[C++-sig] Custom smart pointer with const Types

[Holger Brandsmeier]

Jim,

how do you handle smart_ptr<const T> in boost python? Do you simply
cast away the constness?


For my custom smart pointer I provide a class extending
  to_python_converter<Teuchos::RCP<T>, rcp_to_python<T>, true>
now I decided to also implement
  to_python_converter<Teuchos::RCP<const T>, rcp_to_python_const<T>, true>
where I simply cast away the constness and use the above implementation.

This seems to be working so far. Did you provide a smarter
implementation for shared_ptr<const T>?

-Holger


On Wed, Oct 5, 2011 at 15:44, Jim Bosch <talljimbo at gmail.com> wrote:
> 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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