Serialize JSON as response to HTTP request

Question

I'd like my ESP8266 to provide some JSON endpoints. I want to use ArduinoJson for serialisation. While this works in general if I serialize into a String and return this String afterwards, I was wondering if there is really no way to do this without wasting so much memory. Shouldn't it be possible to do something like this? Any hints?

ESP8266WebServer server;
StaticJsonDocument<768> response;
response["night"] = _playback.isNight();
response["shuffle"] = _playback.isShuffle();
server.send_P(200, "application/json", NULL);
serializeJson(response, server.client());

(this gives the following error:)

no instance of overloaded function "serializeJson" matches the argument list -- argument types are: (ArduinoJson6141_0000010::StaticJsonDocument<768U>, WiFiServer::ClientType)

• Edgar Bonet: Did you look at the documentation of ArduinoJson? Did you try? Did it work? If not, what errors did you get?
• Nitek: Yes I did. Yes I tried. No it did not. I added the error message.
• dandavis: you can just build it as a string. imho, the big advantage of the json lib is parsing, not serializing.
• Edgar Bonet: Weird. WiFiServer::ClientType is defined as WiFiClient, which inherits from Client, which inherits from Stream, which inherits from Print, which you should be able to serialize to. Could you try to serialize to Serial, just to see if it works?
• Nitek: That’s what I thought! I serialized into a char[] and it works fine

Answer 1

serializeJson() accepts a Print&, i.e, a reference to a class that implements the Print interface.

WiFiClient does implement the Print interface, but serializeJson() cannot make a reference to it because ESP8266WebServer::client() returns a temporary. Indeed, C++ forbids references to temporary variables, only const reference to temporaries are allowed.

To fix this compilation error, you must extract a variable:

WiFiClient client = server.client();
serializeJson(doc, client);

The way I see it, ESP8266WebServer::client() should return a WiFiClient& instead of a WiFiClient, just as HTTPClient::getStream() does. I opened issue #7075 about that.

Anyway, even it this fixes the compilation, I'm not sure it will work as you intend because ESP8266WebServer may not allow writing directly to the WiFiClient.