Qt 5.9 | Qt Core | C++ Classes | QJsonValue

Qt 5.9.9 Reference Documentation

# QJsonValue Class

The QJsonValue class encapsulates a value in JSON. More...

Header: | #include <QJsonValue>
qmake: | QT += core
Since: | Qt 5.0

- List of all members, including inherited members

Note: All functions in this class are reentrant.

## Public Types

enum | Type { Null, Bool, Double, String, ..., Undefined }

## Public Functions

| QJsonValue(Type type = Null)
| QJsonValue(bool b)
| QJsonValue(double n)
| QJsonValue(int n)
| QJsonValue(qint64 n)
| QJsonValue(const QString &s)
| QJsonValue(QLatin1String s)
| QJsonValue(const char *s)
| QJsonValue(const QJsonArray &a)
| QJsonValue(const QJsonObject &o)
| QJsonValue(const QJsonValue &other)
| ~QJsonValue()
bool | isArray() const
bool | isBool() const
bool | isDouble() const
bool | isNull() const
bool | isObject() const
bool | isString() const
bool | isUndefined() const
QJsonArray | toArray(const QJsonArray &defaultValue) const
QJsonArray | toArray() const
bool | toBool(bool defaultValue = false) const
double | toDouble(double defaultValue = 0) const
int | toInt(int defaultValue = 0) const
QJsonObject | toObject(const QJsonObject &defaultValue) const
QJsonObject | toObject() const
QString | toString() const
QString | toString(const QString &defaultValue) const
QVariant | toVariant() const
Type | type() const
bool | operator!=(const QJsonValue &other) const
QJsonValue & | operator=(const QJsonValue &other)
bool | operator==(const QJsonValue &other) const

## Static Public Members

QJsonValue | fromVariant(const QVariant &variant)

## Detailed Description

The QJsonValue class encapsulates a value in JSON.

A value in JSON can be one of 6 basic types:

JSON is a format to store structured data. It has 6 basic data types:

- bool QJsonValue::Bool
- double QJsonValue::Double
- string QJsonValue::String
- array QJsonValue::Array
- object QJsonValue::Object
- null QJsonValue::Null

A value can represent any of the above data types. In addition, QJsonValue has one special flag to represent undefined values. This can be queried with isUndefined().

The type of the value can be queried with type() or accessors like isBool(), isString(), and so on. Likewise, the value can be converted to the type stored in it using the toBool(), toString() and so on.

Values are strictly typed internally and contrary to QVariant will not attempt to do any implicit type conversions. This implies that converting to a type that is not stored in the value will return a default constructed return value.

### QJsonValueRef

QJsonValueRef is a helper class for QJsonArray and QJsonObject. When you get an object of type QJsonValueRef, you can use it as if it were a reference to a QJsonValue. If you assign to it, the assignment will apply to the element in the QJsonArray or QJsonObject from which you got the reference.

The following methods return QJsonValueRef:

- QJsonArray::operator[](int i)
- QJsonObject::operator[](const QString & key) const

See also JSON Support in Qt and JSON Save Game Example.

## Member Type Documentation

### enum QJsonValue::Type

This enum describes the type of the JSON value.

Constant | Value | Description
--- | --- | ---
`QJsonValue::Null` | `0x0` | A Null value
`QJsonValue::Bool` | `0x1` | A boolean value. Use toBool() to convert to a bool.
`QJsonValue::Double` | `0x2` | A double. Use toDouble() to convert to a double.
`QJsonValue::String` | `0x3` | A string. Use toString() to convert to a QString.
`QJsonValue::Array` | `0x4` | An array. Use toArray() to convert to a QJsonArray.
`QJsonValue::Object` | `0x5` | An object. Use toObject() to convert to a QJsonObject.
`QJsonValue::Undefined` | `0x80` | The value is undefined. This is usually returned as an error condition, when trying to read an out of bounds value in an array or a non existent key in an object.

## Member Function Documentation

### QJsonValue::QJsonValue(Type type = Null)

Creates a QJsonValue of type type.

The default is to create a Null value.

### QJsonValue::QJsonValue(bool b)

Creates a value of type Bool, with value b.

### QJsonValue::QJsonValue(double n)

Creates a value of type Double, with value n.

### QJsonValue::QJsonValue(int n)

This is an overloaded function.

Creates a value of type Double, with value n.

### QJsonValue::QJsonValue(qint64 n)

This is an overloaded function.

Creates a value of type Double, with value n. NOTE: the integer limits for IEEE 754 double precision data is 2^53 (-9007199254740992 to +9007199254740992). If you pass in values outside this range expect a loss of precision to occur.

### QJsonValue::QJsonValue(const QString &s)

Creates a value of type String, with value s.

### QJsonValue::QJsonValue(QLatin1String s)

Creates a value of type String, with value s.

### QJsonValue::QJsonValue(const char *s)

Creates a value of type String with value s, assuming UTF-8 encoding of the input.

You can disable this constructor by defining `QT_NO_CAST_FROM_ASCII` when you compile your applications.

This function was introduced in Qt 5.3.

### QJsonValue::QJsonValue(const QJsonArray &a)

Creates a value of type Array, with value a.

### QJsonValue::QJsonValue(const QJsonObject &o)

Creates a value of type Object, with value o.

### QJsonValue::QJsonValue(const QJsonValue &other)

Creates a copy of other.

### QJsonValue::~QJsonValue()

Destroys the value.

### `[static]` QJsonValue QJsonValue::fromVariant(const QVariant &variant)

Converts variant to a QJsonValue and returns it.

The conversion will convert QVariant types as follows:

Source type | Destination type
--- | ---

- QMetaType::Nullptr

QJsonValue::Null

- QMetaType::Bool

QJsonValue::Bool

- QMetaType::Int
- QMetaType::UInt
- QMetaType::LongLong
- QMetaType::ULongLong
- QMetaType::Float
- QMetaType::Double

QJsonValue::Double

- QMetaType::QString

QJsonValue::String

- QMetaType::QStringList
- QMetaType::QVariantList

QJsonValue::Array

- QMetaType::QVariantMap
- QMetaType::QVariantHash

QJsonValue::Object

For all other QVariant types a conversion to a QString will be attempted. If the returned string is empty, a Null QJsonValue will be stored, otherwise a String value using the returned QString.

See also toVariant().

### bool QJsonValue::isArray() const

Returns `true` if the value contains an array.

See also toArray().

### bool QJsonValue::isBool() const

Returns `true` if the value contains a boolean.

See also toBool().

### bool QJsonValue::isDouble() const

Returns `true` if the value contains a double.

See also toDouble().

### bool QJsonValue::isNull() const

Returns `true` if the value is null.

### bool QJsonValue::isObject() const

Returns `true` if the value contains an object.

See also toObject().

### bool QJsonValue::isString() const

Returns `true` if the value contains a string.

See also toString().

### bool QJsonValue::isUndefined() const

Returns `true` if the value is undefined. This can happen in certain error cases as e.g. accessing a non existing key in a QJsonObject.

### QJsonArray QJsonValue::toArray(const QJsonArray &defaultValue) const

Converts the value to an array and returns it.

If type() is not Array, the defaultValue will be returned.

### QJsonArray QJsonValue::toArray() const

This is an overloaded function.

Converts the value to an array and returns it.

If type() is not Array, a QJsonArray() will be returned.

### bool QJsonValue::toBool(bool defaultValue = false) const

Converts the value to a bool and returns it.

If type() is not bool, the defaultValue will be returned.

### double QJsonValue::toDouble(double defaultValue = 0) const

Converts the value to a double and returns it.

If type() is not Double, the defaultValue will be returned.

### int QJsonValue::toInt(int defaultValue = 0) const

Converts the value to an int and returns it.

If type() is not Double or the value is not a whole number, the defaultValue will be returned.

### QJsonObject QJsonValue::toObject(const QJsonObject &defaultValue) const

Converts the value to an object and returns it.

If type() is not Object, the defaultValue will be returned.

### QJsonObject QJsonValue::toObject() const

This is an overloaded function.

Converts the value to an object and returns it.

If type() is not Object, the QJsonObject() will be returned.

### QString QJsonValue::toString() const

Converts the value to a QString and returns it.

If type() is not String, a null QString will be returned.

See also QString::isNull().

### QString QJsonValue::toString(const QString &defaultValue) const

Converts the value to a QString and returns it.

If type() is not String, the defaultValue will be returned.

### QVariant QJsonValue::toVariant() const

Converts the value to a QVariant().

The QJsonValue types will be converted as follows:

Constant | Description
--- | ---
`Null` | QMetaType::Nullptr
`Bool` | QMetaType::Bool
`Double` | QMetaType::Double
`String` | QString
`Array` | QVariantList
`Object` | QVariantMap
`Undefined` | QVariant()

See also fromVariant().

### Type QJsonValue::type() const

Returns the type of the value.

See also QJsonValue::Type.

### bool QJsonValue::operator!=(const QJsonValue &other) const

Returns `true` if the value is not equal to other.

### QJsonValue &QJsonValue::operator=(const QJsonValue &other)

Assigns the value stored in other to this object.

### bool QJsonValue::operator==(const QJsonValue &other) const

Returns `true` if the value is equal to other.