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April 2018

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Boost::python and PyQt5 issues (lvalue to-python converter?)
by Chris Djali April 26, 2018

April 26, 2018

Hi, I’m working on Mod Organizer’s Python-based plugin support. If anyone needs context, the base branch is at https://github.com/Modorganizer2/modorganizer-plugin_python and my expansion is at https://github.com/AnyOldName3/modorganizer-plugin_python/tree/implement-mo…. Headers that get exposed to Python live in another repository at https://github.com/Modorganizer2/modorganizer-uibase/tree/Develop/src As part of my general tidying up of things, I found and replaced an incomplete boost::python::class_ definition for QDir which should have been using SIP/PyQt5 as it’s a Qt class. The change is https://github.com/AnyOldName3/modorganizer-plugin_python/commit/8dd9a710b4… This broke some things (and lead me to realise some other things were already broken). Specifically, I noticed that when a C++ function returns a Qt class by value and this function gets called from Python, the C++ part goes out of scope and is deallocated, despite still being used in Python. This is happening because the to-Python converter for these classes (https://github.com/AnyOldName3/modorganizer-plugin_python/blob/implement-mo…) is building the Python object around the existing C++ object instead of copy constructing a new one, and I guess to-Python converters are supposed to create something that will work even once the object being converted is deallocated. Unfortunately, that isn’t an option here, as some Qt types can’t be copy constructed (and even if they could, stuff might break which expected the thing it received to stay in sync). What I’d like to be able to do is register separate to-Python converters, some of which require the C++ object to be kept alive, but return an l-value, and others which make a copy of the C++ object (although if Boost could use the former as the latter by doing the copying itself, that would be good, too). I can prevent these from leaking memory as SIP has the ability to let me transfer ownership at will. As far as I can tell, this option doesn’t exist. I wouldn’t be too surprised if some other workaround existed, but I don’t know what to put into Google to make it appear. I suppose it might be possible to make wrappers returning pointers for all functions that return by value, but I don’t know how I’d make a static function a boost::python::class_ member function. Any help would be greatly appreciated, Many thanks, AnyOldName3 PS: If you do go looking through the whole repository rather than focusing on the specific bits I mentioned, don’t blame me for weirdness like there being wrappers for classes which will never be extended in Python as this project is years old, and I’ve only been working on it for a little bit.

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