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

[Jim Bosch]

On 10/05/2011 11:31 AM, Holger Brandsmeier wrote:
> 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>?
>

Personally, I pretty much always use shared_ptr, and I've written a 
rather large extension to support constness on the Python side, and 
dealing with shared_ptr<const T> is a side effect of that.

I'm welcome to pass it on if you're interested, but I don't think it 
really addresses your problem.

Jim



>
> 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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