On Thu, Sep 10, 2020 at 10:01 AM Edward Diener via Boost <boost@lists.boost.org> wrote:
Did I miss something or did std::optional ( or boost::optional ) go away as a means of saying that some value does not exist ? The nullptr is for a null pointer. Using it otherwise seems wrong, even if the value is the same size of a nullptr.
"null" in JSON does not mean "value does not exist." It means... well, it is a null. Some people use the word monostate, which I find pretentious. optional, std or otherwise, has nothing to do with this. A JSON variant container needs a way to have a null assigned to it, just like all the other types (integers, floating point). Example: json::value jv; jv = 1; // assign an integer assert( jv.is_int64() ); jv = 3.14; // assign a double assert( jv.kind() == json::kind::double_ ); jv = nullptr; // assign a null assert( jv.is_null() ); I could have gone with this syntax: jv = json::null; by providing my own type and constant: namespace json { struct null_t {}; inline static null_t null; } but nullptr is quite close to it and entirely accessible by the common folk that this library targets, and this avoids an unnecessary proliferation of small types. Regards