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

[Stodge]

I've exposed a C++ class to Python using Boost Python. The class,
let's say it's called Entity, contains private static data, which is
an array of strings. Though I think it implements it using MFC's
CPtrArray.

I've also exposed a public function from Entity - let's say it's
called foo. This function accesses the private static data (the string
array).

I have multiple instances of Entity stored in a custom C++ container,
which is also exposed to Python as class EntityList.

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.

The static data is setup at initialisation - my Python code is only
called long after initialisation is complete.

I added a static dump() function to the Entity class that dumps the
string array, and even if I just do the following in Python:

Entity.dump()

in Python, the private static data is empty. Doing the same from C++
works fine. Weird.

I know this is an impossible question to ask, but can anyone think of
something obvious I need to look into?

Thanks

 

[Giuseppe Ottaviano]

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

 

[Stodge]

Thanks. Maybe it's a DLL boundary issue? I'll look into this too.

 

[Stodge]

I wonder if it's a DLL boundary problem.

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

 

[Stodge]

Could it be a boundary problem? The static data is initialised by the
application. The problem arises when the python module tries to access
it.

 

[Stodge]

Oops - I didn't see my post so I thought something had gone wrong and
reposted. Apologies for the multiple posts.