[C++-sig] Exposing std::vector using vector_indexing_suite

[Jim Bosch]

On Nov 26, 2014 6:37 AM, "James Bird" <jsb at dneg.com> wrote:
>
> Hi,
>
> We have a simple data structure like this, that we would like to expose
via boost python:
>
> struct Layer
> {
>     double value;
> };
>
> struct Document
> {
>     std::vector< Layer > layer;
> };
>
> If we were writing this in pure python, then it could be written as
follows:
>
> class Layer:
>     def __init__( self ):
>         self.value = 0.0
>
> class Document:
>     def __init__( self ):
>         self.layers = []
>
> We would like our C++ bindings to match the behaviour of python as
closely as possible. But we have encountered a scenario in which it
doesn't, and we wanted to get some advice on the best way forward.
>
> In pure python, the following code produces an output of '123.0':
>
> l = Layer( )
> d = Document( )
> d.layers.append( l )
> l.value = 123.0
> print d.layers[0].value
>
> When exposing C++ classes via boost python (using the
vector_indexing_suite for Document.layers), it produces a result of '0.0'.
>
> The 'append' is making a copy such that 'l' and 'd.layers[0]' are two
separate objects. We've noticed that one way to make this work is by using
shared_ptr in our C++ data structure, but we feel we would have to
compromise on our C++ API as a result. Do we have any other options?
>

I don't think you have any good ones.

Any simple C++ version of your Python snippet would also deep-copy the
Layer object; append() is copying here simply because it's calling
std::vector::push_back(), and that's what the std::vector does.  If you
want this to work across the C++/Python boundary, you'll need to make it
work in pure C++ first, and I do think using shared_ptr seems like the most
natural way to do that.

Jim
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