boost:shared_ptr base class cast.

blueblood · Jan 31st 2007, 3:33am

Hey,

I'm starting to try out boosts smart pointers, and so far I love'em.

But I have come to a problem that I have a hard time solving, and I might be blind, but I can't find any information about it, so I come her.

The thing I try to do is to cast a class to one of it's childs.
This is how it looks:
SceneNodeBase -> RootNode (RootNode inherits from SceneNodeBase)

The thing is that my scene graph manager got a function that's called

Source Code

typedef boost::shared_ptr<SceneNodeBase> SceneNodeBasePointer;
typedef boost::shared_ptr<RootNode> RootNodePointer;
SceneNodeBasePointer FindByName(const char *pName)

When I want to find my root node, I type,

Source Code

RootNodePointer r = sgm->FindByName("root");

The trouble here is that my compiler complains about...

Source Code

...error C2440: 'initializing' : cannot convert from 'SceneNodeBase *const ' to 'RootNode *'

I have found boost:static_pointer_cast() and boost:dynamic_pointer_cast(), but I can't figure out if they are right, or how I use them.

Anyone here that could help me with this problem?

Best regards
Jesper, Sweden.

Turing · Jan 31st 2007, 3:07pm

Yes those are the function you'll need for downcasting.
Because you know for sure that the function will return a RootNode it's safe to use a static_cast instead of a dynamic cast.

blueblood · Jan 31st 2007, 3:35pm

Originally posted by Turing
Yes those are the function you'll need for downcasting.
Because you know for sure that the function will return a RootNode it's safe to use a static_cast instead of a dynamic cast.

Hello Turing,

Would I step over the line if I asked for a simple example from you, on how this could look codewise?

Best regards
Jesper, Sweden

mrmike · Jan 31st 2007, 3:54pm

I think the compiler is complaining that you can't cast away the 'const' part.

Tarviathun · Jan 31st 2007, 4:58pm

I agree with Mike on that one. However, here's a simple example of using static_pointer_cast that's in our game:

Note:
process_pointer is of type ProcessSharedPtr. I'm returning an EntityComponentControllerWeakPtr (heh, yeah, it's a mouth full. VAX rocks), which derives from Process.

Source Code

return boost::static_pointer_cast<EntityComponentController>(process_pointer);

Another example is this:

Source Code

class A
{
//...
};
class B : public class A
{
//...
};
boost::shared_ptr<A> ptr_to_a(new A);
boost::shared_ptr<B> ptr_to_b = boost::static_pointer_cast<B>(ptr_to_a);

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Hope that helps.