How to make a C++ mapping from C struct to int in cython?

Question

How to make a C++ mapping from C struct to int in cython?

I try the following code:

main.py

#!/usr/bin/env python
# encoding: utf-8
import pyximport


pyximport.install()
from foo import Fun

Fun()

foo.pyx

#!/usr/bin/env python
# encoding: utf-8
from libcpp.map cimport map
from libcpp.string cimport string

cdef struct mystruct:
    int i


def Fun():
    cdef:
        map[mystruct,int] dDict
        mystruct A

    A.i=1

    dDict[A]=1

configuration file foo.pyxbld

def make_ext(modname, pyxfilename):
    from distutils.extension import Extension
    return Extension(name=modname,
                     sources=[pyxfilename],
                     language='C++')

I get the following error when run main.py:

foo.cpp
C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\INCLUDE\xlocale(342) : warning C4530: C++ exception handler used, but unwind semantics are not enabled. Specify /EHsc
C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\INCLUDE\functional(143) : error C2784: 'bool std::operator <(const std::basic_string<_Elem,_Traits,_Alloc> &,const _Elem *)' : could not deduce template argument for 'const std::basic_string<_Elem,_Traits,_Alloc> &' from 'const __pyx_t_3foo_mystruct'
        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\INCLUDE\string(150) : see declaration of 'std::operator <'
        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\INCLUDE\functional(142) : while compiling class template member function 'bool std::less<_Ty>::operator ()(const _Ty &,const _Ty &) const'
        with
        [
            _Ty=__pyx_t_3foo_mystruct
        ]
        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\INCLUDE\map(68) : see reference to class template instantiation 'std::less<_Ty>' being compiled
        with
        [
            _Ty=__pyx_t_3foo_mystruct
        ]
        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\INCLUDE\xtree(22) : see reference to class template instantiation 'std::_Tmap_traits<_Kty,_Ty,_Pr,_Alloc,_Mfl>' being compiled
        with
        [
            _Kty=__pyx_t_3foo_mystruct,
            _Ty=int,
            _Pr=std::less<__pyx_t_3foo_mystruct>,
            _Alloc=std::allocator<std::pair<const __pyx_t_3foo_mystruct,int>>,
            _Mfl=false
        ]
        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\INCLUDE\xtree(63) : see reference to class template instantiation 'std::_Tree_nod<_Traits>' being compiled
        with
        [
            _Traits=std::_Tmap_traits<__pyx_t_3foo_mystruct,int,std::less<__pyx_t_3foo_mystruct>,std::allocator<std::pair<const __pyx_t_3foo_mystruct,int>>,false>
        ]
        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\INCLUDE\xtree(89) : see reference to class template instantiation 'std::_Tree_ptr<_Traits>' being compiled
        with
        [
            _Traits=std::_Tmap_traits<__pyx_t_3foo_mystruct,int,std::less<__pyx_t_3foo_mystruct>,std::al

How should I initialize and use this mapping properly? Thanks in advance for your help!

Answer 1

To be able to use a map with key of type mystruct, you need to define a comparison operator for mystruct objects. Indeed, C++ maps are ordered maps (usually backed by some auto balancing trees such as AVL or red-black). A proper syntax for that is

cdef cppclass mystruct:
    int i
    bint lessthan "operator<"(const mystruct t) const:
         return this.i < t.i   

Edit : replaced i < t.i by this.i < t.i because this is more explicit (Zen of Python).