ARDUINOJSON_ENABLE_ARDUINO_STRING

Description

The macro ARDUINOJSON_ENABLE_ARDUINO_STRING enables the support of the String class in the library.

The default is 1 of ARDUINO is defined, 0 otherwise. In other words, ArduinoJson supports the String class as soon as you work in an Arduino-compatible environment.

How to force the value?

If you need to force the support of String, add this at the top of your program:

#define ARDUINOJSON_ENABLE_ARDUINO_STRING 1
#include <ArduinoJson.h>

On the other hand, if you need to disable String, do:

#define ARDUINOJSON_ENABLE_ARDUINO_STRING 0
#include <ArduinoJson.h>

Only 0 and 1 are valid. Any other value (like false or true) will produce a compilation error.

Where ArduinoJson supports the String class?

Once enabled, you can use a String in many places.

1. You can use a String as your JSON input

    // WARNING: ArduinoJson duplicates the String in the JsonDocument
    String input = "{\"sensor\":\"gps\",\"time\":1351824120,\"data\":[48.756080,2.302038]}";
    deserializeJson(doc, input);
   

2. You can use a String to get an element of a JsonObject

    long time = doc[String("time")];
   

3. You can use a String to set an element of a JsonObject

    // WARNING: ArduinoJson duplicates the String in the JsonDocument
    doc[String("time")] = time;
   

4. You can get a String from a JsonObject or JsonArray

    String sensor = doc["sensor"];
   

5. You use a String as a value for a JsonObject or JsonArray

    // WARNING: ArduinoJson duplicates the String in the JsonDocument
    doc["sensor"] = sensor;
   

6. You can also concatenate strings

    // WARNING: ArduinoJson duplicates the String in the JsonDocument
    doc[String("sen") + "sor"] = String("gp") + "s";
   

7. You can compare the content of a JsonObject with a String

    if (doc["sensor"] == sensor) {
        // ...
    }
   

8. You can print the resulting JSON to a String

    String output;
    serializeJson(doc, output);
   

The one place where it doesn’t work ⚠️

Unfortunately, the following doesn’t work (issue #118):

String sensor = "something";
sensor = doc["sensor"]; // <-  error "ambiguous overload for 'operator='"

This line is ambiguous because the compiler cannot tell which constructor of String to call. Is it the one taking a const char*, an int, or a float?

To solve this ambiguity, you must explicitly cast the JsonVariant to a String:

sensor = doc["sensor"].as<String>();

See also

• ARDUINOJSON_ENABLE_STD_STRING
• 8 tips to use the String class efficiently