- Q: I have an object composed of 12 doubles. A const& to this object is - returned by a member function of another class. From the viewpoint of - using the returned object in Python I do not care if I get a copy or a - reference to the returned object. In Boost.Python Version 2 I have the - choice of using copy_const_reference or return_internal_reference. Are - there considerations that would lead me to prefer one over the other, - such as size of generated code or memory overhead? + Q: I have an object composed of 12 doubles. A const& to + this object is returned by a member function of another class. From the + viewpoint of using the returned object in Python I do not care if I get + a copy or a reference to the returned object. In Boost.Python Version 2 + I have the choice of using copy_const_reference or + return_internal_reference. Are there considerations that would lead me + to prefer one over the other, such as size of generated code or memory + overhead? --A: copy_const_reference will make an instance with storage for one of - your objects, size = base_size + 12 * sizeof(double). - return_internal_reference will make an instance with storage for a - pointer to one of your objects, size = base_size + sizeof(void*). - However, it will also create a weak reference object which goes in the - source object's weakreflist and a special callback object to manage the - lifetime of the internally-referenced object. My guess? - copy_const_reference is your friend here, resulting in less overall - memory use and less fragmentation, also probably fewer total cycles.
+A: copy_const_reference will make an instance with storage + for one of your objects, size = base_size + 12 * sizeof(double). + return_internal_reference will make an instance with storage for a + pointer to one of your objects, size = base_size + sizeof(void*). + However, it will also create a weak reference object which goes in the + source object's weakreflist and a special callback object to manage the + lifetime of the internally-referenced object. My guess? + copy_const_reference is your friend here, resulting in less overall + memory use and less fragmentation, also probably fewer total + cycles.
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+Ralf W. Grosse-Kunstleve provides these notes:
+ +class_<> wrapper:
+
+class_<std::vector<double> >("std_vector_double")
+ .def(...)
+ ...
+ ;
+
+ 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).
++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. +
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:
+ +C++:
+
+void foo(std::vector<double>& array)
+{
+ for(std::size_t i=0;i<array.size();i++) {
+ array[i] *= 2;
+ }
+}
+
+ Python:
++>>> 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. + +
P.S.:
+ +The "scitbx" files referenced above are available via anonymous + CVS:
++cvs -d:pserver:anonymous@cvs.cctbx.sourceforge.net:/cvsroot/cctbx login +cvs -d:pserver:anonymous@cvs.cctbx.sourceforge.net:/cvsroot/cctbx co scitbx +
Revised