parsing: switching from json to ordered_json

[robinsch]

Imagine I have this code piece which parses a json to a lua table:

void settable_from_json(LuaState* state, nlohmann::json json)
{
	lua_createtable(state, 0, 0);
	lua_checkstack(state, 2);

	for (auto const& item : json.items())
		parse_json_item(item, state);
}

parse_json_item contains this and can call itself recursivly to parse arrays/objects:

void parse_json_item(nlohmann::json_abi_v3_11_2::detail::iteration_proxy_value<nlohmann::json_abi_v3_11_2::detail::iter_impl<nlohmann::json_abi_v3_11_2::json>> const& val, LuaState* state)
{
	// key
	if (is_number(val.key()))
		lua_pushinteger(state, std::atoi(val.key().c_str()) + 1);
	else
		lua_pushstring(state, val.key().c_str());

	// value
	switch (val.value().type())
	{
		case nlohmann::json::value_t::array:
		case nlohmann::json::value_t::object:
		{
			lua_createtable(state, 0, 0);
			lua_checkstack(state, 2);

			for (auto const& val : val.value().items())
				parse_json_item(val, state);
			break;
		}
		case nlohmann::json::value_t::boolean:
			lua_pushboolean(state, val.value().get<bool>());
			break;
		case nlohmann::json::value_t::string:
			lua_pushstring(state, val.value().get<std::string>().c_str());
			break;
		case nlohmann::json::value_t::number_integer:
		case nlohmann::json::value_t::number_unsigned:
			lua_pushinteger(state, val.value().get<int32>());
			break;
		case nlohmann::json::value_t::number_float:
			lua_pushnumber(state, val.value().get<float>());
			break;
		default:
			lua_pushnil(state);
	}

	lua_settable(state, -3);
}

When changing the type to ordered_json in settable_from_json and parse_json_item I get greeted by this error on MSVC 19:

Error	C2039	'difference_type': is not a member of 'nlohmann::json_abi_v3_11_2::detail::iter_impl<nlohmann::json_abi_v3_11_2::ordered_json>'

Any hints on how to solve this?

[falbrechtskirchinger]

A few observations upfront:

• Don't use types from the detail namespace directly (and json_abi_v3_11_2 is an inline namespace and should be omitted in this context). I recommend you pass item.key() and item.value() to parse_json_item().
• You should pass any nlohmann::basic_json by (const) reference.
• lua_pushstring(state, val.value().get<std::string>().c_str()) – I hope lua_pushstring creates a copy of the string, otherwise your string will point to invalid memory after the end of the statement. .get_ref<const std::string&>().c_str() avoids this issue and skips creating an unnecessary temporary.

For your primary issue, I'll need to take a look at the source and will update this post shortly.

Edit: Can you provide the full error message or at least point out the line that triggers the error? difference_type should always be defined.

[robinsch]

json.hpp(19908,58): error C2039: 'difference_type': is not a member of 'nlohmann::json_abi_v3_11_2::detail::iter_impl<nlohmann::json_abi_v3_11_2::ordered_json>'
FrameScript.cpp(9): message : see declaration of 'nlohmann::json_abi_v3_11_2::detail::iter_impl<nlohmann::json_abi_v3_11_2::ordered_json>'
json.hpp(12843): message : see reference to class template instantiation 'nlohmann::json_abi_v3_11_2::basic_json<nlohmann::json_abi_v3_11_2::ordered_map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::json_abi_v3_11_2::adl_serializer,std::vector<uint8_t,std::allocator<uint8_t>>>' being compiled
json.hpp(5122): message : see reference to class template instantiation 'nlohmann::json_abi_v3_11_2::detail::iter_impl<nlohmann::json_abi_v3_11_2::ordered_json>' being compiled
FrameScript.cpp(12): message : see reference to class template instantiation 'nlohmann::json_abi_v3_11_2::detail::iteration_proxy_value<nlohmann::json_abi_v3_11_2::detail::iter_impl<nlohmann::json_abi_v3_11_2::ordered_json>>' being compiled

https://github.com/nlohmann/json/blob/develop/single_include/nlohmann/json.hpp#L19948

FrameScript:9 is the parse_json_item definition.
FrameScript:12 is the first access to passed val

[robinsch]

I adjust the code to your suggestions, I am unsure how to pass item.value() (what type should I use to pass it for ordered_json)?

EDIT:

I think I solved the issue by passing key and value, thanks for the fast and good hints!

void parse_json_item(std::string const& key, nlohmann::ordered_json::iterator::reference const& value, LuaState* state)

Is this the correct way to pass value?

[falbrechtskirchinger]

I adjust the code to your suggestions, I am unsure how to pass item.value() (what type should I use to pass it for ordered_json)?

item.value() is the nlohmann::basic_json type, i.e., nlohmann::ordered_json.

I'm not sure what's causing this error...

[falbrechtskirchinger]

void parse_json_item(std::string const& key, nlohmann::ordered_json::iterator::reference const& value, LuaState* state)

Is this the correct way to pass value?

That's fine. You can simplify it as suggested above, but that's not required.

I'm now able to reproduce the core issue. Looks like you uncovered a bug. Glad you were able to work around it!