JsonVariantConst is an immutable version of JsonVariant; only use it if you want to enforce immutability.

Description

JsonVariantConst::is<T>() tests if the variant is currently holding a value of type T.

Signatures

bool is<bool>() const;

bool is<float>() const;
bool is<double>() const;

bool is<signed char>() const;
bool is<unsigned char>() const;
bool is<signed int>() const;
bool is<unsigned int>() const;
bool is<signed short>() const;
bool is<unsigned short>() const;
bool is<signed long>() const;
bool is<unsigned long>() const;
bool is<unsigned long long>() const;   // ⚠️ may require ARDUINOJSON_USE_LONG_LONG
bool is<signed long long>() const;     // ⚠️ may require ARDUINOJSON_USE_LONG_LONG
bool is<signed __int64>() const;       // ⚠️ may require ARDUINOJSON_USE_INT64
bool is<unsigned __int64>() const;     // ⚠️ may require ARDUINOJSON_USE_INT64

bool is<char*>() const;  // ⚠️ deprecated since 6.18
bool is<const char*>() const;

bool is<JsonArray>() const;    // ⚠️ always returns false
bool is<JsonObject>() const;   // ⚠️ always returns false
bool is<JsonVariant>() const;  // ⚠️ always returns false

bool is<JsonArrayConst>() const;
bool is<JsonObjectConst>() const;
bool is<JsonVariantConst>() const;

bool is<TEnum>() const; // 🆕 alias of as<int>() (added in 6.15.2)
bool is<T>() const;     // 🆕 calls user-defined function (added in 6.18.0)

Return value

  • true if the variant is currently holding a value of type T,
  • false if not

JSON types vs. C++ types

Different C++ types can store the same JSON value, so is<T>() can return true for several Ts. For example, is<float>() always returns the same value as is<double>() .

The table below gives the correspondence between the JSON type and the C++ types:

JSON type T
Floating point float, double
Integer int, short, long, long long
String const char*, char*
Boolean bool
Array JsonArray
Object JsonObject

Caution: is<float>() and is<double>() return true for integers too.

Integer overflows

JsonVariantConst::is<T>() is aware of integer overflows and only returns true if the specified type can store the value.

For example if the variant contains 512, is<char>() returns false, but is<int>() return true.

This feature was added in ArduinoJson 6.10.0

User-defined types 🆕

JsonVariantConst::is<T>() supports user-defined types by calling canConvertFromJson(). For example, to support an hypothetical Complex class, we could write:

bool canConvertFromJson(JsonVariantConst src, const Complex&) {
  return src["real"].is<double>() && src["imag"].is<double>();
}

The second parameter of canConvertFromJson() is required to trigger ADL but must not be used by the function.

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

See also