Adding value_from support without including a Boost.JSON header

[pdimov]

It's important for class authors to be able to write the appropriate tag_invoke overload for their types without needing to include a Boost.JSON header (in order to not require Boost.JSON to be present when not used.) A minimal example is https://godbolt.org/z/jqMj4o4j4. The immediate problem is that value_from can't be forward declared because it has a default argument.

[vinniefalco]

These declarations should have comments explaining the rationale for their signatures

[pdimov]

The easiest fix here is to change

template<class T> value value_from( T&& t, storage_ptr sp = {} );

to

template<class T> value value_from( T&& t );
template<class T> value value_from( T&& t, storage_ptr sp );

[pdimov]

... however, doing so still doesn't enable the code to compile, because value_from returns value by value, and value is incomplete, so the API as-is can't be used with only forward declarations.

So instead of changing the above declaration, we need to introduce another API that works when value is incomplete:

template<class T> void value_from_( value& v, T&& t );

with some better name.

The tag_invoke overload then becomes

template<class W> void tag_invoke( boost::json::value_from_tag const&, W& w, X const& x )
{
    auto& obj = w.emplace_object();
    value_from_( obj[ "m" ], x.m );
}

which is even easier to use and automatically uses the right storage. (And the signature matches that of tag_invoke.)

[vinniefalco]

we need to introduce another API

Is this a public API?

[pdimov]

How would anyone use it otherwise?