When the JsonVariant points to an array, JsonVariant::add() appends a value to the array. Otherwise, this function does nothing.

In other words, JsonVariant::add() reproduces JsonArray::add().


bool add(bool value) const;

bool add(float value) const;
bool add(double value) const;

bool add(signed char value) const;
bool add(signed long value) const;
bool add(signed int value) const;
bool add(signed short value) const;
bool add(unsigned char value) const;
bool add(unsigned long value) const;
bool add(unsigned int value) const;
bool add(unsigned short value) const;

bool add(char *value) const;                       // stores by copy
bool add(const char *value) const;                 // stores by pointer
bool add(const __FlashStringHelper *value) const;  // stores by copy

bool add(const String &value) const;       // stores by copy
bool add(const std::string &value) const;  // stores by copy
bool add(const Printable& value)           // 🆕 (added in 6.18.0)

bool add(JsonArray array) const;       // stores by copy
bool add(JsonObject object) const;     // stores by copy
bool add(JsonVariant variant) const;   // stores by copy

bool add(TEnum value) const;  // 🆕 alias of add(int) (added in 6.15.2)
bool add(T value) const;      // 🆕 calls user-defined function (added in 6.18.0)


value: the value to append to the array.

If you pass a JsonArray, a JsonObject, or a JsonVariant, JsonVariant::add() makes a complete clone of the argument. In other words, the value is stored by copy, not by reference.

As usual, ArduinoJson makes a copy of a string in the JsonDocument, except if it’s a const char*.

Return value

JsonVariant::add() returns a bool that tells whether the operation was successful or not:

  • true if the value was successfully added.
  • false if there was not enough memory in the JsonDocument.

String duplication

ArduinoJson makes a copy of the string when you call this function with one of the following types:

This duplication consumes some space in the JsonDocument; don’t forget to increase its capacity accordingly.

User-defined types 🆕

JsonVariant::add() supports user-defined types by calling convertToJson(). For example, to support tm, you must define:

bool convertToJson(const tm& src, JsonVariant dst) {
  char buf[32];
  strftime(buf, sizeof(buf), "%FT%TZ", &src);
  return dst.set(buf);

This feature was added in ArduinoJson 6.18.0, see article for details.

See also