He's asking about python - I don't know if C++ even has MScriptUtil, since
it's a class created purely for python wrapping issues.
He's asking about destroying objects because, unfortunately, MScriptUtil is
evil, and creates "ptr" objects which refer to storage contained within the
MScriptUtil itself... but NOT in any sort of way that python is aware of.
The net result is that it's possible for python to destroy the MScriptUtil,
and it's associated C++ objects... but still have a ptr object created by
that MScriptUtil, which tries to use that now-freed space.
As an example, code like this can cause a crash:
def getPtr():
return MScriptUtil().asDoublePtr()
ptr = getPtr()
maya.OpenMaya.someFuncThatNeedsADoublePtr(ptr)
The problem is that the MScriptUtil object created within the function will
get deleted as soon as the function exits... but the returned pointer
object still points at the storage associated with that (now freed)
MScriptUtil. This is probably one of the many reasons why it was removed
completely in the python 2.0 api (yay!)
However, because we still need to use the 1.0 api for many things... pymel
contains a small wrapper class for dealing with MScriptUtil pointers.
Basically, it just holds a (python) ref to the MScriptUtil object along
with the pointer, to ensure that it isn't destroyed prematurely. Full
details are in the docs / comments in pymel.api.allapi.SafeApiPtr
<https://github.com/LumaPictures/pymel/blob/master/pymel/api/allapi.py#L30>,
but here's some example usage:
ptr = pymel.api.allapi.SafeApiPtr('double')
maya.OpenMaya.someFuncThatNeedsADoublePtr(ptr())
Note that you need to manually get the pointer object by "calling" it - the
general idea is that you should hold onto the SafeApiPtr object in python,
and get the "raw" MScriptUtil pointer only immediately before passing to
the OpenMaya call. However, the SafeApiPtr still can't guarantee that you
won't get in trouble - for instance, you can't do this:
maya.OpenMaya.someFuncThatNeedsADoublePtr(SafeApiPtr('double')())
...as the SafeApiPtr, and thus the MScriptUtil, may be destroyed by the
time the OpenMaya function is called. Essentially, you need to make sure
the SafeApiPtr object is kept around until the OpenMaya C++ functions are
"done with it".
- Paul
On Fri, Feb 24, 2017 at 2:02 AM Justin Israel <[email protected]>
wrote:
>
>
> On Thu, Feb 23, 2017, 9:04 PM justin hidair <[email protected]>
> wrote:
>
> Quick questions here :
>
> How do you properly destroy your objects and MScriptutils.. ?
>
>
> I assume this is a C++ question, since Python is garbage collected?
>
> "When an MScriptUtil object is destroyed any pointers to its data
> immediately become invalid."
>
> So any data being managed by MScriptUtil should get cleaned up when the
> dtor is called. Is that what you are after?
>
>
> Any optimization tricks ?
>
>
> What specifically are you after?
>
>
> Finally how many of you guys use Python API 2.O ?
>
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