Re: Passing References: Is this correct and according to standard?

"Daniel T." <>
Thu, 10 Jul 2008 22:31:17 -0400
Akshay Loke <> wrote:

I have this function from a class MFnDagNode,
        addChild( MObject & child, unsigned int index = kNextPos, bool
keepExistingParents = false );
which takes an object reference as first input parameter (ignore the
rest since those are default)

Because 'child' is a *non-const* reference, the only objects that it can
refer to are l-values.

now in another class, I have:

MFnDagNode parent;
        Where node is MfnDependencyNode&
        And node.object() returns MObject

This gives me error
?error: no matching function for call to
        Note: candidates are: Mstatus MFnDagNode::addChild(MObject&, unsigned
int, bool)

Now I am not sure why this doesn?t work and the object doesn?t get
implicitly cast to the MObject& reference argument, but I am assuming
that could be because the node.object() returns a temporary const
object and it cannot be cast to a MObject& reference?

Correct, but even if node.object() returned a temporary *non-const*
object, it still wouldn't work.

So this is the way I have done this?

const MObject& nodeObjectConstRef = node.object();
MObject& nodeObjectRef = const_cast<MObject&> (nodeObjectConstRef);

and This works!

Your unlucky, it's undefined behavior.

Initially I tried this,
MObject& nodeObjectConstRef = node.object();
parent.addChild(nodeObjectConstRef );

but that didn?t work ? it gives the error,
        ?error: invalid initialization of non-const reference of type
?MObject&? from a temporary of type ?MObject??

Right, a non-const reference can only refer to an l-value.

The first method above is working, but am not sure if I am suppressing
something using the const_cast?

Better to do this:

MObject obj = node.object(); // makes a modifiable copy of the return
addChild( obj );

My guess though is that even this won't really work. I suspect that
'addChild' keeps a pointer to the object passed in and unless you are
very careful, obj is going out of scope before the pointer to it does.

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