[C++-sig] call policies help needed

Roman Yakovenko roman.yakovenko at gmail.com
Wed May 24 12:06:39 CEST 2006

Hi. Please consider next case:

class A{ ... };

struct B{
    B( A& a )
    : a( a )

    A& a;

In order to export class B, I have to create access function for
member variable a.

A& get_a( B& b ){ return b.a };

    class_< A > ...;
    class_< B >( "B", bp::init< A& >( )[with_custodian_and_ward<1,2>()] )
        .add_property( "a", make_funtion( &get_a, ???????? ) );

I don't know what call policies pyplusplus, code generator,  should
give for make_function
by default. Why? Because there are different use case:

1. Python code:
    a = module.A()
    b = module.B( a )
    In this case life time of "a" is already managed by boost.python,
so the return value
    policy for make_function could be "return_existing_object". I
think, that in this case
    it is safe to use it.( Am I wrong ? )

2. Python code:
    a = module.get_the_a_object() # returns reference to existing
object a, that was created
                                                 # from C++, life time
of this object is not managed
                                                 # by boost.python
    b = module.B( a )
    In this case it is unsafe to use "return_existing_object". The
safer alternatives are:
    1. to use "copy_[non]_const_reference", will not work for all classes, also
        this is not an expected behaviour:
        b.a.s = 2
        will not take effect, because b.a will return copy of a

    2. to use "return_internal_reference". Here I make small
assumption, that developer
        has some guaranties: object "a" will not be destroyed, if
there is at least one "b"
        Obviously this guarantee exists in C++ code, but C++ code does not have
        garbage collector. In Python, object "b", could leave longer
then object "a".
        So I am not sure about this option too.

    3. "return_existing_object"

    4. I am sure, I missed something.


Thanks for help.

Roman Yakovenko
C++ Python language binding

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