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

ARDUINOJSON_ENABLE_STD_STRING | ArduinoJson 6

The macro ARDUINOJSON_ENABLE_STD_STRING controls the support of the type std::string in the library.

If ARDUINOJSON_ENABLE_STD_STRING is defined 1, then std::string is supported.

Default value

On C++17-capable compilers, ArduinoJson can detect the presence of the <string> header. In that case, the default value of ARDUINOJSON_ENABLE_STD_STRING is:

1 if <string> is available,
0 otherwise.

On older compilers, ArduinoJson tries to guess whether std::string is available or not, based on the presence of the ARDUINO macro. In that case, the default value of ARDUINOJSON_ENABLE_STD_STRING is:

0 if ARDUINO is defined,
1 otherwise.

In other words, ArduinoJson assumes that std::string is available as soon as you work outside of Arduino.

Examples

If you need to force the support of std::string, add the following line at the top of your program:

#define ARDUINOJSON_ENABLE_STD_STRING 1
#include <ArduinoJson.h>

If for some reason, you need to disable the support for std::string, add the following line at the top of your program:

#define ARDUINOJSON_ENABLE_STD_STRING 0
#include <ArduinoJson.h>

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

Where can I use the std::string class?

Once enabled, you can use a std::string in many places.

As your JSON input:

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

As your JSON output:

 std::string output;
 serializeJson(doc, output);

As a key to extract a value from a JsonDocument:
```
 long time = doc[std::string("time")];
```

As a new key in JsonDocument:

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

To extract a value from a JsonVariant:
```
 std::string sensor = doc["sensor"];
```

To set the value of a JsonVariant:

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

You can also concatenate strings

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

You can compare the content of a JsonVariant with a std::string

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

The one place where it doesn’t work ⚠️

Unfortunately, the following doesn’t work:

std::string sensor(doc["sensor"]); // <-  error: call of overloaded 'basic_string(...)' is ambiguous

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

In C++11, we get a similar problem with the brace-initializer (issue #1498):

std::string sensor{doc["sensor"]}; // <-  error: no matching function for call to 'variantAs(...)'

To solve this ambiguity, the simplest is to use the assignment operator:

std::string sensor = doc["sensor"];
// or
auto sensor = doc["sensor"].as<std::string>();

Alternatively, you can explicitly case the JsonVariant to a const char*:

std::string sensor(doc["sensor"].as<const char*>());