Technique for using std::ifstream, std::ofstream in python via SWIG?

Question

Is there a way to use std::[io]fstream\'s in python via swig?

I have a c-class with functions like:

void readFrom(std::istream& istr         


        

Answer 1

I made a few tweaks to the solution provided by @Flexo

The main change was to use a boost::iostream::stream owned by a unique pointer

Some other differences:

• used a fragment for the iostream header requirements
• removed the "io" namespace alias to avoid conflicts with code where that name is already used
• took advantage of fileno() returning -1 for NULL or invalid FILE* arguments
• use "%#include" "%#else" "%#endif" so the directives are in the wrap file and the same wrap file can be used for python2 and python3
• included his suggestion for flushing the output stream because it violated the principle of least surprise without it
• call Py_DECREF on the result from the python output stream flush() method call - this result is None so the world won't catch fire if None's reference counts grow but it's cleaner and guards against overflowing the refcount

%fragment("iostream_header", "header") %{
#include 
#include 
#include 
#include 
using boost_ofd_stream = boost::iostreams::stream;
using boost_ifd_stream = boost::iostreams::stream;
%}  

%typemap(in, fragment="iostream_header") std::ostream& (std::unique_ptr stream) {
    PyObject *flush_result = PyObject_CallMethod($input, const_cast("flush"), nullptr);
    if (flush_result) Py_DECREF(flush_result);
%#if PY_VERSION_HEX < 0x03000000
    int fd = fileno(PyFile_AsFile($input));
%#else
    int fd = PyObject_AsFileDescriptor($input);
%#endif
    if (fd < 0) { SWIG_Error(SWIG_TypeError, "File object expected."); SWIG_fail; }
    stream = std::make_unique(fd, boost::iostreams::never_close_handle);
    $1 = stream.get();
}   

%typemap(in, fragment="iostream_header") std::istream& (std::unique_ptr stream) {
%#if PY_VERSION_HEX < 0x03000000
    int fd = fileno(PyFile_AsFile($input));
%#else
    int fd = PyObject_AsFileDescriptor($input);
%#endif
    if (fd < 0) { SWIG_Error(SWIG_TypeError, "File object expected.");  SWIG_fail; }
    stream = std::make_unique(fd, boost::iostreams::never_close_handle);
    $1 = stream.get();
}