How to SWIG std::string& to C# ref string

Question

I'm trying to convert a C++ function with std::string reference to C#.

My API looks like this:

void GetStringDemo(std::string& str);

Ideally I would like to see something like this from C#

void GetStringDemoWrap(ref string);

I understand I need to create a typemap for this, and I tried a few things by playing around with the std_string.i file, but I don't think I'm getting anywhere. Does anyone has any example. I'm new to both Swig and C# so I'm not able to come up with any genuine ideas.

Answer 1

Just in case somebody is looking for this in the future, I created std_string.i like this for C#. Seems to work for me. Note that I changed the ref to out since it was more appropriate in my case but ref should work as well.

I called %include "std_string.i" from the .i file

/* -----------------------------------------------------------------------------
 * std_string_ref.i
 *
 * Typemaps for std::string& and const std::string&
 * These are mapped to a C# String and are passed around by reference
 *
 * ----------------------------------------------------------------------------- */

%{
#include <string>
%}

namespace std {

%naturalvar string;

class string;

// string &

%typemap(ctype) std::string & "char**"
%typemap(imtype) std::string & "/*imtype*/ out string"
%typemap(cstype) std::string & "/*cstype*/ out string"

//C++
%typemap(in, canthrow=1) std::string &
%{  //typemap in
    std::string temp;
    $1 = &temp; 
 %}

//C++
%typemap(argout) std::string & 
%{ 
    //Typemap argout in c++ file.
    //This will convert c++ string to c# string
    *$input = SWIG_csharp_string_callback($1->c_str());
%}

%typemap(argout) const std::string & 
%{ 
    //argout typemap for const std::string&
%}

%typemap(csin) std::string & "out $csinput"

%typemap(throws, canthrow=1) string &
%{ SWIG_CSharpSetPendingException(SWIG_CSharpApplicationException, $1.c_str());
   return $null; %}

}

The reason I need to define argout for const std::string& is because SWIG will get confused and override const std::string& also with the typemap. So I explicitly tell it not to override in my case (You may have a different use case)

For Python I created something like this:

%typemap(argout)std::string&
{
    //typemap argout std::string&
    PyObject* obj = PyUnicode_FromStringAndSize((*$1).c_str(),(*$1).length());

    $result=SWIG_Python_AppendOutput($result, obj);
}

%typemap(argout) const std::string & 
%{ 
    //argout typemap for const std::string&
%}