On 10/05/2011 03:08 AM, Holger Brandsmeier wrote:
Dear list,
how should I export functions to python which return smart pointers to
const-pointers, e.g. shared_ptr<const T>?
For my own classes I always tried to avoid this problem by always
providing a methods which returns shared_ptr<T>.
Now I need to export the following method in a class provided by some
other software package (Trilinos). Its implementation I do not want to
change, the function is declared as
Teuchos::RCP< const Teuchos::Comm< int> > getComm () const
(if you need details:
http://trilinos.sandia.gov/packages/docs/r10.6/packages/tpetra/doc/html/classTpetra_1_1MpiPlatform.html
)
I believe I already exported the custom smart pointer `Teuchos::RCP`
to python, I also exported the class `Teuchos::Comm< int>` to python,
but I get the error
No to_python (by-value) converter found for C++ type:
Teuchos::RCP<Teuchos::Comm<int> const>
which is perfectly true, as I did not export the class `const
Teuchos::Comm<int>`.
I briefly tried to export also the const version of this class (all
methods that I need are provided are available for the const version),
but I failed exporting the class with varying error message, depending
on how I tried to export it. I realized that I don't know how or even
if I should export a const version of a type?
Is there another workaround to this problem? Is there something I'm
missing in the implementation of my custom smart pointer?
I could wrap the function getComm() above to cast away the const'ness,
but do I need to?
I also found some old messages on the list titled "[Boost.Python]
shared_ptr<const T>" with some workaround proposed by providing
get_pointer for const T, but I believe that I have a different problem
that I can not solve by modifying get_pointer.
I'm afraid you do have a different problem, though some of those ideas
help. Essentially, Boost.Python's support for custom smart pointers
isn't as complete as its support for shared_ptr, and its support for
const smart pointers is basically nonexistent. You're pretty much in
not-currently-supported territory.
I think wrapping getComm() to cast away the constness is going to be the
easiest way to make this work, but if you have many such functions it
may be worth digging deeper into the Boost.Python internals to try and
trick it into working by specializing some templates.
Jim
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