Cythonic way to wrap boost::geometry::Point accessors - c++

Question

What is the correct cythonic way to wrap the following member functions of a boost::geometry::Point? The code snippet comes from here.
/// #brief Get a coordinate
/// #tparam K coordinate to get
/// #return the coordinate
template <std::size_t K>
inline CoordinateType const& get() const
{
#if defined(BOOST_GEOMETRY_ENABLE_ACCESS_DEBUGGING)
BOOST_GEOMETRY_ASSERT(m_created == 1);
BOOST_GEOMETRY_ASSERT(m_values_initialized[K] == 1);
#endif
BOOST_STATIC_ASSERT(K < DimensionCount);
return m_values[K];
}
/// #brief Set a coordinate
/// #tparam K coordinate to set
/// #param value value to set
template <std::size_t K>
inline void set(CoordinateType const& value)
{
#if defined(BOOST_GEOMETRY_ENABLE_ACCESS_DEBUGGING)
BOOST_GEOMETRY_ASSERT(m_created == 1);
m_values_initialized[K] = 1;
#endif
BOOST_STATIC_ASSERT(K < DimensionCount);
m_values[K] = value;
}
I first attempted using:
cdef extern from "s57/data/geometries.h" namespace "bg::model":
cdef cppclass _Geo2 "bg::model::point<double, 2, bg::cs::spherical_equatorial<bg::degree>>":
_Geo2()
_Geo2( const _Geo2& other )
const double get[size_t]() except +
void set[size_t](double) except +
but then I don't know where to go since something like this:
property x:
def __set__(self, double value):
deref(self._p).set[0](value)
gives me this failure:
Error compiling Cython file:
------------------------------------------------------------
...
property x:
def __set__(self, double value):
deref(self._p).set[0](value)
^
------------------------------------------------------------
c:\XXXX\x.pyx:24:31: not parsable as a type
My current working solution has been to create some helper functions like:
double get_geo2_x( geo2& pnt );
double get_geo2_y( geo2& pnt );
void set_geo2_x( geo2& pnt, double value );
void set_geo2_y( geo2& pnt, double value );
Does somebody have in mind a more elegant solution?

Answer 1

You're running into problems with Cython's handling of non-type template parameters. You can manually specify function names using strings, which are inserted directly into the generated C code (see http://docs.cython.org/src/userguide/external_C_code.html#resolving-naming-conflicts-c-name-specifications)
As a very simple example
// example.hpp
#include <iostream>
class C {
template <int K>
void set(double value) {
std::cout << K << ": " << value << std::endl;
}
And the Cython code
cdef extern from "example.hpp":
cdef cppclass C:
void set0 "set<0>"(double) except +
void set1 "set<1>"(double) except +
def do_set():
# very simple illustrative example
cdef C c
c.set0(1.0)
c.set1(1.5)
}
Whenever Cython sees set0 called on a C it substitutes set<0>, calling the template function directly. You can then use properties, as you were trying to do.
This probably isn't significantly better than creating helper functions, but it might be a little easier.