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How can I wrap functions which take C++ containers as arguments?

Ralf W. Grosse-Kunstleve provides these notes:

  1. Using the regular class_<> wrapper:

    class_<std::vector<double> >("std_vector_double")

    This can be moved to a template so that several types (double, int, long, etc.) can be wrapped with the same code. This technique is used in the file scitbx/include/scitbx/array_family/boost_python/flex_wrapper.h in the "scitbx" package. The file could easily be modified for wrapping std::vector<> instantiations. This type of C++/Python binding is most suitable for containers that may contain a large number of elements (>10000).

  2. Using custom rvalue converters. Boost.Python "rvalue converters" match function signatures such as:

    void foo(std::vector<double> const &array); // pass by const-reference
    void foo(std::vector<double> array); // pass by value

    Some custom rvalue converters are implemented in the file scitbx/include/scitbx/boost_python/container_conversions.h This code can be used to convert from C++ container types such as std::vector<> or std::list<> to Python tuples and vice versa. A few simple examples can be found in the file scitbx/array_family/boost_python/regression_test_module.cpp Automatic C++ container <-> Python tuple conversions are most suitable for containers of moderate size. These converters generate significantly less object code compared to alternative 1 above.

A disadvantage of using alternative 2 is that operators such as arithmetic +,-,*,/,% are not available. It would be useful to have custom rvalue converters that convert to a "math_array" type instead of tuples. This is currently not implemented but is possible within the framework of Boost.Python V2 as it will be released in the next couple of weeks. [ed.: this was posted on 2002/03/10]

It would also be useful to also have "custom lvalue converters" such as std::vector<> <-> Python list. These converters would support the modification of the Python list from C++. For example:


void foo(std::vector<double> &array)
  for(std::size_t i=0;i&lt;array.size();i++) {
    array[i] *= 2;


>>> l = [1, 2, 3]
>>> foo(l)
>>> print l
[2, 4, 6]

Custom lvalue converters require changes to the Boost.Python core library and are currently not available.


The "scitbx" files referenced above are available via anonymous CVS:

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cvs co scitbx