[C++-sig] boost.python: implicit type conversion

[Stefan Seefeld]

Mihail Konstantinov wrote:
>> Roman Yakovenko wrote:
>>     
>
>   
>> It is not a good idea. Python string is immutable. May be you should
>> consider custom converter.
>> http://www.boost.org/doc/libs/1_37_0/libs/python/doc/v2/faq.html#custom_string
>>     
>
> I don't see why it is not a good idea. I am nowhere accessing any string derived classes in a mutable fashion, am I?
> Also I have no option to change the class hierarchy. The code fragments I am posting are simplified from a large project that I try to pythonify, reduced to what I think is the cause of the problems. I want to avoid modifying the underlying project under all circumstances.
>   

You are attempting to make Python aware of the inheritance relationship 
between std::string and your own class. That is what doesn't work, not 
your own deriving from std::string.
In order to make 'bases<std::string>' work, you would have to export the 
std::string class to Python first. But as that isn't what you actually 
want, you may as well remove the 'bases<std::string>' from your code, 
and instead rely on (implicit) conversion.


>   
>>> Can I modify the declaration inside BOOST_PYTHON_MODULE so that the implicit 
>>>       
>> conversion is recognized and the above call boost_ext.printme(b) succeeds?
>>
>> Yes: http://www.boost.org/doc/libs/1_37_0/libs/python/doc/v2/implicit.html
>>     
>
> This looks very helpful. But I don't understand the C++ module definition example on that page. It ends with
>
>   
>>    implicitly_convertible<X,int>();
>>    implicitly_convertible<int,X>();
>> }
>>     
> I see that an int can be implicitly converted to X. But how can an X be converted to an int? I can't believe that boost.python searches all implemented functions to find that make_x is capable of converting an int to X?
>   

I believe it simply expects an 'operator int ()' to be available in that 
case.

Regards,
       Stefan

-- 

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