraSimpleNode(static_cast<ISceneNode*>(smgr), smgr, id) instead, and it should work.

Cheers
hearsedriver

[MindGames]

I could be wrong here but I think your segmentation issue is actually due to the way the memory manager works. I have had this issue before with dlls where the memory is allocated either within the dll or within your application's memory space and then deleted from the other. Try overloading the drop function in your class and handle the deletion yourself by just copying Niko's method. If this causes the problem to go away then I am most likely correct and you will have to mess around with overloading delete operators etc.

[DrAnonymous]

Uggh no go.

I can't compile using static_cast. The compiler says invalid cast type of ISceneManger to ISceneNode. Ugggh. Why is there an interface, aren't you only going have 1 scenemanger ever?

I'm compiling on linux, so I'm not sure the dll would apply. My executable is staticly linked, so I think my memory allocs and free should all be in the same memory space.

Any other ideas?

Regards,
Dr. A>

[MindGames]

Yeah you are right, the problem I mentioned was windows only with a dll.

Anyway, why are you calling drop after calling new. This will cause it to be deleted. Maybe I am misunderstanding this somehow, but AFAIK you only call drop when you are using something created with a createXXX function and only after you are finished with it. If you delete the object and then try to use it you are going to get problems.

[DrAnonymous]

True, about dropping it. I'm still not keen on the whole drop/grab thing. I understand the intent of bookkeeping, but it doesn't seem intuitive enough.

Any way, the drop issue aside, the parent pointer is still not right. Basically if you copy the exact code out of the create method and put it into your main.cpp, it won't work. The cast of the scenemanager isn't right. The only work around I found was to use the getParent() method of another camera and pass that pointer to the constructor of my custom camera for a parent ie -

new CustomCamera(firstCamera->getParent(), sceneManager, 100)

Any other ideas?

Cheers,
Dr. A>

[mohaps]

wildgues...

your app is single-threaded and your irrlicht dll is multithreaded...

is that the case...

look in project-->Settings-->C/C++---> Code Generation...

[MindGames]

I don't think it is a threading issue, he said he was statically linking it.

Right, here come some new theories. This time I have had a look at the Irrlicht code (as I am not at work).

smgr = Device->getSceneManager();
ICameraSceneNode* node = new CCameraSimpleNode((ISceneNode*)smgr, smgr, id);

This, as far as I can see is invalid. You cannot cast a ISceneManager into an ISceneNode, they are unrelated. As you are just using a c style cast you are not getting to see this as an error. Try doing a static_cast<ISceneNode>(smgr) or if you prefer a dynamic_cast<ISceneNode>(smgr). Either one of these would have thrown up a compiler error. When the compiler told you it was an invalid cast earlier, it was right. I imagine that this is the source of your problems so I have not looked further.

[mohaps]

I think it is a multithreaded dll / singlethreaded app issue... cause i have faced such symptoms before...

[MindGames]

When you statically link you do not use a dll. Therefor there can only be one threading library in use. If it is a case of requiring specific threading related functions from the threading library then I would expect the compiler to complain, though I have not used GCC (I presume that is what he is using).

Of course, I could be misunderstanding what he meant exactly by statically linking.

[mohaps]

i think he does static linking on linux and links to the dll on win32..... is the problem occuring on both platforms...same symptom?? or just on windows??

[MindGames]

Well I questioned the windows dll issue earlier and he only mentioned the linux statically linked system. I too have had the issues you mentioned though only when linking to a dll.

However, unless I am very much mistaken, he is attempting an invalid cast and, due to the fact that he uses c style casts, he is unaware of this (a very good arguement for never using them BTW). This error is almost certain to cause issues so should be resolved before worrying about more complex issues.

Unfortunately I have not delved very much into this area so am not really qualified to tell him what exactly he is meant to be doing to correct the issue.

I suspect however, that what he was meant to have written was something like;

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smgr = Device->getSceneManager();
ICameraSceneNode* node = new CCameraSimpleNode(smgr->getRootSceneNode(), smgr, id); 
setActiveCamera(node); 

But as I say, until I have a proper look at this area I will not really be certain.

[Guest]

Mindgames is right.

The "parent" of a scenenode is always supposed to be another scenenode, or none (0).

The scenemanager is set separately. There is absolutely no reason to pass it twice.

Setting the parent probably also modifies a list in the parent scenenode itself.
Since the scene manager is not a scenenode, that likely overwrites something that should not be overwritten.

If you don't want to set a parent for your new camera scenenode, just set it to 0.