json schema support default value

[jbbjarnason]

Looking at the specification of json schema https://json-schema.org/understanding-json-schema/reference/generic.html I see that the keyword default is supported.

From glaze/json/schema.hpp

      struct schema_ref
      {
         std::string_view ref{};
         std::optional<std::string_view> description{};

         struct glaze
         {
            using T = schema_ref;
            static constexpr auto value = glz::object("$ref", &T::ref,                //
                                                      "description", &T::description  //
            );
         };
      };

      struct schema
      {
         std::optional<std::vector<std::string_view>> type{};
         std::optional<std::string_view> _const{};
         std::optional<std::string_view> description{};
         std::optional<std::map<std::string_view, schema_ref, std::less<>>> properties{};  // glaze_object
         std::optional<schema_ref> items{};                                                // array
         std::optional<std::variant<bool, schema_ref>> additionalProperties{};             // map
         std::optional<std::map<std::string_view, schema, std::less<>>> defs{};
         std::optional<std::vector<std::string_view>> _enum{};  // enum
         std::optional<std::vector<schema>> oneOf{};
      };

I am not sure whether declaring default of a schema variable is supported by glaze?

[mwalcott3]

No its not supported. The json schema stuff is fairly limited and was mainly to help auto generate a schema for a meta object that can be used when filling out config files for autocomplete/linting. The full schema spec is pretty large so I imagine less than 5% of it is supported. Id be more apt to support a limited spec like https://jsontypedef.com/ if it was more widely used.

We could 100% support default constructing the object (or it being passed in as an optional parameter) and get the default values from it if that would be useful.

[jbbjarnason]

We could 100% support default constructing the object (or it being passed in as an optional parameter) and get the default values from it if that would be useful.

I think it would be beneficial to default to default construction, and being able to pass optional parameter with explicit value of the given type(being converted to schema) as default.

Something like the following:

struct foo {
  int a{11};
  int b{32};
  struct glaze{
    static constexpr auto value{ "a", &foo::a, "b", &foo::b };
  }
}

void function() {
  glz::write_json_schema<foo>(); // would use foo{} as defaults
  glz::write_json_schema<foo>(foo{.a=2,.b=3}); // would use the input param defaults
}

[stephenberry]

While feasible, I think this has the potential of being confusing, as this allows the meaning of default to change on the C++ side. There are also a lot of complexities when it comes to walking large structures and the need to be able to default initialize structures. It also does not affect validation, and autocomplete can be annoying for large objects that are given defaults.

I think it is best to write out default values in the JSON comments until we have a well thought out approach on how this should be handled. I do not think this is a priority, but I'm open to seeing more use cases if this is seen by others as a significant issue.