[C++-sig] Boost.Python indexing suite version 2?

[Matthew Scouten (TT)]

The documentation points out (with very good reason) that just using
"::with_policies(bp::return_internal_reference)" can cause major
problems if the container in question reallocates things between the
time that you extract the object and the time you try to access it. 

v = VectorofFoo()
v.extend([Foo() for x in range(10)])
f = v[5] #returns a reference to a foo on the heap
f.extend([Foo() for x in range(1000)]) #almost certainly causes
reallocation of underlying array in c++

f.function() #accesses a no longer valid reference, sky falls.

The standard indexing suite uses clever proxy magic to work around this.


On second Look, it looks like the intended usage of container_proxy is
something like this:

class_< container_proxy <std::vector<foo> > >("class_< std::vector<foo>
>("VectorOfFoo") )
    .def( indexing::container_suite< container_proxy <std::vector<foo> >
>() )
    ;

Which seems kind of clunky to me.

I think defaulting to by-value is the wrong behavior. The idiom
"v[n].some_attribute = blah" works seamlessly on python lists, and c++
vectors. The indexing suite V1 is the default behavior, and can be
disabled if needed. 


I am using container_suite on a "std::map<std::string,
std::vector<std::string> >" and the only problem I have is a compiler
warning about a very long decorated name. using foo_vector_exposer_t
directly seems kind of clunky to me. 

While I was trying to use indexing suite v2 to wrap a non-STL but
STL-like container class, I noticed that the documentation for doing so
is badly out of date. This is the main (almost the only) advantage that
V2 has over V1, so these docs badly need to be updated. 


-----Original Message-----
From:
cplusplus-sig-bounces+matthew.scouten=tradingtechnologies.com at python.org
[mailto:cplusplus-sig-bounces+matthew.scouten=tradingtechnologies.com at py
thon.org] On Behalf Of Roman Yakovenko
Sent: Saturday, July 25, 2009 2:19 PM
To: Development of Python/C++ integration
Subject: Re: [C++-sig] Boost.Python indexing suite version 2?

On Thu, Jul 23, 2009 at 6:59 PM, Matthew Scouten
(TT)<Matthew.Scouten at tradingtechnologies.com> wrote:
> This is a sketch of what I want to do. This seems to work seamlessly
> with the
> included indexing suite. I cannot figure out how the make it work with
> the advanced one.

Okey. By default the indexing suite v2 returns objects by value (
default call policy ).
It is very simple to change this behavior:

        namespace bp = boost::python;

        typedef bp::class_< std::vector< foo > > foo_vector_exposer_t;
        foo_vector_exposer_t foo_vector_exposer =
foo_vector_exposer_t( "foo_vector");
        bp::scope foo_vector_scope( foo_vector_exposer );
        foo_vector_exposer.def( bp::indexing::vector_suite<
std::vector< foo > >::with_policies(bp::return_internal_reference<
>()) );

And than all your code will work as expected.

I suggest you not to use "container_suite":
* the compilation is longer
* it is also unable to guess/resolve between map and multimap
containers.

P.S. Let me know if you still want\need to use container_proxy. I
never used that class, so it could take me some time to understand it.

-- 
Roman Yakovenko
C++ Python language binding
http://www.language-binding.net/
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