Re: [Swig-user] boost::optional

[William S F. <ws...@fu...>]

ta...@gm... wrote:
> I want to use boost::optional for optional values in C++.
> Wrapping them with SWIG does not work "out of the box".
> Did anyone manage wrapping boost::optional with SWIG?
> 
> There exists a class boost::none which represents an unset value (comparable 
> to C++'s NULL or Python's None) which I would like to have represented with 
> None in Python.
> 
> Example:
> C++
> ---------
> class Test {
>   public:
>     void setValue(const boost::optional<int>& v);
>     const boost::optional<int>& value() const;
>   private:
>     boost::optional<int> _value;
> };
> 
> In Python I want to work with this class like this:
> 
> t = Test()
> t.setValue(5)  # sets the value to 5
> t.setValue(None)  # sets the value to boost::none
> 
> Maybe there is a way using typemaps?

There is of course a way to do this with typemaps.

The typemap for input int parameters could be:

%typemap(in) int {
  long v = PyLong_AsLong($input);
  $1 = static_cast<int>(v);
}

The above is a rather simplified version of what is in SWIG's library
for int. You'd then modify it slightly for boost::optional:

%typemap(in) const boost::optional<int>& (boost::optional<int> tmp) {
  if ($input == Py_None) {
    tmp = boost::optional<int>();
  } else {
    long v = PyLong_AsLong($input);
    tmp = boost::optional<int>(static_cast<int>(v));
  }
  $1 = &tmp;
}

Of course this could be improved to add in error checking and to take a
generic type and to use with %template. The the out typemap needs
writing too, but this should be enough for you to get going...

William