About API Guide - C++

These APIs were added to AllJoyn® 14.12 release for using the About Feature in an older release please see: Legay C++ About API Guide

Reference code

Classes used to send AboutData

Class Description
AboutObj Class that implements the org.alljoyn.About interface as a BusObject.
AboutIconObj Class that implements the org.alljoyn.Icon interface as a BusObject.
AboutDataListener Interface that supplies the MsgArg containing the AboutData fields required for the Announce signal payload and GetAboutData().
AboutData A default implementation of the AboutDataListener interface. For most developers this implementation will be sufficient.
AboutIcon A container class that holds the icon sent by the AboutIconObj

Classes used to receive AboutData

Class Description
AboutProxy Class used to get proxy access to the AboutObj.
AboutIconProxy Class used to get proxy access to the AboutIconObj.
AboutListener Abstract base class implemented by AllJoyn users to receive About interface related events
AboutData A default implementation of the AboutDataListener interface. This class can be used to read the contents of an org.alljoyn.About.Announce signal
AboutObjectDescription A helper class for accessing the fields of the ObjectDescription MsgArg that is sent as part of the org.alljoyn.About.Announce signal
BusAttachment Used to register AboutListeners and specify interfaces of interest

Setting Up an application to send an Announce signal

The following is the high-level process to build an application that will broadcast an Announce signal. Steps marked with a * are unique to applications using the About Feature.

Setting Up the AllJoyn Framework to receive an Announce signal

The following is the high-level process to build an application that will receive an Announce signal. Steps marked with a * are unique to applications using the About Feature.

Sample code for sending an Announce signal

Create a BusAttachment

Create a new BusAttachment.

BusAttachment bus("About Service Example");

Start the BusAttachment and Connect to the routing node.

status = bus.Start();
if (ER_OK != status) {
    printf("FAILED to start BusAttachment (%s)\n", QCC_StatusText(status));

status = bus.Connect();
if (ER_OK != status) {
    printf("FAILED to connect to router node (%s)\n", QCC_StatusText(status));

Bind a session port that will be used to communicate. the value for ASSIGNED_SESSION_PORT is chosen by the developer. The value itself is unimportant. What is important is that the session port bound to is part of the Announce signal.

SessionOpts opts(SessionOpts::TRAFFIC_MESSAGES, false, SessionOpts::PROXIMITY_ANY, TRANSPORT_ANY);
SessionPort sessionPort = ASSIGNED_SESSION_PORT;
MySessionPortListener sessionPortListener;
bus.BindSessionPort(sessionPort, opts, sessionPortListener);
if (ER_OK != status) {
    printf("Failed to BindSessionPort (%s)", QCC_StatusText(status));

Create interfaces

The interface is a collection methods, signals, and properties. The interface can be specified in code or using xml notation.

For this sample the following xml interface was used.

<interface name='com.example.about.feature.interface.sample' >
  <method name='Echo'>
    <arg name='out_arg' type='s' direction='in' />
    <arg name='return_arg' type='s' direction='out' />

C++ code showing adding the interface to the BusAttachment using xml. The INTERFACE_NAME is coded to be the string com.example.about.feature.interface.sample.

qcc::String interface = "<node>"
                        "<interface name='" + qcc::String(INTERFACE_NAME) + "'>"
                        "  <method name='Echo'>"
                        "    <arg name='out_arg' type='s' direction='in' />"
                        "    <arg name='return_arg' type='s' direction='out' />"
                        "  </method>"

status = bus.CreateInterfacesFromXml(interface.c_str());
if (ER_OK != status) {
    printf("Failed to parse the xml interface definition (%s)", QCC_StatusText(status));

Alternative C++ code showing adding the interface with out using xml notation.

/* Add org.alljoyn.Bus.method_sample interface */
InterfaceDescription* intf = NULL;
status = bus.CreateInterface(INTERFACE_NAME, intf);

if (status == ER_OK) {
    printf("Interface created.\n");
    intf->AddMethod("Echo", "s",  "s", "out_arg,return_arg", 0);
} else {
    printf("Failed to create interface '%s'.\n", INTERFACE_NAME);

Create BusObjects for interfaces

Sample implementation of a BusObject that announces the interface defined above. When adding the interface to the BusObject you can specify if that interface is announced by adding the ANNOUNCED value to the AddInterface() member function.

NOTE: The BusObject adds method handlers for methods specified in the com.example.about.feature.interface.sample interface. If it contained any properties it would also be responsible for add Get/Set handler functions for the properties as well. The code lets the object path be passed in at runtime. The path could have also been hard coded into the BusObject.

class MyBusObject : public BusObject {
    MyBusObject(BusAttachment& bus, const char* path)
        : BusObject(path) {
        QStatus status;
        const InterfaceDescription* iface = bus.GetInterface(INTERFACE_NAME);
        assert(iface != NULL);

        // Here the value ANNOUNCED tells AllJoyn that this interface
        // should be announced
        status = AddInterface(*iface, ANNOUNCED);
        if (status != ER_OK) {
            printf("Failed to add %s interface to the BusObject\n", INTERFACE_NAME);

        /* Register the method handlers with the object */
        const MethodEntry methodEntries[] = {
            { iface->GetMember("Echo"), static_cast<MessageReceiver::MethodHandler>(&MyBusObject::Echo) }
        AddMethodHandlers(methodEntries, sizeof(methodEntries) / sizeof(methodEntries[0]));

    // Respond to remote method call `Echo` by returning the string back to the
    // sender.
    void Echo(const InterfaceDescription::Member* member, Message& msg) {
        printf("Echo method called: %s", msg->GetArg(0)->v_string.str);
        const MsgArg* arg((msg->GetArg(0)));
        QStatus status = MethodReply(msg, arg, 1);
        if (status != ER_OK) {
            printf("Failed to created MethodReply.\n");

Register the BusObjects with the BusAttachment

MyBusObject busObject(bus, "/example/path");
status = bus.RegisterBusObject(busObject);
if (ER_OK != status) {
    printf("Failed to register BusObject (%s)", QCC_StatusText(status));

AboutData fields

Field name Required Announced Localized Signature
AppId yes yes no ay
DefaultLanguage yes yes no s
DeviceName no yes yes s
DeviceId yes yes no s
AppName yes yes yes s
Manufacturer yes yes yes s
ModelNumber yes yes no s
SupportedLanguages yes no no as
Description yes no yes s
DateofManufacture no no no s
SoftwareVersion yes no no s
AJSoftwareVersion yes no no s
HardwareVersion no no no s
SupportUrl no no no s

Fields marked as Announced are part of the Announce signal. If a value is not announced then you must use the org.alljoyn.About.GetAboutData method to access those values.

Fields marked as Required must all be supplied to send an Announce signal. They are required even if the value is not part of the Announce signal.

Fields marked as Localized should supply localization values for every language listed in the SupportedLanguages

AppId is a 128-bit UUID (16-bytes) as specified in RFC 4122.

Fill in your AboutData

The AboutData is an instance of the AboutDataListener interface. For most developers, the AboutData will provide the AboutDataListener dictionary of key/value pairs (a{sv}). This is needed to send an `Announce signal.

// Setup the about data
// The default language is specified in the constructor. If the default language
// is not specified any Field that should be localized will return an error
AboutData aboutData("en");
//AppId is a 128bit uuid
uint8_t appId[] = { 0x01, 0xB3, 0xBA, 0x14,
                    0x1E, 0x82, 0x11, 0xE4,
                    0x86, 0x51, 0xD1, 0x56,
                    0x1D, 0x5D, 0x46, 0xB0 };
aboutData.SetAppId(appId, 16);
aboutData.SetDeviceName("My Device Name");
//DeviceId is a string encoded 128bit UUIDf
aboutData.SetDescription("A poetic description of this application");

Localized values like DeviceName, AppName, etc are automatically set to the default language specified in the constructor unless a different language tag is passed in when setting the values. For example, to add the Spanish language to the AboutData the following would be done. All strings must be UTF-8 encoded.

aboutData.SetDeviceName("Mi dispositivo Nombre", "es");
aboutData.SetAppName("aplicación", "es");
aboutData.SetManufacturer("fabricante", "es");
aboutData.SetDescription("Una descripción poética de esta aplicación", "es");

Any new language specified, including the default language, is automatically added to the SupportedLanguages by the AboutData implementation.

The AJSoftwareVersion is also automatically filled in by the AboutData implementation.

Create an AboutObj and Announce

AboutObj aboutObj(bus);
status = aboutObj.Announce(sessionPort, aboutData);
if (ER_OK != status) {
    printf("AboutObj Announce failed (%s)\n", QCC_StatusText(status));

The ObjectDesciprition part of the announced signal is found automatically by introspectin the the BusObjects that were registered with the BusAttachment.

Any time a new interface is added or the AboutData is changed the Announce member function should be called again.

Sample code for receiving an Announce signal

Code that receives an Announce signal will need to create, start, and connect a BusAttachment the same as the code that sent the Announce signal. The application that receives the Announce signal does not need to bind a session port. See create a BusAttachment

create an AboutListener

The AboutListener interface is responsible for responding to Announce signals.

class MyAboutListener : public AboutListener {
    void Announced(const char* busName, uint16_t version, SessionPort port,
                   const MsgArg& objectDescriptionArg, const MsgArg& aboutDataArg) {
        // Place code here to handle Announce signal.

The AboutListener is called by the AllJoyn routing node when an Announce signal is found. The Announced call back contains all the information contained in the received Announce signal as well as the unique BusName of the BusAttachment that emitted the Announce signal. This information can be used to form a session with the remote device; and Make a ProxyBus object based on the interfaces reported in the objectDescriptionArg.

Register the new AboutListener and call WhoImplements

MyAboutListener aboutListener;

const char* interfaces[] = { INTERFACE_NAME };
status = bus.WhoImplements(interfaces, sizeof(interfaces) / sizeof(interfaces[0]));
if (ER_OK != status) {
    printf("WhoImplements call FAILED with status %s\n", QCC_StatusText(status));

Although it is possible to register multiple AboutListeners it is unlikely that a program will need more than one AboutListener.

The WhoImplements member function

The WhoImplements member function is used to declare your interest in one or more specific interfaces. If a remote device is announcing the interface(s) then all Registered AboutListeners will be called.

For example, if you need both com.example.Audio and com.example.Video interfaces then do the following.

RegisterAboutListener once:

const char* interfaces[] = {"com.example.Audio", "com.example.Video"};
WhoImplements(interfaces, sizeof(interfaces) / sizeof(interfaces[0]));

If the AboutListener should be called if com.example.Audio or com.example.Video interfaces are found then call WhoImplements multiple times:

const char* audioInterface[] = {"com.example.Audio"};
WhoImplements(audioInterface, sizeof(audioInterface) / sizeof(audioInterface[0]));
const char* videoInterface[] = {"com.example.Video"};
WhoImplements(videoInterface, sizeof(videoInterface) / sizeof(videoInterface[0]));

The interface name may be a prefix followed by a *. Using this, the example where we are interested in com.example.Audio or com.example.Video interfaces could be written as:

const char* exampleInterface[] = {"com.example.*"};
WhoImplements(exampleInterface, sizeof(exampleInterface) / sizeof(exampleInterface[0]));

The AboutListener will receive any announcement that implements an interface beginning with the com.example. name.

It is the AboutListeners responsibility to parse through the reported interfaces to figure out what should be done in response to the Announce signal.

Calls to WhoImplements is ref counted. If WhoImplements is called with the same list of interfaces multiple times then CancelWhoImplements must also be called multiple times with the same list of interfaces.

Specifying NULL for the implementsInterfaces parameter is allowed, however, it could have significant impact on network performance and should be avoided unless all announcements are needed.

Add an AboutIcon (optional)

An application that sends an Announce signal can be extended to broadcast a device icon using an instance of the AboutIconObj class.

Provision for the Icon content and URL

An Icon is published directly as a byte array or a reference URL, and is provisioned as follows:

Create an icon using a byte array. An Icon size of 72 pixels x 72 pixels is recommended.

uint8_t aboutIconContent[] = { 0x89, 0x50, 0x4E, 0x47, 0x0D /* Add relevant data here */ };
AboutIcon icon;
status = icon.SetContent("image/png", aboutIconContent, sizeof(aboutIconContent) / sizeof(aboutIconContent[0]));

Create an icon using a URL.

AboutIcon icon;
status = icon.SetUrl("image/png", "http://www.example.com");

As long as the MimeType of the Url and the icon content are the same. Both the Url and icon content can be set.


The AboutIconObj will create and register a BusObject to handle remote method calls made on the org.alljoyn.Icon interface. The AboutIconObj is announced by default. Applications interested in the org.alljoyn.Icon interface can call WhoImplements(org.alljoyn.Icon) to find applications that broadcast device icon information.

Announce the org.alljoyn.Icon interface:

AboutIconObj aboutIconObj(bus, icon);
aboutObj.Announce(port, aboutData);

Discover the org.alljoyn.Icon interface


Using Ping to determine presence

The BusAttachment Ping member function can be used to determine if a device is responsive. Contents of an Announce signal can be stale so it is recommended to ping the device to see if it is still present and responsive before attempting to form a connection.

NOTE: The BusAttachment::Ping member function makes a bus call. If Ping is called inside an AllJoyn callback, BusAttachment::EnableConcurrentCallbacks must be called first.

// when pinging a remote bus wait a max of 5 seconds
#define PING_WAIT_TIME    5000
QStatus status = bus.Ping(busName.c_str(), PING_WAIT_TIME);
if( ER_OK == status) {

Request non-announced data

If there is a need to request information that is not contained in the announcement, perform the following steps.

  1. Join the session

    Create a session with the application by calling BusAttachment::JoinSession.

    NOTE: The variables name and port are obtained from the AboutListener::Announced member function.

    SessionId sessionId;
    SessionOpts opts(SessionOpts::TRAFFIC_MESSAGES, false,
                     SessionOpts::PROXIMITY_ANY, TRANSPORT_ANY);
    QStatus status = bus.JoinSession(name, port, NULL, sessionId, opts);
    if (status == ER_OK) {
       printf("JoinSession SUCCESS (Session id=%d)", sessionId);
    } else {
       printf("JoinSession failed");
  2. Create an AboutProxy

    Generate an About ProxyBusObject by passing the local BusAttachment, the name of the remote BusAttachment, and the SessionId obtained from the BusAttachment::JoinSession call.

    AboutProxy aboutProxy(bus, busName, sessionId);
    MsgArg arg;
    status = aboutProxy.GetAboutData("", arg);
    if(ER_OK != status) {
        //handle error
  3. Create AboutIconProxy (optional)

    Generate an Icon ProxyBusObject by passing the local BusAttachment, the name of the remote BusAttachment, and the SessionId obtained from the BusAttachment::JoinSession call.

    AboutIconProxy aiProxy(bus, busName, sessionId);
    AboutIcon retIcon;
    status = aiProxy.GetIcon(retIcon);
    if(ER_OK != status) {
        //handle error
    // Get the Url
    // Get the content size
    // Get a pointer to the icon content
    // Get the MimeType