Add feature to convert function to subroutine with "optional" result

[sethrj]

HDF5 and other C apps often use error codes for return values. It's also not uncommon in C++ code to return a value that won't necessarily be used (e.g. std::map.insert).

I propose a new %feature("fortran:subroutine") funcname; that will convert a function's return type to an intent(out), optional-qualified dummy variable, turning the function into a subroutine.

[aprokop]

Definitely for it, it gives users of the tool more control.

[sethrj]

This could help with the subroutine/function "generic" conflict, but it could also cause failures if the subroutine with the "optional" output parameter conflicts with another subroutine:

class Foo {
  int myvalue_;
public:
  int myvalue() const { return myvalue_; }
  void myvalue(int new_value) { myvalue_ = new_value; }
};

The getter/setter overload is idiomatic of C++ standard library; so perhaps we could make the parameter not optional if it's a const method with no input parameters? That seems pretty sensible and would allow us to differentiate from:

status_t calculate();

[tjfulle]

Couldn’t you use an interface block for the case of conflicting function names?

   INTERFACE myvalue
      SUBROUTINE myvalue_setter(in_value)
        integer, intent(in) :: in_value
      END SUBROUTINE

      SUBROUTINE myvalue_getter(out_value)
        integer, intent(out) :: out_value
      END SUBROUTINE
    END INTERFACE

[sethrj]

@tjfulle I didn't realize Fortran's module interfaces would allow multiple subroutines where the only difference is the intent. This might not be too bad then.

[tjfulle]

@sethrj, I’m not sure that the intent can distinguish one type from another and allow overloading. I think I jumped the gun.