Mbed OS Reference | GattCharacteristic.h Source File

 /* mbed Microcontroller Library
  * Copyright (c) 2006-2020 ARM Limited
  *
  * SPDX-License-Identifier: Apache-2.0
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at
  *
  *     http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
 
 #ifndef __GATT_CHARACTERISTIC_H__
 #define __GATT_CHARACTERISTIC_H__
 
 #include "ble/common/FunctionPointerWithContext.h"
 
 #include "ble/gatt/GattAttribute.h"
 #include "ble/gatt/GattCallbackParamTypes.h"
 
 // Forward declare ble::impl::GattServer
 namespace ble {
 
 #if !defined(DOXYGEN_ONLY)
 namespace impl {
 class GattServer;
 }
 #endif // !defined(DOXYGEN_ONLY)
 }
 
 /**
  * @addtogroup ble
  * @{
  * @addtogroup gatt
  * @{
  * @addtogroup server
  * @{
  */
 
 /**
  * Representation of a GattServer characteristic.
  *
  * A characteristic is a typed value enclosed in a GATT service (GattService).
  *
  * @par Type
  *
  * The type of the value defines the purpose of the characteristic, and a
  * UUID represents it. Standard characteristic types may be consulted at
  * https://www.bluetooth.com/specifications/gatt/characteristics
  *
  * @par Supported operations
  * A set of properties define what client operations the characteristic
  * supports. See GattServer::Properties_t
  *
  * @par Descriptors
  *
  * Additional information, such as the unit of the characteristic value, a
  * description string or a client control point, can be added to the
  * characteristic.
  *
  * See BLUETOOTH SPECIFICATION Version 4.2 [Vol 3, Part G] - 3.3.1.1
  *
  * One of the most important types of descriptor is the Client Characteristic
  * Configuration Descriptor (CCCD) that must be present if the characteristic
  * properties allow a client to subscribe to updates of the characteristic
  * value.
  *
  * @par Characteristic breakdown
  *
  * A characteristic is composed of several GATT attributes (GattAttribute):
  *   - Characteristic declaration: It contains the properties of the
  *     characteristic, its type and the handle of its value.
  *   - Characteristic value: The value of the characteristic.
  *   - Descriptors: A single GATT attribute stores each descriptor.
  *
  * When the GattService containing the characteristic is registered in the
  * GattServer, a unique attribute handle is assigned to the various attributes
  * of the characteristic. Clients use this handle to interact with the
  * characteristic. This handle is used locally in GattServer APIs.
  *
  * @par Security requirements
  *
  * Verification of security requirements happens whenever a client request to
  * read the characteristic; write it or even register to its updates. Different
  * requirements may be defined for these three type of operation. As an example:
  * it is possible to define a characteristic that do not require security to be
  * read and require an authenticated link to be written.
  *
  * By default all security requirements are set to att_security_requirement_t::NONE
  * except if the characteristic supports signed write; in such case the security
  * requirement for write operations is set to att_security_requirement_t::UNAUTHENTICATED.
  *
  * @note If a peer uses an operation that is not set in the characteristic
  * properties then the request request is discarded regardless of the security
  * requirements and current security level. The only exception being signed
  * write: signed write are converted into regular write without response if
  * the link is encrypted.
  */
 class GattCharacteristic {
 public:
 
     /*
      * Enumeration of characteristic UUID defined by the Bluetooth body.
      */
     enum {
         /**
          * Not used in actual BLE service.
          */
         UUID_BATTERY_LEVEL_STATE_CHAR = 0x2A1B,
 
         /**
          * Not used in actual BLE service.
          */
         UUID_BATTERY_POWER_STATE_CHAR = 0x2A1A,
 
         /**
          * Not used in actual BLE service.
          */
         UUID_REMOVABLE_CHAR = 0x2A3A,
 
         /**
          * Not used in actual BLE service.
          */
         UUID_SERVICE_REQUIRED_CHAR = 0x2A3B,
 
         /**
          * Not used as a characteristic UUID.
          */
         UUID_ALERT_CATEGORY_ID_CHAR = 0x2A43,
 
         /**
          * Not used as a characteristic UUID.
          */
         UUID_ALERT_CATEGORY_ID_BIT_MASK_CHAR = 0x2A42,
 
         /**
          * Control point of the Immediate Alert service that allows the client to
          * command the server to alert to a given level.
          */
         UUID_ALERT_LEVEL_CHAR = 0x2A06,
 
         /**
          * Control point of the Alert Notification service that allows the client
          * finely tune the notification configuration.
          */
         UUID_ALERT_NOTIFICATION_CONTROL_POINT_CHAR = 0x2A44,
 
         /**
          * Part of the Alert Notification service, which exposes the count of
          * unread alert events existing in the server.
          */
         UUID_ALERT_STATUS_CHAR = 0x2A3F,
 
         /**
          * Characteristic of the Battery service, which exposes the current
          * battery level as a percentage.
          */
         UUID_BATTERY_LEVEL_CHAR = 0x2A19,
 
         /**
          * Describe the features supported by the blood pressure sensor exposed
          * by the Blood Pressure service.
          */
         UUID_BLOOD_PRESSURE_FEATURE_CHAR = 0x2A49,
 
         /**
          * Characteristic of the Blood Pressure service that exposes the
          * measurement of the blood sensor.
          */
         UUID_BLOOD_PRESSURE_MEASUREMENT_CHAR = 0x2A35,
 
         /**
          * Characteristic of the Heart Rate service that indicate the intended
          * location of the heart rate monitor.
          */
         UUID_BODY_SENSOR_LOCATION_CHAR = 0x2A38,
 
         /**
          * Part of the Human Interface Device service.
          */
         UUID_BOOT_KEYBOARD_INPUT_REPORT_CHAR = 0x2A22,
 
         /**
          * Part of the Human Interface Device service.
          */
         UUID_BOOT_KEYBOARD_OUTPUT_REPORT_CHAR = 0x2A32,
 
         /**
          * Part of the Human Interface Device service.
          */
         UUID_BOOT_MOUSE_INPUT_REPORT_CHAR = 0x2A33,
 
         /**
          * Characteristic of the Current Time service that contains the current
          * time.
          */
         UUID_CURRENT_TIME_CHAR = 0x2A2B,
 
         /**
          * Not used in a service as a characteristic.
          */
         UUID_DATE_TIME_CHAR = 0x2A08,
 
         /**
          * Not used in a service as a characteristic.
          */
         UUID_DAY_DATE_TIME_CHAR = 0x2A0A,
 
         /**
          * Not used in a service as a characteristic.
          */
         UUID_DAY_OF_WEEK_CHAR = 0x2A09,
 
         /**
          * Not used in a service as a characteristic.
          */
         UUID_DST_OFFSET_CHAR = 0x2A0D,
 
         /**
          * Not used in a service as a characteristic.
          */
         UUID_EXACT_TIME_256_CHAR = 0x2A0C,
 
         /**
          * Characteristic of the Device Information Service that contains a
          * UTF8 string representing the firmware revision for the firmware within
          * the device.
          */
         UUID_FIRMWARE_REVISION_STRING_CHAR = 0x2A26,
 
         /**
          * Characteristic of the Glucose service that exposes features supported
          * by the server.
          */
         UUID_GLUCOSE_FEATURE_CHAR = 0x2A51,
 
         /**
          * Characteristic of the Glucose service that exposes glucose
          * measurements.
          */
         UUID_GLUCOSE_MEASUREMENT_CHAR = 0x2A18,
 
         /**
          * Characteristic of the Glucose service that sends additional
          * information related to the glucose measurements.
          */
         UUID_GLUCOSE_MEASUREMENT_CONTEXT_CHAR = 0x2A34,
 
         /**
          * Characteristic of the Device Information Service that contains a
          * UTF8 string representing the hardware revision of the device.
          */
         UUID_HARDWARE_REVISION_STRING_CHAR = 0x2A27,
 
         /**
          * Characteristic of the Heart Rate service used by the client to control
          * the service behavior.
          */
         UUID_HEART_RATE_CONTROL_POINT_CHAR = 0x2A39,
 
         /**
          * Characteristic of the Heart Rate that sends heart rate measurements to
          * registered clients.
          */
         UUID_HEART_RATE_MEASUREMENT_CHAR = 0x2A37,
 
         /**
          * Part of the Human Interface Device service.
          */
         UUID_HID_CONTROL_POINT_CHAR = 0x2A4C,
 
         /**
          * Part of the Human Interface Device service.
          */
         UUID_HID_INFORMATION_CHAR = 0x2A4A,
 
         /**
          * Characteristic of the Environmental Sensing service, which exposes
          * humidity measurements.
          */
         UUID_HUMIDITY_CHAR = 0x2A6F,
 
         /**
          * Characteristic of the Device Information Service, which exposes
          * various regulatory or certification compliance items to which the
          * device claims adherence.
          */
         UUID_IEEE_REGULATORY_CERTIFICATION_DATA_LIST_CHAR = 0x2A2A,
 
         /**
          * Characteristic of the Blood Pressure service, which exposes intermediate
          * cuff pressure measurements.
          */
         UUID_INTERMEDIATE_CUFF_PRESSURE_CHAR = 0x2A36,
 
         /**
          * Characteristic of the Health Thermometer service that sends intermediate
          * temperature values while the measurement is in progress.
          */
         UUID_INTERMEDIATE_TEMPERATURE_CHAR = 0x2A1E,
 
         /**
          * Characteristic of the current Time service that exposes information
          * about the local time.
          */
         UUID_LOCAL_TIME_INFORMATION_CHAR = 0x2A0F,
 
         /**
          * Characteristic of the Device Information Service that contains a
          * UTF8 string representing the manufacturer name of the device.
          */
         UUID_MANUFACTURER_NAME_STRING_CHAR = 0x2A29,
 
         /**
          * Characteristic of the Health Thermometer service that exposes the
          * interval time between two measurements.
          */
         UUID_MEASUREMENT_INTERVAL_CHAR = 0x2A21,
 
         /**
          * Characteristic of the Device Information Service that contains a
          * UTF8 string representing the model number of the device assigned by
          * the vendor.
          */
         UUID_MODEL_NUMBER_STRING_CHAR = 0x2A24,
 
         /**
          * Characteristic of the Alert Notification Service that shows how many
          * numbers of unread alerts exist in the specific category in the device.
          */
         UUID_UNREAD_ALERT_CHAR = 0x2A45,
 
         /**
          * Characteristic of the Alert Notification Service that defines the
          * category of the alert and how many new alerts of that category have
          * occurred in the server.
          */
         UUID_NEW_ALERT_CHAR = 0x2A46,
 
         /**
          * Characteristic of the Device Information Service; it is a set of
          * values used to create a device ID that is unique for this device.
          */
         UUID_PNP_ID_CHAR = 0x2A50,
 
         /**
          * Characteristic of the Environmental Sensing Service that exposes the
          * pressure measured.
          */
         UUID_PRESSURE_CHAR = 0x2A6D,
 
         /**
          * Part of the Human Interface Device service.
          */
         UUID_PROTOCOL_MODE_CHAR = 0x2A4E,
 
         /**
          * Pulse Oxymeter, Glucose and Continuous Glucose Monitoring services
          * use this control point to provide basic management of the patient
          * record database.
          */
         UUID_RECORD_ACCESS_CONTROL_POINT_CHAR = 0x2A52,
 
         /**
          * Characteristic of the Current Time service that exposes information
          * related to the current time served (accuracy, source, hours since
          * update and so on).
          */
         UUID_REFERENCE_TIME_INFORMATION_CHAR = 0x2A14,
 
         /**
          * Part of the Human Interface Device service.
          */
         UUID_REPORT_CHAR = 0x2A4D,
 
         /**
          * Part of the Human Interface Device service.
          */
         UUID_REPORT_MAP_CHAR = 0x2A4B,
 
         /**
          * Characteristic of the Phone Alert Status service that allows a client
          * to configure operating mode.
          */
         UUID_RINGER_CONTROL_POINT_CHAR = 0x2A40,
 
         /**
          * Characteristic of the Phone Alert Status service that returns the
          * ringer setting when read.
          */
         UUID_RINGER_SETTING_CHAR = 0x2A41,
 
         /**
          * Characteristic of the Scan Parameter service that stores the client's
          * scan parameters (scan interval and scan window).
          */
         UUID_SCAN_INTERVAL_WINDOW_CHAR = 0x2A4F,
 
         /**
          * Characteristic of the Scan Parameter service that sends a notification
          * to a client when the server requires its latest scan parameters.
          */
         UUID_SCAN_REFRESH_CHAR = 0x2A31,
 
         /**
          * Characteristic of the Device Information Service that contains a
          * UTF8 string representing the serial number of the device.
          */
         UUID_SERIAL_NUMBER_STRING_CHAR = 0x2A25,
 
         /**
          * Characteristic of the Device Information Service that contains an
          * UTF8 string representing the software revision of the device.
          */
         UUID_SOFTWARE_REVISION_STRING_CHAR = 0x2A28,
 
         /**
          * Characteristic of the Alert Notification Service that notifies the
          * count of new alerts for a given category to a subscribed client.
          */
         UUID_SUPPORTED_NEW_ALERT_CATEGORY_CHAR = 0x2A47,
 
         /**
          * Characteristic of the Alert Notification service, which exposes
          * categories of unread alert supported by the server.
          */
         UUID_SUPPORTED_UNREAD_ALERT_CATEGORY_CHAR = 0x2A48,
 
         /**
          * Characteristic of the Device Information Service that exposes a
          * structure containing an Organizationally Unique Identifier (OUI)
          * followed by a manufacturer-defined identifier. The value of the
          * structure is unique for each individual instance of the product.
          */
         UUID_SYSTEM_ID_CHAR = 0x2A23,
 
         /**
          * Characteristic of the Environmental Sensing service that exposes the
          * temperature measurement with a resolution of 0.01 degree Celsius.
          */
         UUID_TEMPERATURE_CHAR = 0x2A6E,
 
         /**
          * Characteristic of the Health Thermometer service that sends temperature
          * measurement to clients.
          */
         UUID_TEMPERATURE_MEASUREMENT_CHAR = 0x2A1C,
 
         /**
          * Characteristic of the Health Thermometer service that describes
          * where the measurement takes place.
          */
         UUID_TEMPERATURE_TYPE_CHAR = 0x2A1D,
 
         /**
          * Not used in a service as a characteristic.
          */
         UUID_TIME_ACCURACY_CHAR = 0x2A12,
 
         /**
          * Not used in a service as a characteristic.
          */
         UUID_TIME_SOURCE_CHAR = 0x2A13,
 
         /**
          * Characteristic of the Reference Time service that allows clients to
          * control time update.
          */
         UUID_TIME_UPDATE_CONTROL_POINT_CHAR = 0x2A16,
 
         /**
          * Characteristic of the Reference Time service that informs clients of
          * the status of the time update operation.
          */
         UUID_TIME_UPDATE_STATE_CHAR = 0x2A17,
 
         /**
          * Characteristic of the Next DST Change service that returns to clients
          * the time with DST.
          */
         UUID_TIME_WITH_DST_CHAR = 0x2A11,
 
         /**
          * Not used in a service as a characteristic.
          */
         UUID_TIME_ZONE_CHAR = 0x2A0E,
 
         /**
          * Characteristic of the TX Power service that exposes the current
          * transmission power in dBm.
          */
         UUID_TX_POWER_LEVEL_CHAR = 0x2A07,
 
         /**
          * Characteristic of the Cycling Speed and Cadence (CSC) service that
          * exposes features supported by the server.
          */
         UUID_CSC_FEATURE_CHAR = 0x2A5C,
 
         /**
          * Characteristic of the Cycling Speed and Cadence (CSC) service that
          * exposes measurements made by the server.
          */
         UUID_CSC_MEASUREMENT_CHAR = 0x2A5B,
 
         /**
          * Characteristic of the Running Speed and Cadence (RSC) service that
          * exposes features supported by the server.
          */
         UUID_RSC_FEATURE_CHAR = 0x2A54,
 
         /**
          * Characteristic of the Running Speed and Cadence (RSC) service that
          * exposes measurements made by the server.
          */
         UUID_RSC_MEASUREMENT_CHAR = 0x2A53
     };
 
     /**
      * Unit type of a characteristic value.
      *
      * These unit types are used to describe what the raw numeric data in a
      * characteristic actually represents. A server can expose that information
      * to its clients by adding a Characteristic Presentation Format descriptor
      * to relevant characteristics.
      *
      * @note See https://developer.bluetooth.org/gatt/units/Pages/default.aspx
      */
     enum {
 
         /**
          * No specified unit type.
          */
         BLE_GATT_UNIT_NONE = 0x2700,
 
         /**
          * Length, meter.
          */
         BLE_GATT_UNIT_LENGTH_METRE = 0x2701,
 
         /**
          * Mass, kilogram.
          */
         BLE_GATT_UNIT_MASS_KILOGRAM = 0x2702,
 
         /**
          * Time, second.
          */
         BLE_GATT_UNIT_TIME_SECOND = 0x2703,
 
         /**
          * Electric current, ampere.
          */
         BLE_GATT_UNIT_ELECTRIC_CURRENT_AMPERE = 0x2704,
 
         /**
          * Thermodynamic temperature, kelvin.
          */
         BLE_GATT_UNIT_THERMODYNAMIC_TEMPERATURE_KELVIN = 0x2705,
 
         /** Amount of substance, mole.
          *
         */
         BLE_GATT_UNIT_AMOUNT_OF_SUBSTANCE_MOLE = 0x2706,
 
         /**
          * Luminous intensity, candela.
          */
         BLE_GATT_UNIT_LUMINOUS_INTENSITY_CANDELA = 0x2707,
 
         /**
          * Area, square meters.
          */
         BLE_GATT_UNIT_AREA_SQUARE_METRES = 0x2710,
 
         /**
          * Volume, cubic meters.
          */
         BLE_GATT_UNIT_VOLUME_CUBIC_METRES = 0x2711,
 
         /**
          * Velocity, meters per second.
          */
         BLE_GATT_UNIT_VELOCITY_METRES_PER_SECOND = 0x2712,
 
         /**
          * Acceleration, meters per second squared.
          */
         BLE_GATT_UNIT_ACCELERATION_METRES_PER_SECOND_SQUARED = 0x2713,
 
         /**
          * Wave number reciprocal, meter.
          */
         BLE_GATT_UNIT_WAVENUMBER_RECIPROCAL_METRE = 0x2714,
 
         /**
          * Density, kilogram per cubic meter.
          */
         BLE_GATT_UNIT_DENSITY_KILOGRAM_PER_CUBIC_METRE = 0x2715,
 
         /**
          * Surface density (kilogram per square meter).
          */
         BLE_GATT_UNIT_SURFACE_DENSITY_KILOGRAM_PER_SQUARE_METRE = 0x2716,
 
         /**
          * Specific volume (cubic meter per kilogram).
          */
         BLE_GATT_UNIT_SPECIFIC_VOLUME_CUBIC_METRE_PER_KILOGRAM = 0x2717,
 
         /**
          * Current density (ampere per square meter).
          */
         BLE_GATT_UNIT_CURRENT_DENSITY_AMPERE_PER_SQUARE_METRE = 0x2718,
 
         /**
          * Magnetic field strength, ampere per meter.
          */
         BLE_GATT_UNIT_MAGNETIC_FIELD_STRENGTH_AMPERE_PER_METRE = 0x2719,
 
         /**
          * Amount concentration (mole per cubic meter).
          */
         BLE_GATT_UNIT_AMOUNT_CONCENTRATION_MOLE_PER_CUBIC_METRE = 0x271A,
 
         /**
          * Mass concentration (kilogram per cubic meter).
          */
         BLE_GATT_UNIT_MASS_CONCENTRATION_KILOGRAM_PER_CUBIC_METRE = 0x271B,
 
         /**
          * Luminance (candela per square meter).
          */
         BLE_GATT_UNIT_LUMINANCE_CANDELA_PER_SQUARE_METRE = 0x271C,
 
         /**
          * Refractive index.
          */
         BLE_GATT_UNIT_REFRACTIVE_INDEX = 0x271D,
 
         /**
          * Relative permeability.
         */
         BLE_GATT_UNIT_RELATIVE_PERMEABILITY = 0x271E,
 
         /**
          * Plane angle (radian).
          */
         BLE_GATT_UNIT_PLANE_ANGLE_RADIAN = 0x2720,
 
         /**
          * Solid angle (steradian).
         */
         BLE_GATT_UNIT_SOLID_ANGLE_STERADIAN = 0x2721,
 
         /**
          * Frequency, hertz.
          */
         BLE_GATT_UNIT_FREQUENCY_HERTZ = 0x2722,
 
         /**
          * Force, newton.
          */
         BLE_GATT_UNIT_FORCE_NEWTON = 0x2723,
 
         /**
          * Pressure, pascal.
          */
         BLE_GATT_UNIT_PRESSURE_PASCAL = 0x2724,
 
         /**
          * Energy, joule.
          */
         BLE_GATT_UNIT_ENERGY_JOULE = 0x2725,
 
         /**
          * Power, watt.
          */
         BLE_GATT_UNIT_POWER_WATT = 0x2726,
 
         /**
          * Electrical charge, coulomb.
          */
         BLE_GATT_UNIT_ELECTRIC_CHARGE_COULOMB = 0x2727,
 
         /**
          * Electrical potential difference, voltage.
          */
         BLE_GATT_UNIT_ELECTRIC_POTENTIAL_DIFFERENCE_VOLT = 0x2728,
 
         /**
          * Capacitance, farad.
          */
         BLE_GATT_UNIT_CAPACITANCE_FARAD = 0x2729,
 
         /**
          * Electric resistance, ohm.
          */
         BLE_GATT_UNIT_ELECTRIC_RESISTANCE_OHM = 0x272A,
 
         /**
          * Electric conductance, siemens.
          */
         BLE_GATT_UNIT_ELECTRIC_CONDUCTANCE_SIEMENS = 0x272B,
 
         /**
          * Magnetic flux, weber.
          */
         BLE_GATT_UNIT_MAGNETIC_FLUX_WEBER = 0x272C,
 
         /**
          * Magnetic flux density, tesla.
          */
         BLE_GATT_UNIT_MAGNETIC_FLUX_DENSITY_TESLA = 0x272D,
 
         /**
          * Inductance, henry.
          */
         BLE_GATT_UNIT_INDUCTANCE_HENRY = 0x272E,
 
         /**
          * Celsius temperature, degree Celsius.
          */
         BLE_GATT_UNIT_THERMODYNAMIC_TEMPERATURE_DEGREE_CELSIUS = 0x272F,
 
         /**
          * Luminous flux, lumen.
          */
         BLE_GATT_UNIT_LUMINOUS_FLUX_LUMEN = 0x2730,
 
         /**
          * Illuminance, lux.
          */
         BLE_GATT_UNIT_ILLUMINANCE_LUX = 0x2731,
 
         /**
          * Activity referred to a radionuclide, becquerel.
          */
         BLE_GATT_UNIT_ACTIVITY_REFERRED_TO_A_RADIONUCLIDE_BECQUEREL = 0x2732,
 
         /**
          * Absorbed dose, gray.
          */
         BLE_GATT_UNIT_ABSORBED_DOSE_GRAY = 0x2733,
 
         /**
          * Dose equivalent, sievert.
          */
         BLE_GATT_UNIT_DOSE_EQUIVALENT_SIEVERT = 0x2734,
 
         /**
          * Catalytic activity, katal.
          */
         BLE_GATT_UNIT_CATALYTIC_ACTIVITY_KATAL = 0x2735,
 
         /**
          * Dynamic viscosity, pascal second.
          */
         BLE_GATT_UNIT_DYNAMIC_VISCOSITY_PASCAL_SECOND = 0x2740,
 
         /**
          * Moment of force, newton meter.
          */
         BLE_GATT_UNIT_MOMENT_OF_FORCE_NEWTON_METRE = 0x2741,
 
         /**
          * Surface tension, newton per meter.
          */
         BLE_GATT_UNIT_SURFACE_TENSION_NEWTON_PER_METRE = 0x2742,
 
         /**
          * Angular velocity, radian per second.
          */
         BLE_GATT_UNIT_ANGULAR_VELOCITY_RADIAN_PER_SECOND = 0x2743,
 
         /**
          * Angular acceleration, radian per second squared.
          */
         BLE_GATT_UNIT_ANGULAR_ACCELERATION_RADIAN_PER_SECOND_SQUARED = 0x2744,
 
         /**
          * Heat flux density, watt per square meter.
          */
         BLE_GATT_UNIT_HEAT_FLUX_DENSITY_WATT_PER_SQUARE_METRE = 0x2745,
 
         /**
          * Heat capacity, joule per kelvin.
          */
         BLE_GATT_UNIT_HEAT_CAPACITY_JOULE_PER_KELVIN = 0x2746,
 
         /**
          * Specific heat capacity, joule per kilogram kelvin.
          */
         BLE_GATT_UNIT_SPECIFIC_HEAT_CAPACITY_JOULE_PER_KILOGRAM_KELVIN = 0x2747,
 
         /**
          * Specific energy, joule per kilogram.
          */
         BLE_GATT_UNIT_SPECIFIC_ENERGY_JOULE_PER_KILOGRAM = 0x2748,
 
         /**
          * Thermal conductivity, watt per meter kelvin.
          */
         BLE_GATT_UNIT_THERMAL_CONDUCTIVITY_WATT_PER_METRE_KELVIN = 0x2749,
 
         /**
          * Energy density, joule per cubic meter.
          */
         BLE_GATT_UNIT_ENERGY_DENSITY_JOULE_PER_CUBIC_METRE = 0x274A,
 
         /**
          * Electric field strength, volt per meter.
          */
         BLE_GATT_UNIT_ELECTRIC_FIELD_STRENGTH_VOLT_PER_METRE = 0x274B,
 
         /**
          * Electric charge density, coulomb per cubic meter.
          */
         BLE_GATT_UNIT_ELECTRIC_CHARGE_DENSITY_COULOMB_PER_CUBIC_METRE = 0x274C,
 
         /**
          * Surface charge density, coulomb per square meter.
          */
         BLE_GATT_UNIT_SURFACE_CHARGE_DENSITY_COULOMB_PER_SQUARE_METRE = 0x274D,
 
         /**
          * Electric flux density, coulomb per square meter.
          */
         BLE_GATT_UNIT_ELECTRIC_FLUX_DENSITY_COULOMB_PER_SQUARE_METRE = 0x274E,
 
         /**
          * Permittivity, farad per meter.
          */
         BLE_GATT_UNIT_PERMITTIVITY_FARAD_PER_METRE = 0x274F,
 
         /**
          * Permeability, henry per meter.
          */
         BLE_GATT_UNIT_PERMEABILITY_HENRY_PER_METRE = 0x2750,
 
         /**
          * Molar energy, joule per mole.
          */
         BLE_GATT_UNIT_MOLAR_ENERGY_JOULE_PER_MOLE = 0x2751,
 
         /**
          * Molar entropy, joule per mole kelvin.
          */
         BLE_GATT_UNIT_MOLAR_ENTROPY_JOULE_PER_MOLE_KELVIN = 0x2752,
 
         /**
          * Exposure, coulomb per kilogram.
          */
         BLE_GATT_UNIT_EXPOSURE_COULOMB_PER_KILOGRAM = 0x2753,
 
         /**
          * Absorbed dose rate, gray per second.
          */
         BLE_GATT_UNIT_ABSORBED_DOSE_RATE_GRAY_PER_SECOND = 0x2754,
 
         /**
          * Radiant intensity, watt per steradian.
          */
         BLE_GATT_UNIT_RADIANT_INTENSITY_WATT_PER_STERADIAN = 0x2755,
 
         /**
          * Radiance, watt per square meter steradian.
          */
         BLE_GATT_UNIT_RADIANCE_WATT_PER_SQUARE_METRE_STERADIAN = 0x2756,
 
         /**
          * Catalytic activity concentration, katal per cubic meter.
          */
         BLE_GATT_UNIT_CATALYTIC_ACTIVITY_CONCENTRATION_KATAL_PER_CUBIC_METRE = 0x2757,
 
         /**
          * Time, minute.
          */
         BLE_GATT_UNIT_TIME_MINUTE = 0x2760,
 
         /**
          * Time, hour.
          */
         BLE_GATT_UNIT_TIME_HOUR = 0x2761,
 
         /**
          * Time, day.
          */
         BLE_GATT_UNIT_TIME_DAY = 0x2762,
 
         /**
          * Plane angle, degree.
          */
         BLE_GATT_UNIT_PLANE_ANGLE_DEGREE = 0x2763,
 
         /**
          * Plane angle, minute.
          */
         BLE_GATT_UNIT_PLANE_ANGLE_MINUTE = 0x2764,
 
         /**
          * Plane angle, seconds.
          */
         BLE_GATT_UNIT_PLANE_ANGLE_SECOND = 0x2765,
 
         /**
          * Area, hectare.
          */
         BLE_GATT_UNIT_AREA_HECTARE = 0x2766,
 
         /**
          * Volume, liter.
          */
         BLE_GATT_UNIT_VOLUME_LITRE = 0x2767,
 
         /**
          * Mass, ton.
          */
         BLE_GATT_UNIT_MASS_TONNE = 0x2768,
 
         /**
          * Pressure, bar.
          */
         BLE_GATT_UNIT_PRESSURE_BAR = 0x2780,
 
         /**
          * Pressure, millimeter of mercury.
          */
         BLE_GATT_UNIT_PRESSURE_MILLIMETRE_OF_MERCURY = 0x2781,
 
         /**
          * Length, ngstrm.
          */
         BLE_GATT_UNIT_LENGTH_ANGSTROM = 0x2782,
 
         /**
          * Length, nautical mile.
          */
         BLE_GATT_UNIT_LENGTH_NAUTICAL_MILE = 0x2783,
 
         /**
          * Area, barn.
          */
         BLE_GATT_UNIT_AREA_BARN = 0x2784,
 
         /**
          * Velocity, knot.
          */
         BLE_GATT_UNIT_VELOCITY_KNOT = 0x2785,
 
         /**
          * Logarithmic radio quantity, neper.
          */
         BLE_GATT_UNIT_LOGARITHMIC_RADIO_QUANTITY_NEPER = 0x2786,
 
         /**
          * Logarithmic radio quantity, bel.
          */
         BLE_GATT_UNIT_LOGARITHMIC_RADIO_QUANTITY_BEL = 0x2787,
 
         /**
          * Length, yard.
          */
         BLE_GATT_UNIT_LENGTH_YARD = 0x27A0,
 
         /**
          * Length, parsec.
          */
         BLE_GATT_UNIT_LENGTH_PARSEC = 0x27A1,
 
         /**
          * Length, inch.
          */
         BLE_GATT_UNIT_LENGTH_INCH = 0x27A2,
 
         /**
          * Length, foot.
          */
         BLE_GATT_UNIT_LENGTH_FOOT = 0x27A3,
 
         /**
          * Length, mile.
          */
         BLE_GATT_UNIT_LENGTH_MILE = 0x27A4,
 
         /**
          * Pressure, pound-force per square inch.
          */
         BLE_GATT_UNIT_PRESSURE_POUND_FORCE_PER_SQUARE_INCH = 0x27A5,
 
         /**
          * Velocity, kilometer per hour.
          */
         BLE_GATT_UNIT_VELOCITY_KILOMETRE_PER_HOUR = 0x27A6,
 
         /** Velocity, mile per hour.
          *
          */
         BLE_GATT_UNIT_VELOCITY_MILE_PER_HOUR = 0x27A7,
 
         /**
          * Angular Velocity, revolution per minute.
          */
         BLE_GATT_UNIT_ANGULAR_VELOCITY_REVOLUTION_PER_MINUTE = 0x27A8,
 
         /**
          * Energy, gram calorie.
          */
         BLE_GATT_UNIT_ENERGY_GRAM_CALORIE = 0x27A9,
 
         /**
          * Energy, kilogram calorie.
          */
         BLE_GATT_UNIT_ENERGY_KILOGRAM_CALORIE = 0x27AA,
 
         /**
          * Energy, killowatt hour.
          */
         BLE_GATT_UNIT_ENERGY_KILOWATT_HOUR = 0x27AB,
 
         /**
          * Thermodynamic temperature, degree Fahrenheit.
          */
         BLE_GATT_UNIT_THERMODYNAMIC_TEMPERATURE_DEGREE_FAHRENHEIT = 0x27AC,
 
         /**
          * Percentage.
          */
         BLE_GATT_UNIT_PERCENTAGE = 0x27AD,
 
         /**
          * Per mille.
          */
         BLE_GATT_UNIT_PER_MILLE = 0x27AE,
 
         /**
          * Period, beats per minute.
          */
         BLE_GATT_UNIT_PERIOD_BEATS_PER_MINUTE = 0x27AF,
 
         /**
          * Electric charge, ampere hours.
          */
         BLE_GATT_UNIT_ELECTRIC_CHARGE_AMPERE_HOURS = 0x27B0,
 
         /**
          * Mass density, milligram per deciliter.
          */
         BLE_GATT_UNIT_MASS_DENSITY_MILLIGRAM_PER_DECILITRE = 0x27B1,
 
         /**
          * Mass density, millimole per liter.
          */
         BLE_GATT_UNIT_MASS_DENSITY_MILLIMOLE_PER_LITRE = 0x27B2,
 
         /**
          * Time, year.
          */
         BLE_GATT_UNIT_TIME_YEAR = 0x27B3,
 
         /**
          * Time, month.
          */
         BLE_GATT_UNIT_TIME_MONTH = 0x27B4,
 
         /**
          * Concentration, count per cubic meter.
          */
         BLE_GATT_UNIT_CONCENTRATION_COUNT_PER_CUBIC_METRE = 0x27B5,
 
         /**
          * Irradiance, watt per square meter.
          */
         BLE_GATT_UNIT_IRRADIANCE_WATT_PER_SQUARE_METRE = 0x27B6
     };
 
     /**
      * Presentation format of a characteristic.
      *
      * It determines how the value of a characteristic is formatted. A server
      * can expose that information to its clients by adding a Characteristic
      * Presentation Format descriptor to relevant characteristics.
      *
      * @note See Bluetooth Specification 4.0 (Vol. 3), Part G, Section 3.3.3.5.2.
      */
     enum {
         /**
          * Reserved for future use.
          */
         BLE_GATT_FORMAT_RFU = 0x00,
 
         /**
          * Boolean.
          */
         BLE_GATT_FORMAT_BOOLEAN = 0x01,
 
         /**
          * Unsigned 2-bit integer.
          */
         BLE_GATT_FORMAT_2BIT = 0x02,
 
         /**
          * Unsigned 4-bit integer.
          */
         BLE_GATT_FORMAT_NIBBLE = 0x03,
 
         /**
          * Unsigned 8-bit integer.
          */
         BLE_GATT_FORMAT_UINT8 = 0x04,
 
         /**
          * Unsigned 12-bit integer.
          */
         BLE_GATT_FORMAT_UINT12 = 0x05,
 
         /**
          * Unsigned 16-bit integer.
          */
         BLE_GATT_FORMAT_UINT16 = 0x06,
 
         /**
          * Unsigned 24-bit integer.
          */
         BLE_GATT_FORMAT_UINT24 = 0x07,
 
         /**
          * Unsigned 32-bit integer.
          */
         BLE_GATT_FORMAT_UINT32 = 0x08,
 
         /**
          * Unsigned 48-bit integer.
          */
         BLE_GATT_FORMAT_UINT48 = 0x09,
 
         /**
          * Unsigned 64-bit integer.
          */
         BLE_GATT_FORMAT_UINT64 = 0x0A,
 
         /**
          * Unsigned 128-bit integer.
          */
         BLE_GATT_FORMAT_UINT128 = 0x0B,
 
         /**
          * Signed 8-bit integer.
          */
         BLE_GATT_FORMAT_SINT8 = 0x0C,
 
         /**
          * Signed 12-bit integer.
          */
         BLE_GATT_FORMAT_SINT12 = 0x0D,
 
         /**
          * Signed 16-bit integer.
          */
         BLE_GATT_FORMAT_SINT16 = 0x0E,
 
         /**
          * Signed 24-bit integer.
          */
         BLE_GATT_FORMAT_SINT24 = 0x0F,
 
         /**
          * Signed 32-bit integer.
          */
         BLE_GATT_FORMAT_SINT32 = 0x10,
 
         /**
          * Signed 48-bit integer.
          */
         BLE_GATT_FORMAT_SINT48 = 0x11,
 
         /**
          * Signed 64-bit integer.
          */
         BLE_GATT_FORMAT_SINT64 = 0x12,
 
         /**
          * Signed 128-bit integer.
          */
         BLE_GATT_FORMAT_SINT128 = 0x13,
 
         /**
          * IEEE-754 32-bit floating point.
          */
         BLE_GATT_FORMAT_FLOAT32 = 0x14,
 
         /**
          * IEEE-754 64-bit floating point.
          */
         BLE_GATT_FORMAT_FLOAT64 = 0x15,
 
         /**
          * IEEE-11073 16-bit SFLOAT.
          */
         BLE_GATT_FORMAT_SFLOAT = 0x16,
 
         /**
          * IEEE-11073 32-bit FLOAT.
          */
         BLE_GATT_FORMAT_FLOAT = 0x17,
 
         /**
          * IEEE-20601 format.
          */
         BLE_GATT_FORMAT_DUINT16 = 0x18,
 
         /**
          * UTF8 string.
          */
         BLE_GATT_FORMAT_UTF8S = 0x19,
 
         /**
          * UTF16 string.
          */
         BLE_GATT_FORMAT_UTF16S = 0x1A,
 
         /**
          * Opaque Structure.
          */
         BLE_GATT_FORMAT_STRUCT = 0x1B
     };
 
     /*!
      * Characteristic properties.
      *
      * It is a bitfield that determines how a characteristic value can be used.
      *
      * @note See Bluetooth Specification 4.0 (Vol. 3), Part G, Section 3.3.1.1
      * and Section 3.3.3.1 for Extended Properties.
      */
     enum Properties_t {
         /**
          * No property defined.
          */
         BLE_GATT_CHAR_PROPERTIES_NONE = 0x00,
 
         /**
          * Permits broadcasts of the characteristic value using the Server
          * Characteristic Configuration descriptor.
          */
         BLE_GATT_CHAR_PROPERTIES_BROADCAST = 0x01,
 
         /**
          * Permits reads of the characteristic value.
          */
         BLE_GATT_CHAR_PROPERTIES_READ = 0x02,
 
         /**
          * Permits writes of the characteristic value without response.
          */
         BLE_GATT_CHAR_PROPERTIES_WRITE_WITHOUT_RESPONSE = 0x04,
 
         /**
          * Permits writes of the characteristic value with response.
          */
         BLE_GATT_CHAR_PROPERTIES_WRITE = 0x08,
 
         /**
          * Permits notifications of a characteristic value without acknowledgment.
          */
         BLE_GATT_CHAR_PROPERTIES_NOTIFY = 0x10,
 
         /**
          * Permits indications of a characteristic value with acknowledgment.
          */
         BLE_GATT_CHAR_PROPERTIES_INDICATE = 0x20,
 
         /**
          * Permits signed writes to the characteristic value.
          */
         BLE_GATT_CHAR_PROPERTIES_AUTHENTICATED_SIGNED_WRITES = 0x40,
 
         /**
          * The Characteristic Extended Properties descriptor
          * defines additional characteristic properties.
          */
         BLE_GATT_CHAR_PROPERTIES_EXTENDED_PROPERTIES = 0x80
     };
 
     /**
      * Indicates if the properties has at least one of the writable flags.
      *
      * @param[in] properties The properties to inspect.
      *
      * @return True if the properties set at least one of the writable flags and
      * false otherwise.
      */
     static bool isWritable(uint8_t properties)
     {
         const uint8_t writable =
              BLE_GATT_CHAR_PROPERTIES_WRITE_WITHOUT_RESPONSE |
              BLE_GATT_CHAR_PROPERTIES_WRITE |
              BLE_GATT_CHAR_PROPERTIES_AUTHENTICATED_SIGNED_WRITES;
 
         return properties & writable;
     }
 
     /**
      * Indicates if the properties is readable.
      *
      * @param[in] properties The properties to inspect.
      *
      * @return True if the properties has its readable flag set and false
      * otherwise.
      */
     static bool isReadable(uint8_t properties)
     {
         const uint8_t readable = BLE_GATT_CHAR_PROPERTIES_READ;
         return properties & readable;
     }
 
     /**
      * Value of a Characteristic Presentation Format descriptor.
      *
      * Characteristic Presentation Format descriptor expresses the format of a
      * characteristic value.
      *
      * @note See Bluetooth Specification 4.0 (Vol. 3), Part G, Section 3.3.3.5.
      */
     struct PresentationFormat_t {
         /**
          * Format of the value.
          */
         uint8_t gatt_format;
 
         /**
          * Exponent for integer data types.
          *
          * Example: if Exponent = -3 and the char value is 3892, the actual
          * value is 3.892
          */
         int8_t exponent;
 
         /**
          * Unit of the characteristic value.
          *
          * It is a UUID from Bluetooth Assigned Numbers.
          */
         uint16_t gatt_unit;
 
         /**
          * Namespace of the description field.
          *
          * This field identifies the organization that is responsible for
          * defining the enumerations for the description field.
          *
          * The namespace of the Bluetooth Body is 0x01.
          */
         uint8_t gatt_namespace;
 
         /**
          * Description.
          *
          * @note The value 0x0000 means unknown in the Bluetooth namespace.
          */
         uint16_t gatt_nsdesc;
 
     };
 
     /**
      * Security level applied to GATT operations.
      */
     typedef ble::att_security_requirement_t SecurityRequirement_t;
 
     /**
      * @brief  Constructs a new GattCharacteristic.
      *
      * @param[in] uuid The UUID of this characteristic.
      * @param[in] valuePtr Memory buffer holding the initial value. The value is
      * copied into the Bluetooth subsytem when the enclosing service is added.
      * Thereafter, the stack maintains it internally.
      * @param[in] len The length in bytes of this characteristic's value.
      * @param[in] maxLen The capacity in bytes of the characteristic value
      * buffer.
      * @param[in] props An 8-bit field that contains the characteristic's
      * properties.
      * @param[in] descriptors A pointer to an array of descriptors to be included
      * within this characteristic. The caller owns the memory for the descriptor
      * array, which must remain valid at least until the enclosing service is
      * added to the GATT table.
      * @param[in] numDescriptors The number of descriptors presents in @p
      * descriptors array.
      * @param[in] hasVariableLen Flag that indicates if the attribute's value
      * length can change throughout time.
      *
      * @note If valuePtr is nullptr, length is equal to 0 and the characteristic
      * is readable, then that particular characteristic may be considered
      * optional and dropped while instantiating the service with the underlying
      * BLE stack.
      *
      * @note A Client Characteristic Configuration Descriptor (CCCD) should not
      * be allocated if either the notify or indicate flag in the @p props bit
      * field; the underlying BLE stack handles it.
      *
      * @attention GattCharacteristic registered in a GattServer must remain
      * valid for the lifetime of the GattServer.
      */
     GattCharacteristic(
         const UUID &uuid,
         uint8_t *valuePtr = nullptr,
         uint16_t len = 0,
         uint16_t maxLen = 0,
         uint8_t props = BLE_GATT_CHAR_PROPERTIES_NONE,
         GattAttribute *descriptors[] = nullptr,
         unsigned numDescriptors = 0,
         bool hasVariableLen = true
     ) : _valueAttribute(uuid, valuePtr, len, maxLen, hasVariableLen),
         _properties(props),
         _descriptors(descriptors),
         _descriptorCount(numDescriptors),
         readAuthorizationCallback(),
         writeAuthorizationCallback(),
         _update_security(SecurityRequirement_t::NONE) {
         _valueAttribute.allowWrite(isWritable(_properties));
         _valueAttribute.allowRead(isReadable(_properties));
 
 #if BLE_FEATURE_SECURITY
         // signed writes requires at least an unauthenticated CSRK or an
         // unauthenticated ltk if the link is encrypted.
         if (_properties & BLE_GATT_CHAR_PROPERTIES_AUTHENTICATED_SIGNED_WRITES) {
             _valueAttribute.setWriteSecurityRequirement(
                 SecurityRequirement_t::UNAUTHENTICATED
             );
         }
 #endif // BLE_FEATURE_SECURITY
     }
 
     GattCharacteristic(const GattCharacteristic &) = delete;
     GattCharacteristic& operator=(const GattCharacteristic &) = delete;
     
     ~GattCharacteristic() {
             delete _implicit_cccd;
             _implicit_cccd = nullptr;
     }
 
 public:
 
     /**
      * Set all security requirements of the characteristic.
      *
      * @param read_security The security requirement of the read operations.
      * @param write_security The security requirement of write operations.
      * @param update_security The security requirement of update operations.
      */
     void setSecurityRequirements(
         SecurityRequirement_t read_security,
         SecurityRequirement_t write_security,
         SecurityRequirement_t update_security
     ) {
         setReadSecurityRequirement(read_security);
         setWriteSecurityRequirement(write_security);
         setUpdateSecurityRequirement(update_security);
     }
 
     /**
      * Set the security of the read operation.
      *
      * @param[in] security The security requirement of the read operation.
      */
     void setReadSecurityRequirement(SecurityRequirement_t security)
     {
         _valueAttribute.setReadSecurityRequirement(security);
     }
 
     /**
      * Get the security requirement of the read operation.
      *
      * @return The security requirement of the read operation.
      */
     SecurityRequirement_t getReadSecurityRequirement() const
     {
         return _valueAttribute.getReadSecurityRequirement();
     }
 
     /**
      * Set the security requirement of the write operations.
      *
      * @note If the signed write flag is set in the characteristic properties
      * then the security requirement applied to write operation must be either
      * AUTHENTICATED or UNAUTHENTICATED. Security requirements NONE and
      * SC_AUTHENTICATED are not applicable to signing operation.
      *
      * @param[in] security The security requirement of write operations.
      */
     void setWriteSecurityRequirement(SecurityRequirement_t security)
     {
 #if BLE_FEATURE_SECURITY
         MBED_ASSERT(
             ((_properties & BLE_GATT_CHAR_PROPERTIES_AUTHENTICATED_SIGNED_WRITES) &&
             ((security == SecurityRequirement_t::NONE) ||
             (security == SecurityRequirement_t::SC_AUTHENTICATED))) == false
         );
 #endif // BLE_FEATURE_SECURITY
         _valueAttribute.setWriteSecurityRequirement(security);
     }
 
     /**
      * Get the security requirement of write operations.
      *
      * @return The security requirement of write operations.
      */
     SecurityRequirement_t getWriteSecurityRequirement() const
     {
         return _valueAttribute.getWriteSecurityRequirement();
     }
 
     /**
      * Set the security requirement of update operations.
      *
      * @note This security requirement is also applied to the write operation of
      * the Client Characteristic Configuration Descriptor.
      *
      * @param[in] security The security requirement that must be met to send
      * updates and accept write of the CCCD.
      */
     void setUpdateSecurityRequirement(SecurityRequirement_t security)
     {
         _update_security = security.value();
     }
 
     /**
      * Get the security requirement of update operations.
      *
      * @note This security requirement is also applied to the write operation of
      * the Client Characteristic Configuration Descriptor.
      *
      * @return The security requirement that must be met to send updates and
      * accept write of the CCCD.
      */
     SecurityRequirement_t getUpdateSecurityRequirement() const
     {
         return static_cast<SecurityRequirement_t::type>(_update_security);
     }
 
 public:
     /**
      * Register a callback handling client's write requests or commands.
      *
      * The callback registered is invoked when the client attempts to write the
      * characteristic value; the event handler can accept or reject the write
      * request with the appropriate error code.
      *
      * @param[in] callback Event handler being registered.
      */
     void setWriteAuthorizationCallback(
         void (*callback)(GattWriteAuthCallbackParams *)
     ) {
         writeAuthorizationCallback.attach(callback);
     }
 
     /**
      * Register a callback handling client's write requests or commands.
      *
      * The callback registered is invoked when the client attempts to write the
      * characteristic value; the event handler can accept or reject the write
      * request with the appropriate error code.
      *
      * @param[in] object Pointer to the object of a class defining the event
      * handler (@p member). It must remain valid for the lifetime of the
      * GattCharacteristic.
      * @param[in] member The member function that handles the write event.
      */
     template <typename T>
     void setWriteAuthorizationCallback(
         T *object,
         void (T::*member)(GattWriteAuthCallbackParams *)
     ) {
         writeAuthorizationCallback.attach(object, member);
     }
 
     /**
      * Register the read requests event handler.
      *
      * The callback registered is invoked when the client attempts to read the
      * characteristic value; the event handler can accept or reject the read
      * request with the appropriate error code. It can also set specific outgoing
      * data.
      *
      * @param[in] callback Event handler being registered.
      */
     void setReadAuthorizationCallback(
         void (*callback)(GattReadAuthCallbackParams *)
     ) {
         readAuthorizationCallback.attach(callback);
     }
 
     /**
      * Register the read requests event handler.
      *
      * The callback registered is invoked when the client attempts to read the
      * characteristic value; the event handler can accept or reject the read
      * request with the appropriate error code. It can also set specific outgoing
      * data.
      *
      * @param[in] object Pointer to the object of a class defining the event
      * handler (@p member). It must remain valid for the lifetime of the
      * GattCharacteristic.
      * @param[in] member The member function that handles the read event.
      */
     template <typename T>
     void setReadAuthorizationCallback(
         T *object,
         void (T::*member)(GattReadAuthCallbackParams *)
     ) {
         readAuthorizationCallback.attach(object, member);
     }
 
     /**
      * Invoke the write authorization callback.
      *
      * This function is a helper that calls the registered write handler to
      * determine the authorization reply for a write request.
      *
      * @attention This function is not meant to be called by user code.
      *
      * @param[in] params Context of the write-auth request; it contains an
      * out-parameter used as a reply.
      *
      * @return A GattAuthCallbackReply_t value indicating whether authorization
      * is granted.
      */
     GattAuthCallbackReply_t authorizeWrite(GattWriteAuthCallbackParams *params)
     {
         if (!isWriteAuthorizationEnabled()) {
             return AUTH_CALLBACK_REPLY_SUCCESS;
         }
 
         /* Initialized to no-error by default. */
         params->authorizationReply = AUTH_CALLBACK_REPLY_SUCCESS;
         writeAuthorizationCallback.call(params);
         return params->authorizationReply;
     }
 
     /**
      * Invoke the read authorization callback.
      *
      * This function is a helper that calls the registered read handler to
      * determine the authorization reply for a read request.
      *
      * @attention This function is not meant to be called by user code.
      *
      * @param[in] params Context of the read-auth request; it contains an
      * out-parameter used as a reply and the handler can fill it with outgoing
      * data.
      *
      * @return A GattAuthCallbackReply_t value indicating whether authorization
      * is granted.
      *
      * @note If the read request is approved and params->data remains nullptr, then
      * the current characteristic value is used in the read response payload.
      *
      * @note If the read is approved, the event handler can specify an outgoing
      * value directly with the help of the fields
      * GattReadAuthCallbackParams::data and GattReadAuthCallbackParams::len.
      */
     GattAuthCallbackReply_t authorizeRead(GattReadAuthCallbackParams *params)
     {
         if (!isReadAuthorizationEnabled()) {
             return AUTH_CALLBACK_REPLY_SUCCESS;
         }
 
         /* Initialized to no-error by default. */
         params->authorizationReply = AUTH_CALLBACK_REPLY_SUCCESS;
         readAuthorizationCallback.call(params);
         return params->authorizationReply;
     }
 
 public:
     /**
      * Get the characteristic's value attribute.
      *
      * @return A reference to the characteristic's value attribute.
      */
     GattAttribute& getValueAttribute()
     {
         return _valueAttribute;
     }
 
     /**
      * Get the characteristic's value attribute.
      *
      * @return A const reference to the characteristic's value attribute.
      */
     const GattAttribute& getValueAttribute() const
     {
         return _valueAttribute;
     }
 
     /**
      * Get the characteristic's value attribute handle in the ATT table.
      *
      * @return The value attribute handle.
      *
      * @note The underlying BLE stack assigns the attribute handle when the
      * enclosing service is added.
      */
     GattAttribute::Handle_t getValueHandle() const
     {
         return getValueAttribute().getHandle();
     }
 
     /**
      * Get the characteristic's properties.
      *
      * @note Refer to GattCharacteristic::Properties_t.
      *
      * @return The characteristic's properties.
      */
     uint8_t getProperties() const
     {
         return _properties;
     }
 
     /**
      * Get the total number of descriptors within this characteristic.
      *
      * @return The total number of descriptors.
      */
     uint8_t getDescriptorCount() const
     {
         return (_implicit_cccd == nullptr? _descriptorCount : _descriptorCount+1);
     }
 
     /**
      * Check whether read authorization is enabled.
      *
      * Read authorization is enabled when a read authorization event handler is
      * set up.
      *
      * @return true if read authorization is enabled and false otherwise.
      */
     bool isReadAuthorizationEnabled() const
     {
         return readAuthorizationCallback;
     }
 
     /**
      * Check whether write authorization is enabled.
      *
      * Write authorization is enabled when a write authorization event handler is
      * set up.
      *
      * @return true if write authorization is enabled, false otherwise.
      */
     bool isWriteAuthorizationEnabled() const
     {
         return writeAuthorizationCallback;
     }
 
     /**
      * Get this characteristic's descriptor at a specific index.
      *
      * @param[in] index The index of the descriptor to get.
      *
      * @return A pointer the requested descriptor if @p index is within the
      * range of the descriptor array or nullptr otherwise.
      *
      * @note if this characteristic has an implicitly-created CCCD this
      * descriptor will be available at the highest index
      * (ie: return of getDescriptorCount() - 1)
      */
     GattAttribute *getDescriptor(uint8_t index)
     {
 
         if(index == _descriptorCount) {
             // If _implicit_cccd is nullptr, we want to return that anyway
             return _implicit_cccd;
         }
         else if (index > _descriptorCount) {
             return nullptr;
         }
 
         return _descriptors[index];
     }
 
 
 private:
 
     friend ble::impl::GattServer;
 
 #if !defined(DOXYGEN_ONLY)
     /**
      * Sets this GattCharacteristic's implicitly-created CCCD, if
      * applicable.
      *
      * @note once this is called, the pointed-to GattAttribute
      * is owned by this GattCharacteristic and will be deleted
      * during this object's destructor
      */
     void setImplicitCCCD(GattAttribute *implicit_cccd) {
         _implicit_cccd = implicit_cccd;
     }
 #endif // !defined(DOXYGEN_ONLY)
 
 
 private:
 
     /**
      * Attribute that contains the actual value of this characteristic.
      */
     GattAttribute _valueAttribute;
 
     /**
      * The characteristic's properties. Refer to
      * GattCharacteristic::Properties_t.
      */
     uint8_t _properties;
 
     /**
      * The characteristic's descriptor attributes.
      */
     GattAttribute **_descriptors;
 
     /**
      * The number of descriptors in this characteristic.
      */
     uint8_t _descriptorCount;
 
     /**
      * Pointer to characteristic's implicit CCCD, if applicable
      *
      * @note this is only populated if the stack creates an implicit CCCD
      * for this GattCharacteristic. If the descriptors array passed into
      * the constructor includes a CCCD this field is left as nullptr to
      * indicate the CCCD was explicitly created.
      */
     GattAttribute* _implicit_cccd = nullptr;
 
     /**
      * The registered callback handler for read authorization reply.
      */
     FunctionPointerWithContext<GattReadAuthCallbackParams *>
         readAuthorizationCallback;
 
     /**
      * The registered callback handler for write authorization reply.
      */
     FunctionPointerWithContext<GattWriteAuthCallbackParams *>
         writeAuthorizationCallback;
 
     /**
      * Security requirements of update operations.
      *
      * The peer must meet the security requirement to enable, disable and
      * receive updates
      */
     uint8_t _update_security: SecurityRequirement_t::size;
 };
 
 /**
  * Helper class that represents a read only GattCharacteristic.
  */
 template <typename T>
 class ReadOnlyGattCharacteristic : public GattCharacteristic {
 public:
     /**
      * Construct a ReadOnlyGattCharacteristic.
      *
      * @param[in] uuid The characteristic's UUID.
      * @param[in] valuePtr Pointer to the characteristic's initial value. The
      * pointer is reinterpreted as a pointer to an uint8_t buffer.
      * @param[in] additionalProperties Additional characteristic properties. By
      * default, the properties are set to
      * Properties_t::BLE_GATT_CHAR_PROPERTIES_READ.
      * @param[in] descriptors An array of pointers to descriptors to be added
      * to the new characteristic.
      * @param[in] numDescriptors The total number of descriptors in @p
      * descriptors.
      *
      * @note Instances of ReadOnlyGattCharacteristic have a fixed length
      * attribute value that equals sizeof(T). For a variable length alternative,
      * use GattCharacteristic directly.
      */
     ReadOnlyGattCharacteristic<T>(
         const UUID &uuid,
         T *valuePtr,
         uint8_t additionalProperties = BLE_GATT_CHAR_PROPERTIES_NONE,
         GattAttribute *descriptors[] = nullptr,
         unsigned numDescriptors = 0
     ) : GattCharacteristic(
         uuid,
         reinterpret_cast<uint8_t *>(valuePtr),
         sizeof(T),
         sizeof(T),
         BLE_GATT_CHAR_PROPERTIES_READ | additionalProperties,
         descriptors,
         numDescriptors,
         false
     ) {
     }
 };
 
 /**
  * Helper class that represents a write only GattCharacteristic.
  */
 template <typename T>
 class WriteOnlyGattCharacteristic : public GattCharacteristic {
 public:
     /**
      * Construct a WriteOnlyGattCharacteristic.
      *
      * @param[in] uuid The characteristic's UUID.
      * @param[in] valuePtr Pointer to the characteristic's initial value. The
      * pointer is reinterpreted as a pointer to an uint8_t buffer.
      * @param[in] additionalProperties Additional characteristic properties. By
      * default, the properties are set to
      * Properties_t::BLE_GATT_CHAR_PROPERTIES_WRITE.
      * @param[in] descriptors An array of pointers to descriptors to be added to
      * the new characteristic.
      * @param[in] numDescriptors The total number of descriptors in @p
      * descriptors.
      *
      * @note Instances of WriteOnlyGattCharacteristic have variable length
      * attribute value with maximum size equal to sizeof(T). For a fixed length
      * alternative, use GattCharacteristic directly.
      */
     WriteOnlyGattCharacteristic<T>(
         const UUID &uuid,
         T *valuePtr,
         uint8_t additionalProperties = BLE_GATT_CHAR_PROPERTIES_NONE,
         GattAttribute *descriptors[] = nullptr,
         unsigned numDescriptors = 0
     ) : GattCharacteristic(
         uuid,
         reinterpret_cast<uint8_t *>(valuePtr),
         sizeof(T),
         sizeof(T),
         BLE_GATT_CHAR_PROPERTIES_WRITE | additionalProperties,
         descriptors,
         numDescriptors
     ) {
     }
 };
 
 /**
  * Helper class that represents a readable and writable GattCharacteristic.
  */
 template <typename T>
 class ReadWriteGattCharacteristic : public GattCharacteristic {
 public:
     /**
      * Construct a ReadWriteGattCharacteristic.
      *
      * @param[in] uuid The characteristic's UUID.
      * @param[in] valuePtr Pointer to the characteristic's initial value. The
      * pointer is reinterpreted as a pointer to an uint8_t buffer.
      * @param[in] additionalProperties Additional characteristic properties. By
      * default, the properties are set to
      * Properties_t::BLE_GATT_CHAR_PROPERTIES_WRITE and
      * Properties_t::BLE_GATT_CHAR_PROPERTIES_READ.
      * @param[in] descriptors An array of pointers to descriptors to be added to
      * the new characteristic.
      * @param[in] numDescriptors The total number of descriptors in @p descriptors.
      *
      * @note Instances of ReadWriteGattCharacteristic have variable length
      * attribute value with maximum size equal to sizeof(T). For a fixed length
      * alternative, use GattCharacteristic directly.
      */
     ReadWriteGattCharacteristic<T>(
         const UUID &uuid,
         T *valuePtr,
         uint8_t additionalProperties = BLE_GATT_CHAR_PROPERTIES_NONE,
         GattAttribute *descriptors[] = nullptr,
         unsigned numDescriptors = 0
     ) : GattCharacteristic(
         uuid,
         reinterpret_cast<uint8_t *>(valuePtr),
         sizeof(T),
         sizeof(T),
         BLE_GATT_CHAR_PROPERTIES_READ | BLE_GATT_CHAR_PROPERTIES_WRITE | additionalProperties,
         descriptors,
         numDescriptors
     ) {
     }
 };
 
 /**
  * Helper class that represents a write-only GattCharacteristic with an array
  * value.
  */
 template <typename T, unsigned NUM_ELEMENTS>
 class WriteOnlyArrayGattCharacteristic : public GattCharacteristic {
 public:
     /**
      * Construct a WriteOnlyGattCharacteristic.
      *
      * @param[in] uuid The characteristic's UUID.
      * @param[in] valuePtr Pointer to an array of length NUM_ELEMENTS containing
      * the characteristic's initial value. The pointer is reinterpreted as a
      * pointer to an uint8_t buffer.
      * @param[in] additionalProperties Additional characteristic properties. By
      * default, the properties are set to
      * Properties_t::BLE_GATT_CHAR_PROPERTIES_WRITE.
      * @param[in] descriptors An array of pointers to descriptors to be added to
      * the new characteristic.
      * @param[in] numDescriptors The total number of descriptors in @p descriptors.
      *
      * @note Instances of WriteOnlyGattCharacteristic have variable length
      * attribute value with maximum size equal to sizeof(T) * NUM_ELEMENTS.
      * For a fixed length alternative, use GattCharacteristic directly.
      */
     WriteOnlyArrayGattCharacteristic<T, NUM_ELEMENTS>(
         const UUID &uuid,
         T valuePtr[NUM_ELEMENTS],
         uint8_t additionalProperties = BLE_GATT_CHAR_PROPERTIES_NONE,
         GattAttribute *descriptors[] = nullptr,
         unsigned numDescriptors = 0
     ) : GattCharacteristic(
         uuid,
         reinterpret_cast<uint8_t *>(valuePtr),
         sizeof(T) * NUM_ELEMENTS,
         sizeof(T) * NUM_ELEMENTS,
         BLE_GATT_CHAR_PROPERTIES_WRITE | additionalProperties,
         descriptors,
         numDescriptors
     ) {
     }
 };
 
 /**
  * Helper class that represents a read-only GattCharacteristic with an array
  * value.
  */
 template <typename T, unsigned NUM_ELEMENTS>
 class ReadOnlyArrayGattCharacteristic : public GattCharacteristic {
 public:
     /**
      * Construct a ReadOnlyGattCharacteristic.
      *
      * @param[in] uuid The characteristic's UUID.
      * @param[in] valuePtr Pointer to an array of length NUM_ELEMENTS containing
      * the characteristic's initial value. The pointer is reinterpreted as a
      * pointer to an uint8_t buffer.
      * @param[in] additionalProperties Additional characteristic properties. By
      * default, the properties are set to
      * Properties_t::BLE_GATT_CHAR_PROPERTIES_READ.
      * @param[in] descriptors An array of pointers to descriptors to be added to
      * the new characteristic.
      * @param[in] numDescriptors The total number of descriptors in @p
      * descriptors.
      *
      * @note Instances of ReadOnlyGattCharacteristic have fixed length
      * attribute value that equals sizeof(T) * NUM_ELEMENTS. For a variable
      * length alternative, use GattCharacteristic directly.
      */
     ReadOnlyArrayGattCharacteristic<T, NUM_ELEMENTS>(
         const UUID    &uuid,
         T valuePtr[NUM_ELEMENTS],
         uint8_t additionalProperties = BLE_GATT_CHAR_PROPERTIES_NONE,
         GattAttribute *descriptors[] = nullptr,
         unsigned numDescriptors = 0
     ) : GattCharacteristic(
         uuid,
         reinterpret_cast<uint8_t *>(valuePtr),
         sizeof(T) * NUM_ELEMENTS,
         sizeof(T) * NUM_ELEMENTS,
         BLE_GATT_CHAR_PROPERTIES_READ | additionalProperties,
         descriptors,
         numDescriptors,
         false
     ) {
     }
 };
 
 /**
  * Helper class that represents a readable and writable GattCharacteristic with
  * an array value.
  */
 template <typename T, unsigned NUM_ELEMENTS>
 class ReadWriteArrayGattCharacteristic : public GattCharacteristic {
 public:
     /**
      * Construct a ReadWriteGattCharacteristic.
      *
      * @param[in] uuid The characteristic's UUID.
      * @param[in] valuePtr Pointer to an array of length NUM_ELEMENTS containing
      * the characteristic's initial value. The pointer is reinterpreted as a
      * pointer to an uint8_t buffer.
      * @param[in] additionalProperties Additional characteristic properties. By
      * default, the properties are set to
      * Properties_t::BLE_GATT_CHAR_PROPERTIES_WRITE |
      * Properties_t::BLE_GATT_CHAR_PROPERTIES_READ.
      * @param[in] descriptors An array of pointers to descriptors to be added to
      * the new characteristic.
      * @param[in] numDescriptors The total number of descriptors in @p descriptors.
      *
      * @note Instances of ReadWriteGattCharacteristic have variable length
      * attribute value with maximum size equal to sizeof(T) * NUM_ELEMENTS.
      * For a fixed length alternative, use GattCharacteristic directly.
      */
     ReadWriteArrayGattCharacteristic<T, NUM_ELEMENTS>(
         const UUID    &uuid,
         T valuePtr[NUM_ELEMENTS],
         uint8_t additionalProperties = BLE_GATT_CHAR_PROPERTIES_NONE,
         GattAttribute *descriptors[] = nullptr,
         unsigned numDescriptors = 0
     ) : GattCharacteristic(
         uuid,
         reinterpret_cast<uint8_t *>(valuePtr),
         sizeof(T) * NUM_ELEMENTS,
         sizeof(T) * NUM_ELEMENTS,
         BLE_GATT_CHAR_PROPERTIES_READ | BLE_GATT_CHAR_PROPERTIES_WRITE | additionalProperties,
         descriptors,
         numDescriptors
     ) {
     }
 };
 
 /**
  * @}
  * @}
  * @}
  */
 
 #endif /* ifndef __GATT_CHARACTERISTIC_H__ */
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