Optional support for Boost.Optional?

[akrzemi1]

I have a game parser that uses Boost.Spirit X3 into my own data structure. My data structure is a combination of aggregates, std::vector and boost::optional. Just mechanically migrating to Boost.Parser doesn't work, because Boost.Parser recognizes std::optional but not boost::optional.

Is there a possibility to make Boost.Parser recognize boost::optional, given that it already recognizes Boost.Hana? I know it goes against one of the goals of the library. Maybe it doesn't have to be boost::optional directly, but any type sufficiently optional-like?

[tzlaine]

Well, I'll consider it, and I'll leave this issue open. However, I am not very enthusiastic abou this idea. If there are enough people like you who feel that this would be useful, I can probably add this support. I would almost certainly want to add support for the various variants out there too -- it would be too weird to support multiple optionals but only one variant.

In both cases. I would probably require the user to specialize an opt-in constexpr bool, a la std::enable_borrowed_range (see https://en.cppreference.com/w/cpp/ranges/borrowed_range).

[akrzemi1]

I understand. An I understand that Boost.Parser as standalone component doesn't need it.

It is the migration from Boost.Spirit to Boost.Parser that becomes bumpy. I cannot just replace one optional for another in my data structure, as other parts of the program depend on it.

[tzlaine]

Turns out this was pretty easy, so I just did it.

[akrzemi1]

Would you be willing to go a step further with this support and add something like:

#ifdef BOOST_OPTIONAL_INCLUDED
template<typename T>
constexpr bool boost::parser::enable_optional<boost::optional<T>> = true;
#endif

[akrzemi1]

Why do I need to

 #include <boost/optional/optional_io.hpp>

To make my parser compile? Why does parser need to print my attribute?

[tzlaine]

Why do I need to

 #include <boost/optional/optional_io.hpp>

To make my parser compile? Why does parser need to print my attribute?

Yeah, about that. boost::optional defines an operator<< that static_asserts, telling you to include optional_io.hpp. IOW, it advertizes that it implements op<<, but then just breaks everything. This breaks the op<< detection logic in detail/printing.hpp.

I had a TODO t odocument this, but today I realized I could work around it, by changing the printing logic a bit. The fix is on develop if you want to try it.

[tzlaine]

Would you be willing to go a step further with this support and add something like:

#ifdef BOOST_OPTIONAL_INCLUDED
template<typename T>
constexpr bool boost::parser::enable_optional<boost::optional<T>> = true;
#endif

No, I think it's pretty easy to define that variable specialization, and I don't want to priveledge this non-standard optional over all the others, or encourage people to keep using boost::optional when std::optional is s much more widely used.