How to expose std::vector<int> as a Python list using SWIG?

Question

I'm trying to expose this function to Python using SWIG:

std::vector<int> get_match_stats();

And I want SWIG to generate wrapping code for Python so I can see it as a list of integers.

Adding this to the .i file:

%include "typemaps.i"
%include "std_vector.i"

namespace std
{
  %template(IntVector) vector<int>;
}

I'm running SWIG Version 1.3.36 and calling swig with -Wall and I get no warnings.

I'm able to get access to a list but I get a bunch of warnings when compiling with -Wall (with g++ (GCC) 4.2.4 ) the generated C++ code that say:

  warning: dereferencing type-punned pointer will break strict-aliasing rules

Am I exposing the function correctly? If so, what does the warning mean?

These are the lines before the offending line in the same function:

SWIGINTERN PyObject *_wrap_IntVector_erase__SWIG_0(PyObject *SWIGUNUSEDPARM(self), PyObject *args) {
  PyObject *resultobj = 0;
  std::vector *arg1 = (std::vector *) 0 ;
  std::vector::iterator arg2 ;
  std::vector::iterator result;
  void *argp1 = 0 ;
  int res1 = 0 ;
  swig::PySwigIterator *iter2 = 0 ;
  int res2 ;
  PyObject * obj0 = 0 ;
  PyObject * obj1 = 0 ;

  if (!PyArg_ParseTuple(args,(char *)"OO:IntVector_erase",&obj0,&obj1)) SWIG_fail;
  res1 = SWIG_ConvertPtr(obj0, &argp1,SWIGTYPE_p_std__vectorT_int_std__allocatorT_int_t_t, 0 |  0 );
  if (!SWIG_IsOK(res1)) {
    SWIG_exception_fail(SWIG_ArgError(res1), "in method '" "IntVector_erase" "', argument " "1"" of type '" "std::vector *""'"); 
  }
  arg1 = reinterpret_cast * >(argp1);

And this is the offending line:

  res2 = SWIG_ConvertPtr(obj1, SWIG_as_voidptrptr(&iter2), swig::PySwigIterator::descriptor(), 0);

More code follows that.

The warning generated when compiling with g++ 4.2.4 is:

swig_iss_wrap.cxx: In function ‘PyObject* _wrap_IntVector_erase__SWIG_0(PyObject*, PyObject*)’:
swig_iss_wrap.cxx:5885: warning: dereferencing type-punned pointer will break strict-aliasing rules

More recent swig version still generate code that cause warnings like this. Use -fno-strict-aliasing to get g++ to accept the swig wrapper without issues. — J Quinn
– J Quinn, Commented Nov 4, 2021 at 0:13

Mr Fooz · Accepted Answer · 2008-11-10 11:44:15Z

14

%template(IntVector) vector<int>;

answered Nov 10, 2008 at 11:44

Mr Fooz

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7 Comments

Marcos Lara Over a year ago

That's what I tried and I get a bunch of warnings when compiling with g++. Any ideas?

Mr Fooz Over a year ago

What version of g++ are you using? I'm using 4.1.2 and I don't get any warnings, even with -Wall. I don't remember any warnings back when I was using 3.3 or so either. Note the extra "<int>" in this post that's missing from the original post.

Marcos Lara Over a year ago

I'm using g++ 4.2.4 and trying to build with -Wall -Werror -ansi -pedantic. I just tried building it with just -Wall and still get the warnings.

Mr Fooz Over a year ago

This might be a new 4.2-ism. There have been some big changes under the hood with gcc 4.2. Could you post the generated line of code that's creating this warning?

Seth Johnson Over a year ago

Some of the older versions of SWIG generate sloppy code. GCC 4.2 doesn't like a lot of it.

|

Fergal · Accepted Answer · 2008-12-15 16:40:34Z

0

I don't have much experience with Swig, but are you #including your C++ header file in your .i file? Try one (or both) of

%include "myvector.h"


%{
#   include "myvector.h"
%}

answered Dec 15, 2008 at 16:40

Fergal

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