QDBusArgument Class

The QDBusArgument class is used to marshall and demarshall D-Bus arguments. More...

• List of all members, including inherited members

Public Types

Public Functions

Related Non-Members

Detailed Description

The class is used to send arguments over D-Bus to remote applications and to receive them back. D-Bus offers an extensible type system, based on a few primitive types and associations of them. See the Qt D-Bus Type System page for more information on the type system.

QDBusArgument is the central class in the Qt D-Bus type system, providing functions to marshall and demarshall the primitive types. The compound types are then created by association of one or more of the primitive types in arrays, dictionaries or structures.

The following example illustrates how a structure containing an integer and a string can be constructed using the Qt D-Bus type system:

struct MyStructure
{
    int count;
    QString name;

    // ...
};
Q_DECLARE_METATYPE(MyStructure)

// Marshall the MyStructure data into a D-Bus argument
QDBusArgument &operator<<(QDBusArgument &argument, const MyStructure &myStruct)
{
    argument.beginStructure();
    argument << myStruct.count << myStruct.name;
    argument.endStructure();
    return argument;
}

// Retrieve the MyStructure data from the D-Bus argument
const QDBusArgument &operator>>(const QDBusArgument &argument, MyStructure &myStruct)
{
    argument.beginStructure();
    argument >> myStruct.count >> myStruct.name;
    argument.endStructure();
    return argument;
}

The type has to be registered with qDBusRegisterMetaType() before it can be used with QDBusArgument. Therefore, somewhere in your program, you should add the following code:

qDBusRegisterMetaType<MyStructure>();

Once registered, a type can be used in outgoing method calls (placed with QDBusAbstractInterface::call()), signal emissions from registered objects or in incoming calls from remote applications.

It is important to note that the operator<< and operator>> streaming functions must always produce the same number of entries in case of structures, both in reading and in writing (marshalling and demarshalling), otherwise calls and signals may start to silently fail.

The following example illustrates this wrong usage in context of a class that may contain invalid data:

//bad code
    // Wrongly marshall the MyTime data into a D-Bus argument
    QDBusArgument &operator<<(QDBusArgument &argument, const MyTime &mytime)
    {
        argument.beginStructure();
        if (mytime.isValid)
            argument << true << mytime.hour
                     << mytime.minute << mytime.second;
        else
            argument << false;
        argument.endStructure();
        return argument;
    }

In this example, both the operator<< and the operator>> functions may produce a different number of reads/writes. This can confuse the Qt D-Bus type system and should be avoided.

See also QDBusAbstractInterface, The Qt D-Bus type system, Using Adaptors, and qdbus_cast().

Member Type Documentation

enum QDBusArgument::ElementType

This enum describes the type of element held by the argument.

See also currentType().

Member Function Documentation

QDBusArgument::QDBusArgument(const QDBusArgument &other)

Constructs a copy of the other QDBusArgument object.

Both objects will therefore contain the same state from this point forward. QDBusArguments are explicitly shared and, therefore, any modification to either copy will affect the other one too.

QDBusArgument::QDBusArgument()

Constructs an empty QDBusArgument argument.

An empty QDBusArgument object does not allow either reading or writing to be performed.

QDBusArgument &QDBusArgument::operator=(const QDBusArgument &other)

Copies the other QDBusArgument object into this one.

Both objects will therefore contain the same state from this point forward. QDBusArguments are explicitly shared and, therefore, any modification to either copy will affect the other one too.

QDBusArgument::~QDBusArgument()

Disposes of the resources associated with this QDBusArgument object.

QVariant QDBusArgument::asVariant() const

Returns the current argument in the form of a QVariant. Basic types will be decoded and returned in the QVariant, but for complex types, this function will return a QDBusArgument object in the QVariant. It is the caller's responsibility to decode the argument (for example, by calling asVariant() in it).

For example, if the current argument is an INT32, this function will return a QVariant with an argument of type QMetaType::Int. For an array of INT32, it will return a QVariant containing a QDBusArgument.

If an error occurs or if there are no more arguments to decode (i.e., we are at the end of the argument list), this function will return an invalid QVariant.

See also atEnd().

bool QDBusArgument::atEnd() const

Returns true if there are no more elements to be extracted from this QDBusArgument. This function is usually used in QDBusArgument objects returned from beginMap() and beginArray().

void QDBusArgument::beginArray(QMetaType id)

Opens a new D-Bus array suitable for appending elements of meta-type id.

This function is used usually in operator<< streaming operators, as in the following example:

// Append an array of MyElement types
QDBusArgument &operator<<(QDBusArgument &argument, const MyArray &myArray)
{
    argument.beginArray(qMetaTypeId<MyElement>());
    for (const auto &element : myArray)
        argument << element;
    argument.endArray();
    return argument;
}

If the type you want to marshall is a QList or any of the Qt's Container Classes that take one template parameter, you need not declare an operator<< function for it, since Qt D-Bus provides generic templates to do the job of marshalling the data. The same applies for STL's sequence containers, such as std::list, std::vector, etc.

See also endArray(), beginStructure(), and beginMap().

void QDBusArgument::beginArray() const

Recurses into the D-Bus array to allow extraction of the array elements.

This function is used usually in operator>> streaming operators, as in the following example:

// Extract a MyArray array of MyElement elements
const QDBusArgument &operator>>(const QDBusArgument &argument, MyArray &myArray)
{
    argument.beginArray();
    myArray.clear();

    while (!argument.atEnd()) {
        MyElement element;
        argument >> element;
        myArray.append(element);
    }

    argument.endArray();
    return argument;
}

If the type you want to demarshall is a QList or any of the Qt's Container Classes that take one template parameter, you need not declare an operator>> function for it, since Qt D-Bus provides generic templates to do the job of demarshalling the data. The same applies for STL's sequence containers, such as std::list, std::vector, etc.

See also atEnd(), beginStructure(), and beginMap().

void QDBusArgument::beginMap(QMetaType keyMetaType, QMetaType valueMetaType)

Opens a new D-Bus map suitable for appending elements. Maps are containers that associate one entry (the key) to another (the value), such as Qt's QMap or QHash. The ids of the map's key and value meta types must be passed in keyMetaType and valueMetaType respectively.

This function is used usually in operator<< streaming operators, as in the following example:

// Append a dictionary that associates ints to MyValue types
QDBusArgument &operator<<(QDBusArgument &argument, const MyDictionary &myDict)
{
    argument.beginMap(QMetaType::fromType<int>(), QMetaType::fromType<MyValue>());
    MyDictionary::const_iterator i;
    for (i = myDict.cbegin(); i != myDict.cend(); ++i) {
        argument.beginMapEntry();
        argument << i.key() << i.value();
        argument.endMapEntry();
    }
    argument.endMap();
    return argument;
}

You usually don't need to provide an operator<< or operator>> function for associative containers such as QHash or std::map, since Qt D-Bus provides generic templates to do the job of marshalling the data.

See also endMap(), beginStructure(), beginArray(), and beginMapEntry().

void QDBusArgument::beginMap() const

Recurses into the D-Bus map to allow extraction of the map's elements.

This function is used usually in operator>> streaming operators, as in the following example:

// Extract a MyDictionary map that associates integers to MyElement items
const QDBusArgument &operator>>(const QDBusArgument &argument, MyDictionary &myDict)
{
    argument.beginMap();
    myDict.clear();

    while (!argument.atEnd()) {
        int key;
        MyElement value;
        argument.beginMapEntry();
        argument >> key >> value;
        argument.endMapEntry();
        myDict.insert(key, value);
    }

    argument.endMap();
    return argument;
}

If the type you want to demarshall is a QMap or QHash, you need not declare an operator>> function for it, since Qt D-Bus provides generic templates to do the job of demarshalling the data.

See also endMap(), beginStructure(), beginArray(), and beginMapEntry().

void QDBusArgument::beginMapEntry()

Opens a D-Bus map entry suitable for appending the key and value entries. This function is only valid when a map has been opened with beginMap().

See beginMap() for an example of usage of this function.

See also endMapEntry() and beginMap().

void QDBusArgument::beginMapEntry() const

Recurses into the D-Bus map entry to allow extraction of the key and value pair.

See beginMap() for an example of how this function is usually used.

See also endMapEntry() and beginMap().

void QDBusArgument::beginStructure()

Opens a new D-Bus structure suitable for appending new arguments.

This function is used usually in operator<< streaming operators, as in the following example:

QDBusArgument &operator<<(QDBusArgument &argument, const MyStructure &myStruct)
{
    argument.beginStructure();
    argument << myStruct.member1 << myStruct.member2;
    argument.endStructure();
    return argument;
}

Structures can contain other structures, so the following code is also valid:

QDBusArgument &operator<<(QDBusArgument &argument, const MyStructure &myStruct)
{
    argument.beginStructure();
    argument << myStruct.member1 << myStruct.member2;

    argument.beginStructure();
    argument << myStruct.member3.subMember1 << myStruct.member3.subMember2;
    argument.endStructure();

    argument << myStruct.member4;
    argument.endStructure();
    return argument;
}

See also endStructure(), beginArray(), and beginMap().

void QDBusArgument::beginStructure() const

Opens a D-Bus structure suitable for extracting elements.

This function is used usually in operator>> streaming operators, as in the following example:

const QDBusArgument &operator>>(const QDBusArgument &argument, MyStructure &myStruct)
{
    argument.beginStructure();
    argument >> myStruct.member1 >> myStruct.member2 >> myStruct.member3;
    argument.endStructure();
    return argument;
}

See also endStructure(), beginArray(), and beginMap().

QDBusArgument::ElementType QDBusArgument::currentType() const

Returns the classification of the current element type. If an error decoding the type occurs or if we're at the end of the argument, this function returns QDBusArgument::UnknownType.

This function only makes sense when demarshalling arguments. If it is used while marshalling, it will always return UnknownType.

void QDBusArgument::endArray()

Closes a D-Bus array opened with beginArray(). This function must be called same number of times that beginArray() is called.

See also beginArray(), endStructure(), and endMap().

void QDBusArgument::endArray() const

Closes the D-Bus array and allow extracting of the next element after the array.

See also beginArray().

void QDBusArgument::endMap()

Closes a D-Bus map opened with beginMap(). This function must be called same number of times that beginMap() is called.

See also beginMap(), endStructure(), and endArray().

void QDBusArgument::endMap() const

Closes the D-Bus map and allow extracting of the next element after the map.

See also beginMap().

void QDBusArgument::endMapEntry()

Closes a D-Bus map entry opened with beginMapEntry(). This function must be called same number of times that beginMapEntry() is called.

See also beginMapEntry().

void QDBusArgument::endMapEntry() const

Closes the D-Bus map entry and allow extracting of the next element on the map.

See also beginMapEntry().

void QDBusArgument::endStructure()

Closes a D-Bus structure opened with beginStructure(). This function must be called same number of times that beginStructure() is called.

See also beginStructure(), endArray(), and endMap().

void QDBusArgument::endStructure() const

Closes the D-Bus structure and allow extracting of the next element after the structure.

See also beginStructure().

void QDBusArgument::swap(QDBusArgument &other)

Swaps this QDBusArgument instance with other.

QDBusArgument &QDBusArgument::operator<<(const QStringList &arg)

This is an overloaded function.

Appends the QStringList given by arg as ARRAY of STRING to the D-Bus stream.

QStringList and QByteArray are the only two non-primitive types that are supported directly by QDBusArgument because of their widespread usage in Qt applications.

Other arrays are supported through compound types in Qt D-Bus.

QDBusArgument &QDBusArgument::operator<<(uchar arg)

Appends the primitive value arg of type BYTE to the D-Bus stream.

QDBusArgument &QDBusArgument::operator<<(bool arg)

This is an overloaded function.

Appends the primitive value arg of type BOOLEAN to the D-Bus stream.

QDBusArgument &QDBusArgument::operator<<(short arg)

This is an overloaded function.

Appends the primitive value arg of type INT16 to the D-Bus stream.

QDBusArgument &QDBusArgument::operator<<(ushort arg)

This is an overloaded function.

Appends the primitive value arg of type UINT16 to the D-Bus stream.

QDBusArgument &QDBusArgument::operator<<(int arg)

This is an overloaded function.

Appends the primitive value arg of type INT32 to the D-Bus stream.

QDBusArgument &QDBusArgument::operator<<(uint arg)

This is an overloaded function.

Appends the primitive value arg of type UINT32 to the D-Bus stream.

QDBusArgument &QDBusArgument::operator<<(qlonglong arg)

This is an overloaded function.

Appends the primitive value arg of type INT64 to the D-Bus stream.

QDBusArgument &QDBusArgument::operator<<(qulonglong arg)

This is an overloaded function.

Appends the primitive value arg of type UINT64 to the D-Bus stream.

QDBusArgument &QDBusArgument::operator<<(double arg)

This is an overloaded function.

Appends the primitive value arg of type DOUBLE (double-precision floating-point) to the D-Bus stream.

QDBusArgument &QDBusArgument::operator<<(const QString &arg)

This is an overloaded function.

Appends the primitive value arg of type STRING (Unicode character string) to the D-Bus stream.

QDBusArgument &QDBusArgument::operator<<(const QDBusVariant &arg)

This is an overloaded function.

Appends the primitive value arg of type VARIANT to the D-Bus stream.

A D-Bus variant type can contain any type, including other variants. It is similar to the Qt QVariant type.

QDBusArgument &QDBusArgument::operator<<(const QByteArray &arg)

This is an overloaded function.

Appends the QByteArray given by arg as ARRAY of BYTE to the D-Bus stream.

QStringList and QByteArray are the only two non-primitive types that are supported directly by QDBusArgument because of their widespread usage in Qt applications.

Other arrays are supported through compound types in Qt D-Bus.

const QDBusArgument &QDBusArgument::operator>>(QStringList &arg) const

This is an overloaded function.

Extracts an array of strings from the D-Bus stream and return it as a QStringList.

QStringList and QByteArray are the only two non-primitive types that are supported directly by QDBusArgument because of their widespread usage in Qt applications.

Other arrays are supported through compound types in Qt D-Bus.

const QDBusArgument &QDBusArgument::operator>>(uchar &arg) const

Extracts one D-BUS primitive argument of type BYTE from the D-BUS stream and puts it into arg.

const QDBusArgument &QDBusArgument::operator>>(bool &arg) const

This is an overloaded function.

Extracts one D-Bus primitive argument of type BOOLEAN from the D-Bus stream.

const QDBusArgument &QDBusArgument::operator>>(short &arg) const

This is an overloaded function.

Extracts one D-Bus primitive argument of type INT16 from the D-Bus stream.

const QDBusArgument &QDBusArgument::operator>>(ushort &arg) const

This is an overloaded function.

Extracts one D-Bus primitive argument of type UINT16 from the D-Bus stream.

const QDBusArgument &QDBusArgument::operator>>(int &arg) const

This is an overloaded function.

Extracts one D-Bus primitive argument of type INT32 from the D-Bus stream.

const QDBusArgument &QDBusArgument::operator>>(uint &arg) const

This is an overloaded function.

Extracts one D-Bus primitive argument of type UINT32 from the D-Bus stream.

const QDBusArgument &QDBusArgument::operator>>(qlonglong &arg) const

This is an overloaded function.

Extracts one D-Bus primitive argument of type INT64 from the D-Bus stream.

const QDBusArgument &QDBusArgument::operator>>(qulonglong &arg) const

This is an overloaded function.

Extracts one D-Bus primitive argument of type UINT64 from the D-Bus stream.

const QDBusArgument &QDBusArgument::operator>>(double &arg) const

This is an overloaded function.

Extracts one D-Bus primitive argument of type DOUBLE (double-precision floating point) from the D-Bus stream.

const QDBusArgument &QDBusArgument::operator>>(QString &arg) const

This is an overloaded function.

Extracts one D-Bus primitive argument of type STRING (Unicode character string) from the D-Bus stream.

const QDBusArgument &QDBusArgument::operator>>(QDBusVariant &arg) const

This is an overloaded function.

Extracts one D-Bus primitive argument of type VARIANT from the D-Bus stream.

A D-Bus variant type can contain any type, including other variants. It is similar to the Qt QVariant type.

In case the variant contains a type not directly supported by QDBusArgument, the value of the returned QDBusVariant will contain another QDBusArgument. It is your responsibility to further demarshall it into another type.

const QDBusArgument &QDBusArgument::operator>>(QByteArray &arg) const

This is an overloaded function.

Extracts an array of bytes from the D-Bus stream and return it as a QByteArray.

QStringList and QByteArray are the only two non-primitive types that are supported directly by QDBusArgument because of their widespread usage in Qt applications.

Other arrays are supported through compound types in Qt D-Bus.

#include <QDBusMetaType>

qDBusRegisterMetaType<MyClass>();

MyType item = qdbus_cast<Type>(argument);

MyType item;
argument >> item;