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Showing content from https://arduinojson.org/v6/api/config/enable_arduino_string/ below:

ARDUINOJSON_ENABLE_ARDUINO_STRING | ArduinoJson 6

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.

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);

You can use a String to get an element of a JsonObject
```
 long time = doc[String("time")];
```

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

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

You can get a String from a JsonObject or JsonArray
```
 String sensor = doc["sensor"];
```

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

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

You can also concatenate strings

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

You can compare the content of a JsonObject with a String

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

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>();