add std::formatter for json::string_view and json::string

[hj2000]

We can print json::string and json::string_view with ostream,but we cann't use std::format to format them ,could we have std::formatterjson::string and std::formatterjson::string_view?

[grisumbras]

For future reference:

std::format(fmt, args...) defers to std::vformat(fmt.get(), make_format_args(args...)).

template< class Context = std::format_context), class... Args >
format-arg-store make_format_args( Args&... args );

typename Context::template formatter_type<std::remove_cv<Argi>> should be a BasicFormatter. format_context::formatter_type<T> is std::formatter<T, char>. Curiously, BasicFormatters are only required to work with instantiations of std::basic_format_context as Context.

So, it seems like the formatter should be something like this:

namespace std
{
template<>
struct formatter<boost::json::string, char>
{
    template<class OIt>
    OIt
    format(boost::json::string const&, std::basic_format_context<OIt, char>&);
};
}

If we want to also support {fmt}, we need basically the same thing, only in a different namespace, so, better do:

namespace boost::json::detail
{
struct formatter<string>
{
    template<class Context>
    Context::iterator
    format(string const&, Context&);
};
}
namespace std { template<> struct formatter<boost::json::string, char> : boost::json::detail::formatter<boost::json::string> {}; }
namespace fmt { template<> struct formatter<boost::json::string> : boost::json::detail::formatter<boost::json::string> {}; }