If you look at the C++ Maya API doc, you'll see that the `
MSelectionList::getDagPath` method is defining a parameter `MDagPath &
dagPath`. In C++, the `&` character denotes what is called a reference
<https://en.wikipedia.org/wiki/Reference_%28C%2B%2B%29>. What it means here
is that `dagPath` is an input/output parameter to the `
MSelectionList::getDagPath` method—the method can read but also modify the
content of the `dagPath` argument.
In your code, you initialize your `mDagPath` variable with `
OpenMaya.MDagPath()`. At this point in time, `mDagPath` is like an empty
“shell”, with uninitialized data that is not pointing to any valid DAG node
in your scene, so calling some methods, such as `MDagPath::fullPathName`,
might result in an undefined behaviour (in other words: don't do it). When
you pass `mDagPath` to `MSelectionList::getDagPath`, the method will fill
it with a new “substance”, that is the data required to point to a valid
DAG node. Now you can safely access or modify the DAG node like you've
already done with the `MDagPath::fullPathName` method and others.
Arguably, this whole reference thing is popular in C++ but not so much in
Python. If the Python version of the Maya API v1 would have received a bit
more love, you'd probably only would have had to type `mDagPath =
mSel.getDagPath(0)` instead.
Now, `mDagPath.fullPathName('pCube1')` doesn't make so much sense in itself
as one would expect the method `MDagPath::fullPathName` to return the full
path of the DAG node, nothing else. But it surely would make sense to have
another method allowing to easily retrieve a `MDagPath` object from its
name. Alas there's none available and that's why many develop their own
utilities by wrapping the `MGlobal::getSelectionListByName` function.
On 10 August 2016 at 16:43, Rudi Hammad <[email protected]> wrote:
> So this is the code ( it is quite simple so I didn´t use paste bin, I hope
> it is okey)
>
> import maya.OpenMaya as OpenMaya
>
> cmds.polyCube()
> mSel = OpenMaya.MSelectionList(); mSel.add('pCube1')
> mDagPath = OpenMaya.MDagPath()
> mSel.getDagPath(0, mDagPath)
> mDagPath.fullPathName()
>
> that is what I learn in a cgcircuit course. they say that MObject or
> MDagPath are handles that you need to use to access maya data. Okey. no
> problem.
> The thing that I am trying to understand in the code I posted is this:
> I am creating an object mDagPath that is requiered as an argument -->
> mSel.getDagPath(0, mDagPath)
> by doing that, I can use that object method .fullPathName().
>
> what I want to understand is what is happening there. what happens when
> you pass an object as argument? why doing that allows you to use some
> methods of that object.
> in my mind it would be more intuitive something like :
>
> import maya.OpenMaya as OpenMaya
>
> cmds.polyCube()
> mDagPath = OpenMaya.MDagPath()
> mDagPath.fullPathName('pCube1')
>
> does my question make any sence?
>
> thnks
>
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