CODEWITHSUNDEEP
pythonc++swig
Moses
Moses

Reputation: 32

wrapping c++ vector reference with swig

I have been trying to use swig to wrap a c++ code of the form where the function takes a vector reference (to enable modification):

//cpp_code.cpp

void worker(vector<int> &v){
    // do something
}

// cpp_code.i

/* File: cpp_code.i */
%module cpp_code
%include "std_vector.i"
%include "typemaps.i"
%{ 
    #include "cpp_code.cpp"
%}

%template(MyVector) std::vector<int>;
//%template(MyVectorPtr) std::vector<int &>;

%include "cpp_code.cpp"

void worker(vector<int> &v);

If I uncomment the part //%template(MyVectorPtr) std::vector<int &>;, I receive a very long traceback:

In file included from C:/ProgramData/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/x86_64-w64-mingw32/bit+allocator.h:33,
                 from C:/ProgramData/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/allocator.h:46,
                 from C:/ProgramData/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/string:41,
                 from C:/ProgramData/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/stdexcept:39,
                 from dft_wrap.cxx:2743:
C:/ProgramData/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-rev0/mingw64/lib/gcc/x4-w64-mingw32/8.1.0/include/c++/ext/new_allocator.h: In instantiation of 'cl__gnu_cxx::new_allocator<int&>':
C:/ProgramData/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-rev0/mingw64/lib/gcc/x4-w64-mingw32/8.1.0/include/c++/bits/allocator.h:108:11:   required from 'clstd::allocator<int&>'
C:/ProgramData/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-rev0/mingw64/lib/gcc/x4-w64-mingw32/8.1.0/include/c++/bits/stl_vector.h:84:21:   required from 'st std::_Vector_base<int&, std::allocator<int&> >'
C:/ProgramData/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-rev0/mingw64/lib/gcc/x4-w64-mingw32/8.1.0/include/c++/bits/stl_vector.h:339:11:   required from 'c std::vector<int&, std::allocator<int&> >'
dft_wrap.cxx:4748:45:   required from here
C:/ProgramData/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-rev0/mingw64/lib/gcc/x4-w64-mingw32/8.1.0/include/c++/ext/new_allocator.h:63:26: error: forming por to reference type 'int&'
       typedef _Tp*       pointer;
                          ^~~~~~~
C:/ProgramData/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-rev0/mingw64/lib/gcc/x4-w64-mingw32/8.1.0/include/c++/ext/new_allocator.h:64:26: error: forming por to reference type 'int&'
       typedef const _Tp* const_pointer;
                          ^~~~~~~~~~~~~
In file included from C:/ProgramData/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/string:41,
                 from C:/ProgramData/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/stdexcept:39,
                 from dft_wrap.cxx:2743:
C:/ProgramData/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-rev0/mingw64/lib/gcc/x4-w64-mingw32/8.1.0/include/c++/bits/allocator.h: In instantiation of 'class::allocator<int&>':
C:/ProgramData/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-rev0/mingw64/lib/gcc/x4-w64-mingw32/8.1.0/include/c++/bits/stl_vector.h:84:21:   required from 'st std::_Vector_base<int&, std::allocator<int&> >'
C:/ProgramData/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-rev0/mingw64/lib/gcc/x4-w64-mingw32/8.1.0/include/c++/bits/stl_vector.h:339:11:   required from 'c std::vector<int&, std::allocator<int&> >'
dft_wrap.cxx:4748:45:   required from here
C:/ProgramData/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-rev0/mingw64/lib/gcc/x4-w64-mingw32/8.1.0/include/c++/bits/allocator.h:113:26: error: forming pointo reference type 'int&'
       typedef _Tp*       pointer;
                          ^~~~~~~
C:/ProgramData/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-rev0/mingw64/lib/gcc/x4-w64-mingw32/8.1.0/include/c++/bits/allocator.h:114:26: error: forming pointo reference type 'int&'
       typedef const _Tp* const_pointer;
                          ^~~~~~~~~~~~~
In file included from C:/ProgramData/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/ext/alloc_traits.h:36,
                 from C:/ProgramData/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/basic_string.h:40
                 from C:/ProgramData/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/string:52,
                 from C:/ProgramData/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/stdexcept:39,
                 from dft_wrap.cxx:2743:

The wrapped file was created without issues, it also compiled, now on attempt to use it in python;

PS C:\Users\me\graphy\graphy> python Python 3.7.6 (default, Jan 8 2020, 20:23:39) [MSC v.1916 64 bit (AMD64)] :: Anaconda, Inc. on win32

Warning:
This Python interpreter is in a conda environment, but the environment has
not been activated.  Libraries may fail to load.  To activate this environment
please see https://conda.io/activation

Type "help", "copyright", "credits" or "license" for more information.
>>> from cpp_code import *
>>> worker([5])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\me\graphy\graphy\cpp_code.py", line 233, in cpp_code
    return _cpp_code.worker(v)
TypeError: in method 'cpp_code', argument 1 of type 'vector< int > &'
>>>

How do I 'cast' the reference to a python list, I have checked the documentation and a host of stackOverflow but to no avail, any help at all is welcome.

Thanks.

Upvotes: 0

Views: 832

Answers (1)

Mark Tolonen
Mark Tolonen

Reputation: 177554

To modify a vector, you have to create an instance of it. It can't directly modify the Python list you tried to pass. Here's a working example:

%module test

%{
#include <vector>
void worker(std::vector<int> &v) {
    v.push_back(123);
}
%}

%include <std_vector.i>
%template(MyVector) std::vector<int>;

void worker(std::vector<int> &v);

Demo:

>>> import test
>>> v = test.MyVector([1,2,3])
>>> v
<test.MyVector; proxy of <Swig Object of type 'std::vector< int > *' at 0x000001EDE38F87E0> >
>>> list(v)  # to view it as a Python object
[1, 2, 3]
>>> test.worker(v)
>>> list(v)
[1, 2, 3, 123]

Note v is the proxy of a vector, so you can access vector methods:

>>> v.size()
4
>>> v[-1]
123
>>> v.pop()
123
>>> v.push_back(7)
>>> v.size()
4
>>> list(v)
[1, 2, 3, 7]

Upvotes: 1

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