- Qt 5.15
- Qt Bluetooth
- C++ Classes
- QLowEnergyCharacteristic
- Qt 5.15.18 Reference Documentation

# QLowEnergyCharacteristic Class

The QLowEnergyCharacteristic class stores information about a Bluetooth Low Energy service characteristic. More...

Header: | #include <QLowEnergyCharacteristic>
qmake: | QT += bluetooth
Since: | Qt 5.4

This class was introduced in Qt 5.4.

- List of all members, including inherited members

## Public Types

enum | PropertyType { Unknown, Broadcasting, Read, WriteNoResponse, Write, …, ExtendedProperty }
flags | PropertyTypes

## Public Functions

| QLowEnergyCharacteristic(const QLowEnergyCharacteristic &other)
| QLowEnergyCharacteristic()
QLowEnergyCharacteristic & | operator=(const QLowEnergyCharacteristic &other)
| ~QLowEnergyCharacteristic()
QLowEnergyDescriptor | descriptor(const QBluetoothUuid &uuid) const
QList<QLowEnergyDescriptor> | descriptors() const
QLowEnergyHandle | handle() const
bool | isValid() const
QString | name() const
QLowEnergyCharacteristic::PropertyTypes | properties() const
QBluetoothUuid | uuid() const
QByteArray | value() const
bool | operator!=(const QLowEnergyCharacteristic &other) const
bool | operator==(const QLowEnergyCharacteristic &other) const

## Detailed Description

QLowEnergyCharacteristic provides information about a Bluetooth Low Energy service characteristic's name(), uuid(), value(), properties(), handle() and descriptors(). To obtain the characteristic's specification and information, it is necessary to connect to the device using the QLowEnergyService and QLowEnergyController classes.

The characteristic value may be written via the QLowEnergyService instance that manages the service to which this characteristic belongs. The QLowEnergyService::writeCharacteristic() function writes the new value. The QLowEnergyService::characteristicWritten() signal is emitted upon success. The value() of this object is automatically updated accordingly.

Characteristics may contain none, one or more descriptors. They can be individually retrieved using the descriptor() function. The descriptors() function returns all descriptors as a list. The general purpose of a descriptor is to add contextual information to the characteristic. For example, the descriptor might provide format or range information specifying how the characteristic's value is to be interpreted.

See also QLowEnergyService and QLowEnergyDescriptor.

## Member Type Documentation

### enum QLowEnergyCharacteristic::PropertyType
flags QLowEnergyCharacteristic::PropertyTypes

This enum describes the properties of a characteristic.

Constant | Value | Description
--- | --- | ---
`QLowEnergyCharacteristic::Unknown` | `0x00` | The type is not known.
`QLowEnergyCharacteristic::Broadcasting` | `0x01` | Allow for the broadcasting of Generic Attributes (GATT) characteristic values.
`QLowEnergyCharacteristic::Read` | `0x02` | Allow the characteristic values to be read.
`QLowEnergyCharacteristic::WriteNoResponse` | `0x04` | Allow characteristic values without responses to be written.
`QLowEnergyCharacteristic::Write` | `0x08` | Allow for characteristic values to be written.
`QLowEnergyCharacteristic::Notify` | `0x10` | Permits notification of characteristic values.
`QLowEnergyCharacteristic::Indicate` | `0x20` | Permits indications of characteristic values.
`QLowEnergyCharacteristic::WriteSigned` | `0x40` | Permits signed writes of the GATT characteristic values.
`QLowEnergyCharacteristic::ExtendedProperty` | `0x80` | Additional characteristic properties are defined in the characteristic's extended properties descriptor.

The PropertyTypes type is a typedef for QFlags<PropertyType>. It stores an OR combination of PropertyType values.

See also properties().

## Member Function Documentation

### QLowEnergyCharacteristic::QLowEnergyCharacteristic(const QLowEnergyCharacteristic &other)

Construct a new QLowEnergyCharacteristic that is a copy of other.

The two copies continue to share the same underlying data which does not detach upon write.

### QLowEnergyCharacteristic::QLowEnergyCharacteristic()

Construct a new QLowEnergyCharacteristic. A default-constructed instance of this class is always invalid.

See also isValid().

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

Makes a copy of other and assigns it to this QLowEnergyCharacteristic object. The two copies continue to share the same service and controller details.

### QLowEnergyCharacteristic::~QLowEnergyCharacteristic()

Destroys the QLowEnergyCharacteristic object.

### QLowEnergyDescriptor QLowEnergyCharacteristic::descriptor(const QBluetoothUuid &uuid) const

Returns the descriptor for uuid or an invalid `QLowEnergyDescriptor` instance.

See also descriptors().

### QList<QLowEnergyDescriptor> QLowEnergyCharacteristic::descriptors() const

Returns the list of descriptors belonging to this characteristic; otherwise an empty list.

See also descriptor().

### QLowEnergyHandle QLowEnergyCharacteristic::handle() const

Returns the handle of the characteristic's value attribute; or `0` if the handle cannot be accessed on the platform or if the characteristic is invalid.

Note: On macOS and iOS handles can differ from 0, but these values have no special meaning outside of internal/private API.

### bool QLowEnergyCharacteristic::isValid() const

Returns `true` if the QLowEnergyCharacteristic object is valid, otherwise returns `false`.

An invalid characteristic object is not associated with any service (default-constructed) or the associated service is no longer valid due to a disconnect from the underlying Bluetooth Low Energy device, for example. Once the object is invalid it cannot become valid anymore.

Note: If a QLowEnergyCharacteristic instance turns invalid due to a disconnect from the underlying device, the information encapsulated by the current instance remains as it was at the time of the disconnect. Therefore it can be retrieved after the disconnect event.

### QString QLowEnergyCharacteristic::name() const

Returns the human-readable name of the characteristic.

The name is based on the characteristic's uuid() which must have been standardized. The complete list of characteristic types can be found under Bluetooth.org Characteristics.

The returned string is empty if the uuid() is unknown.

See also QBluetoothUuid::characteristicToString().

### QLowEnergyCharacteristic::PropertyTypes QLowEnergyCharacteristic::properties() const

Returns the properties of the characteristic.

The properties define the access permissions for the characteristic.

### QBluetoothUuid QLowEnergyCharacteristic::uuid() const

Returns the UUID of the characteristic if isValid() returns `true`; otherwise a null UUID.

### QByteArray QLowEnergyCharacteristic::value() const

Returns the cached value of the characteristic.

If the characteristic's properties() permit writing of new values, the value can be updated using QLowEnergyService::writeCharacteristic().

The cache is updated during the associated service's detail discovery, a successful read/write operation or when an update notification is received.

The returned QByteArray always remains empty if the characteristic does not have the read permission. In such cases only the QLowEnergyService::characteristicChanged() or QLowEnergyService::characteristicWritten() may provice information about the value of this characteristic.

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

Returns `true` if other is not equal to this QLowEnergyCharacteristic; otherwise `false`.

Two QLowEnergyCharcteristic instances are considered to be equal if they refer to the same characteristic on the same remote Bluetooth Low Energy device or both instances have been default-constructed.

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

Returns `true` if other is equal to this QLowEnergyCharacteristic; otherwise `false`.

Two QLowEnergyCharacteristic instances are considered to be equal if they refer to the same characteristic on the same remote Bluetooth Low Energy device or both instances have been default-constructed.