Boost Python - C++ class' private static data blown away before accessing in Python?

Giuseppe Ottaviano giuott at gmail.com
Sat Jul 5 23:14:57 EDT 2008


>
> In Python, I retrive an Entity from the EntityList:
>
> elist = EntityList()
> elist.append(Entity())
> elist.append(Entity())
>
> entity = elist.get_at(0)
>
> entity.foo()
>
> But it crashes inside foo() as the private static data is empty; or
> rather the string array is empty. I know before that point that the
> private static data is valid when accessed earlier by the C++ code as
> the program works fine. It just won't work from Python, so somehow the
> private static data has been blown away but I can't work out where or
> why.

Probably it is a problem of lifetime. What is the signature of append?  
Who deletes the appended Entity in C++ code?
If append takes a raw pointer, Boost.Python copies the pointer but  
destroys the Entity object because it is a temporary and its reference  
count went to zero. So the pointer in the list is referring to a  
destroyed object, which results in undefined behaviour.

Did you have a look at the lifetime policies of Boost.Python? The  
simplest way to workaround the problem is using const reference  
arguments, and always use value semantics. If it can result in a  
performance penalty, another simple way is using shared_ptr's, which  
have their own reference count (different from the one in CPython  
lib), but Boost.Python does the magic to make them work together.

HTH,
Giuseppe



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