passing simple pointer with no ownership to boost python

Question

I am moving first steps with boost python to test my class but I am facing a difficulty in declaring the python module for my class.

My class accepts a pointer to another class but I don't know how to declare

class A{ };

class B
{
    B( std::string& name, A* ptr ){ 
        std::cot << ptr->data << std::endl; // no ownership
    }
    void my_foo(){
        // do something!
    }
};

I exported class A to python but I am facing problem with class B

class_< B >("B", init< std::string, A >() )
{
    .def("my_foo", &B::my_foo);   
}

I am having tons of error. What I am doing wrong? I am reading about policy but here I don't think I have to apply some of them, am I right?

Kind Regards

AFG

It probably doesn't know how to extract A* from boost::python::object. Have you wrapped A class? — Maxim Egorushkin
– Maxim Egorushkin, Commented Jan 4, 2013 at 17:49
It shouldn't compile because the constructor of B is private. Post complete code please. — Maxim Egorushkin
– Maxim Egorushkin, Commented Jan 4, 2013 at 17:54
Hi Maxim. It is been few weeks that I am testing it and I agree with you. THe best way to use boost python is to have proper wrapper in place and having as argument for all functions all basic data ( int,long, std::string, const char * ). It looks that works fine also having const reference to object as arguments. — Abruzzo Forte e Gentile
– Abruzzo Forte e Gentile, Commented Jan 16, 2013 at 11:06

jbosch · Accepted Answer · 2013-01-05 18:44:52Z

The first problem with your example is that boost::python::class_ bodies should not go inside curly braces - they aren't a macro or a new kind of function, they're just template classes with a lot of fancy operator overloading, so what you really want is something like this:

class_<B>("B", init<std::string,A*>())
    .def("my_foo", &B::my_foo)
    ;

In that snippet, you can see the fix you need for init to compile - just pass A* as a template parameter instead of 'A'. That should be enough to get it to compile.

Whether that's enough for it to do what you want depends on what the constructor for B does with the pointer it is passed. If it simply uses it in the scope of the constructor, or it deep-copies it, you're fine. But if it holds onto that pointer (e.g. as a data member), then you could run into trouble if the A object goes out of scope before the B object does. If that's the case, you would want to use the with_custodian_and_ward policy, which I think would look something like this:

init<std::string,A*>()[with_custodian_and_ward<1,3>()]

The integer template arguments to with_custodian_and_ward refer to positional arguments to the __init__ method; 1 refers to self, and 3 refers to the A* argument, so this means "keep the A object passed to the constructor alive until the constructed B object is destroyed."

You can find more information here:

http://www.boost.org/doc/libs/1_52_0/libs/python/doc/v2/with_custodian_and_ward.html

Collectives™ on Stack Overflow

passing simple pointer with no ownership to boost python

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related