How to make unordered_map from c struct to int in cython?

Question

How to make unordered_map from c struct to int in cython?

I try to run the following code: Main.py

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


pyximport.install()
from foo import Fun

Fun()

Foo.pyx

# cython: experimental_cpp_class_def=True
#!/usr/bin/env python
# encoding: utf-8
from libcpp.unordered_map cimport unordered_map
from libcpp.map cimport map
from libcpp.string cimport string

cdef cppclass mystruct:
    int i,j

    bint equal_to "operator=="(const mystruct t) const:
         return this.i == t.i and this.j == t.j

    bint myhash "hash"(const mystruct t) const:
         return hash(i)+hash(i) # how to calculate it properly?


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

    A.i=111

    dDict[A]=1234
    print A.i
    print dDict[A]

foo.pyxbld

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

I get the following error:

        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\INCLUDE\functional(109) : while compiling class template member function 'bool std::equal_to<_Ty>::operator ()(const _Ty &,const _Ty &) const'
        with
        [
            _Ty=__pyx_t_3foo_mystruct
        ]
        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\INCLUDE\xhash(184) : see reference to class template instantiation 'std::equal_to<_Ty>' being compiled
        with
        [
            _Ty=__pyx_t_3foo_mystruct
        ]
        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\INCLUDE\unordered_map(74) : see reference to class template instantiation 'stdext::_Hash_compare<_Kty,_Hasher,_Keyeq>' being compiled
        with
        [
            _Kty=__pyx_t_3foo_mystruct,
            _Hasher=std::tr1::hash<__pyx_t_3foo_mystruct>,
            _Keyeq=std::equal_to<__pyx_t_3foo_mystruct>
        ]
        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\INCLUDE\xhash(191) : see reference to class template instantiation 'std::tr1::_Umap_traits<_Kty,_Ty,_Tr,_Alloc,_Mfl>' being compiled
        with
        [
            _Kty=__pyx_t_3foo_mystruct,
            _Ty=int,
            _Tr=stdext::_Hash_compare<__pyx_t_3foo_mystruct,std::tr1::hash<__pyx_t_3foo_mystruct>,std::equal_to<__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\unordered_map(86) : see reference to class template instantiation 'stdext::_Hash<_Traits>' being compiled
        with
        [
            _Traits=std::tr1::_Umap_traits<__pyx_t_3foo_mystruct,int,stdext::_Hash_compare<__pyx_t_3foo_mystruct,std::tr1::hash<__pyx_t_3foo_mystruct>,std::equal_to<__pyx_t_3foo_mystruct>>,std::allocator<std::pair<const __pyx_t_3foo_mystruct,int>>,false>
        ]
        C:\Users\LENOVO\.pyxbld\temp.win-amd64-2.7\Release\pyrex\foo.cpp(796) : see reference to class template instantiation 'std::tr1::unordered_map<_Kty,_Ty>' being compiled
        with
        [
            _Kty=__pyx_t_3foo_mystruct,
            _Ty=int
        ]
C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\INCLUDE\functional(1265) : error C2440: 'type cast' : cannot convert from 'const __pyx_t_3foo_mystruct' to 'size_t'
        No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\INCLUDE\functional(1264) : while compiling class template member function 'size_t std::tr1::hash<_Kty>::operator ()(const _Kty &) const'
        with
        [
            _Kty=__pyx_t_3foo_mystruct
        ]
        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\INCLUDE\xhash(183) : see reference to class template instantiation 'std::tr1::hash<_Kty>' being compiled
        with
        [
            _Kty=__pyx_t_3foo_mystruct
        ]

How should I properly define hash function and correct the program to make it work? Thanks in advance for your help.

Answer 1

The problem is not to calculate properly the hash value; The problem is to tell the std C++ lib to use the one which you defined. For this you either

• have to overload std::hash<mystruct> but this cannot be done from Cython as far as I know

• or to pass the extra template parameter which forces you to re-wrap all the unordered_map stuff since Cython only wrap the two parameter default version. And this is not counting the fact that you will have to hack to pass a non type template parameter from Cython since this is not yet supported.

So my short answer would be : write a real C++ wrapper file.