[Swig-devel] Telling SWIG whether it's a value or a reference

[David P. <dpi...@me...>]

At the beginning of October there was a discussion about how to return a
std::wstring from a synthetic attribute. The problem was that SWIG
assumed the attribute was a reference when it wasn't. For example:

%attribute(Foo, std::wstring, FileName, GetFileName);
%inline %{
   #include <string>
   class Foo {
     public:
       std::wstring GetFileName() { return L"file.txt"; }
   };
%}

SWIG generates this code that doesn't work because it treats FileName as
a reference when it isn't:

SWIGEXPORT void * SWIGSTDCALL CSharp_Foo_FileName_get(void * jarg1) {
  void * jresult ;
  Foo *arg1 =3D (Foo *) 0 ;
  std::wstring *result =3D 0 ;
 =20
  arg1 =3D (Foo *)jarg1;=20
  {
    std::wstring const &_result_ref =3D Foo_FileName_get(arg1);
    result =3D (std::wstring *) &_result_ref;
  }
  jresult =3D SWIG_csharp_wstring_callback(result->c_str());=20
  return jresult;
}

William came up with a solution involving allocating a temporary copy of
the string on the heap--needed only because there was no way to tell
SWIG that the return value was a value, not a reference.

Well, today I have the opposite problem. The .NET Compact Framework's
P/Invoke system is impoverished and does not allow a 'double' or 'float'
to be returned from a function. Consequently, it is only  possible to
return a pointer to the value, and I'm pretty sure I could extract the
value from the pointer when it arrives in C#.

For example, let's say I want SWIG to wrap this class:

%inline %{
 struct PointF {
	 float X;
	 float Y;
 };
 %}

I have changed the typemaps so that the value given to
CSharp_PointF_X_set can be passed by reference. But it appears there's
no way to do the same for CSharp_PointF_X_get, because SWIG always
treats float as a value:

SWIGEXPORT float SWIGSTDCALL CSharp_PointF_X_get(void * jarg1) {
  float jresult ;
  PointF *arg1 =3D (PointF *) 0 ;
  float result;
 =20
  arg1 =3D (PointF *)jarg1;=20
  result =3D (float) ((arg1)->X);
  jresult =3D result;=20
  return jresult;
}

AFAIK, no typemap or feature can affect how X is accessed, or the cast
that happens afterward, or the type of the result variable. Thus, a
pointer to X is currently impossible to obtain.

So, I'd like to suggest that there should be a way to control whether
SWIG treats return values as references or values. But I'm not sure
about how it ought to work. Can anyone offer thoughts?

- David