JsonVariant::as<T>()

Description

JsonVariant::as<T>() casts the value pointed by the JsonVariant to the specified type.

Unlike JsonVariant::to<T>(), this function doesn’t modify the value pointed by the JsonVariant.

Signatures

bool               as<bool>() const;

float              as<float>() const;
double             as<double>() const;

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

const char*        as<char*>() const;  // ⛔ removed in 6.20
const char*        as<const char*>() const;
String             as<String>() const;
std::string        as<std::string>() const;

JsonArray          as<JsonArray>() const;
JsonObject         as<JsonObject>() const;
JsonVariant        as<JsonVariant>() const;

JsonArrayConst     as<JsonArrayConst>() const;
JsonObjectConst    as<JsonObjectConst>() const;
JsonVariantConst   as<JsonVariantConst>() const;

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

Return value

JsonVariant::as<T>() returns the value pointed by the JsonVariant casted to the specified type.

This function returns a default value if the cast is not possible. The default value is:

If you want to change the default value, you need to use JsonVariant::operator| instead.

Integer overflows

JsonVariant::as<T>() is aware of integer overflows and only returns a value if it can fit in the specified type.

For example, if the value contains 512, as<char>() returns 0, but as<int>() returns 512.

This feature was added in ArduinoJson 6.10.0

User-defined types 🆕

JsonVariant::as<T>() supports user-defined types by calling convertFromJson(). For example, to support tm, you must define:

void convertFromJson(JsonVariantConst src, tm& dst) {
  strptime(src.as<const char*>(), "%FT%TZ", &dst);
}

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

as<T>() vs to<T>()

JsonVariant::as<T>() and JsonVariant::to<T>() look similar but are very different: the former reads the value, whereas the latter changes the reference.

Suppose the JsonVariant refers to an object:

  • JsonVariant::as<JsonObject>() returns a JsonObject pointing to this object.
  • JsonVariant::to<JsonObject>():
    1. creates a new object in the JsonDocument,
    2. makes the JsonVariant point to the new object,
    3. returns a JsonObject pointing to it.

JsonVariant::to<T>() allows creating an empty object or an empty array, and it’s probably its only legitimate use. Calling JsonVariant::to<T>() with a T other than JsonArray or JsonObject works but has no practical use.

JsonVariant::as<T>(), on the other hand, allows solving situations where implicit casts don’t work; for example, when you call a function with several matching overloads (like Serial::print()) or when you use type deduction with the auto keyword (see example below).

Remark that JsonVariantConst, which is a read-only version of JsonVariant, supports as<T>() but not to<T>().

Example

StaticJsonDocument<256> doc;
deserializeJson(doc, "{\"answer\":42}");

JsonVariant answer = doc["answer"];
auto i = answer.as<int>();         // i == 42
auto d = answer.as<double>();      // d == 42.0
auto s = answer.as<const char*>(); // s == NULL

See also