Add something like Json::StaticString

[ibc]

• Describe the feature in as much detail as possible.

In JsonCpp I can create a "static" JSON string to be used as a Object key with fast access.

• Include sample usage where appropriate.

void doStuff(Json::Value& data)
{
  static const Json::StaticString JsonStringFoo{ "foo" };

  if (!data[JsonStringFoo].isString())
    // Fail here somehow.

  std::string foo = data[JsonStringFoo].asString();
}

The thing here is that data[JsonStringFoo] is supposed to be much faster than using data["foo"] all the time.

AFAIS nlohmann/json does not provide something like that, although it may provide a different approach to get the same result.

[nlohmann]

I do not see why such a string would be faster. Can you elaborate?

[ibc]

It seems to be useful when adding fields with same name to an JSON object:

Value constructor and objectValue member assignement takes advantage of the StaticString and avoid the cost of string duplication when storing the string or the member name.

http://open-source-parsers.github.io/jsoncpp-docs/doxygen/class_json_1_1_static_string.html#details

I assume the optimization just happens here:

static const Json::StaticString JsonStringFoo{ "foo" };

Json::Value data1;
Json::Value data2;

data1[JsonStringFoo] = 111;
data2[JsonStringFoo] = 222; // <-- here the "foo" string is not duplicated internally.

[ibc]

(and also when storing the same string as value).

[nlohmann]

When you pass a string to operator[] it is not copied, but passed as const reference. Internally, we use a std::map which, in case no value was stored so far for this key, must create an entry. In this case, also the static string would be copied into the map's tree.

[ibc]

Understood. May I ask which would be the proper way to check whether a key exists, check its type is the desired one and read the corresponding type value into a C++ variable (without looking for the given key 3 times into the internal map?

[nlohmann]

You can use find, just like for a map.

[ibc]

Do you mean using the Iterator returned by find() to later check the value type and then reading its native value?

[nlohmann]

I’m not sure what you mean. Can you provide an example?

[ibc]

Sure. In libsdptransform (in which I use nlohmann/json) I encourage users to be careful when reading JSON parsed values:

auto h264Fmtp = sdptransform::parseParams(video.at("fmtp")[0].at("config"));
std::string profileLevelId;

if (
  h264Fmtp.find("profile-level-id") != h264Fmtp.end() &&
  h264Fmtp["profile-level-id"].is_string()
)
{
  profileLevelId = h264Fmtp.at("profile-level-id");
}

In this usage example, I'm forcing json library to inspect a key named "profile-level-id" 3 times (in find(), in [] and in at()).

[nlohmann]

Once you make sure the iterator is not end(), you can dereference it to a reference where you can call all member functions.

[ibc]

I wonder if I could do something like:

auto h264Fmtp = sdptransform::parseParams(video.at("fmtp")[0].at("config"));
std::string profileLevelId;

// Iterator
auto it = h264Fmtp.find("profile-level-id");

// Pseudo code here:
if (it != h264Fmtp.end() && it->second.is_string())
  profileLevelId = it->second.get<std::string>();

[ibc]

oh, so it would be:

auto h264Fmtp = sdptransform::parseParams(video.at("fmtp")[0].at("config"));
std::string profileLevelId;

// Iterator
auto it = h264Fmtp.find("profile-level-id");

// Pseudo code here:
if (it != h264Fmtp.end() && it->is_string())
  profileLevelId = it->get<std::string>();

Am I right?

[ibc]

...or just:

profileLevelId = *it;

[ibc]

And this is properly documented in the API, so let's close this issue. Thanks :)