[Swig-devel] Expanding typemaps in cstype "out" attribute

[Vadim Z. <vz...@ze...>]

 Hello,

 I'm trying to make the following work:

----------------------------------------------------------------------------
%module ov

%inline %{
template <typename T>
struct OptionalValue {
  OptionalValue();
  OptionalValue(const T& value);

  bool IsValid() const;

  const T& Get() const;
};
%}

%define DEFINE_OPTIONAL(T)
    %template(Optional ## T) OptionalValue< T >;
    %typemap(cstype) OptionalValue< T >, const OptionalValue< T >& "$typemap(cstype, T)?"
    %typemap(cstype, out="$typemap(cstype, T)?") OptionalValue< T >* "ref $typemap(cstype, T)?"
%enddef

DEFINE_OPTIONAL(double)

%inline %{
struct S {
    OptionalValue<double> od;
};
%}
----------------------------------------------------------------------------

The idea being that I want to allow handling my OptionalValue<T> objects as
"T?" (a.k.a. Nullable<T>) in C# for the convenience of the code using it.

 There are plenty missing typemaps here, of course, but I already have a
big problem: while the "$typemap(...)" expressions in the main typemap
definition are getting expanded in the output, the value of "out" parameter
appears as is, i.e. just as "$typemap(cstype, double)?" directly in the
output C# code.

 I've tried debugging why does it happen and, after spending an hour or so
(I wish I knew how could I find stuff like this faster in SWIG code base),
found that it was due to replace_embedded_typemap() only being called
inside Swig_typemap_lookup_impl() which is not used for the "out" value
which is used directly by the code in proxyClassFunctionHandler() in
csharp.cxx:

    if ((tm = Swig_typemap_lookup("cstype", n, "", 0))) {
      // Note that in the case of polymorphic (covariant) return types, the method's return type is changed to be the base of the C++ return type
      SwigType *covariant = Getattr(n, "covariant");
      String *cstypeout = Getattr(n, "tmap:cstype:out");	// the type in the cstype typemap's out attribute overrides the type in the typemap
      if (cstypeout)
        tm = cstypeout;
      ...
    }

So I had some hope that I could just call replace_embedded_typemap() from
here (after making it public) and solve my problem. Unfortunately, it's not
as simple as this. First of all, replace_embedded_typemap() expects
"$TYPEMAP()" instead of "$typemap()", which is, according to the comments
in the code, a hack, but I don't know what is this hack for and why is it
needed. Second, even if I use $TYPEMAP() it still doesn't work, apparently
because I don't pass the correct ParmList to it. But I have no idea how to
create a correct one nor even whether I'm trying to do it in the right way,
e.g. perhaps it's the code in Swig_typemap_lookup_impl() itself that should
be doing this expansion? And if so, should it use typemap_replace_vars()
for this?

 I'd really appreciate some advice about how to proceed here because I'm
feeling hopefully lost.

 TIA!
VZ