JsonObject::operator[]

Description

JsonObject::operator[] gets, replaces, or adds a value in the object pointed by the JsonObject.

JsonObject::operator[] is a shortcut for JsonObject::getMember(), JsonObject::getOrAddMember(), and JsonVariant::set().

If the JsonObject is null, this function does nothing.

If you need to know if the insertion succeed, use JsonVariant::set() instead.

Signatures

MemberProxy operator[](char* key) const;
MemberProxy operator[](const char* key) const;
MemberProxy operator[](const __FlashStringHelper* key) const;

MemberProxy operator[](const String& key) const;
MemberProxy operator[](const std::string& key) const;
MemberProxy operator[](std::string_view key) const;  // 🆕 (added in 6.18.1)

Arguments

key: the key that the value will be associated with.

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

Return value

JsonObject::operator[] return a proxy class that allows using the JsonObject like a dictionary.

If this concept of proxy class is unfamiliar to you, just think of a JsonVariant instead.

How does it work?

When used for reading, the MemberProxy calls JsonObject::getMember(). In other words, the two following lines are equivalent:

JsonVariant value = object["key"];
JsonVariant value = object.getMember("key");

When used for writing, the MemberProxy calls JsonObject::getOrAddMember() and JsonVariant::set(). In other words, the two following lines are equivalent:

object["key"] = "value";
object.getOrAddMember("key").set("value");

MemberProxy also exposes the same methods as JsonVariant. For example, all the following lines are correct:

object["key"].as<int>();
object["key"].is<int>();
object["key"]["nested key"] = "nested value";
object["key"].createNestedObject();
object["key"].createNestedArray("nested");
// etc.

String duplication

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

char*
String (or std::string)
const __FlashStringHelper* (i.e. Flash string)

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

Example

StaticJsonDocument<256> doc;
JsonObject object = doc.to<JsonObject>();
object["hello"] = "world";
const char* world = object["hello"];