QMetaType Class Reference
^{[QtCore module]}

Detailed Description

The QMetaType class manages named types in the meta-object system.

The class is used as a helper to marshall types in QVariant and in queued signals and slots connections. It associates a type name to a type so that it can be created and destructed dynamically at run-time. Register new types with qRegisterMetaType(). Any class or struct that has a public default constructor, a public copy constructor, and a public destructor can be registered.

The following code allocates and destructs an instance of MyClass:

    int id = QMetaType::type("MyClass");
    if (id != -1) {
        void *myClassPtr = QMetaType::construct(id);
        ...
        QMetaType::destroy(id, myClassPtr);
        myClassPtr = 0;
    }

The Q_DECLARE_METATYPE() macro can be used to register a type at compile time. This is required to use the type as custom type in QVariant.

If we want the stream operators operator<<() and operator>>() to work on QVariant objects that store custom types, the custom type must provide operator<<() and operator>>() operators and register them using qRegisterMetaTypeStreamOperators().

See also Q_DECLARE_METATYPE(), QVariant::setValue(), QVariant::value(), and QVariant::fromValue().

Member Type Documentation

enum QMetaType::Type

These are the built-in types supported by QMetaType:

Additional types can be registered using qRegisterMetaType().

See also type() and typeName().

Member Function Documentation

void * QMetaType::construct ( int type, const void * copy )   [static]

Returns a copy of copy, assuming it is of type type. If copy is zero, creates a default type.

See also destroy(), isRegistered(), and Type.

void QMetaType::destroy ( int type, void * data )   [static]

Destroys the data, assuming it is of the type given.

See also construct(), isRegistered(), and Type.

bool QMetaType::isRegistered ( int type )   [static]

Returns true if the custom datatype with ID type is registered; otherwise returns false.

See also type(), typeName(), and Type.

bool QMetaType::load ( QDataStream & stream, int type, void * data )   [static]

Reads the object of the specified type from the given stream into data. The type must have been registered with qRegisterMetaType() and qRegisterMetaTypeStreamOperators() beforehand.

Normally, you should not need to call this function directly. Instead, use QVariant's operator>>(), which relies on load() to stream custom types.

See also save() and qRegisterMetaTypeStreamOperators().

bool QMetaType::save ( QDataStream & stream, int type, const void * data )   [static]

Writes the object pointed to by data with the ID type to the given stream. The type must have been registered with qRegisterMetaType() and qRegisterMetaTypeStreamOperators() beforehand.

Normally, you should not need to call this function directly. Instead, use QVariant's operator<<(), which relies on save() to stream custom types.

See also load() and qRegisterMetaTypeStreamOperators().

int QMetaType::type ( const char * typeName )   [static]

Returns a handle to the type called typeName, or 0 if there is no such type.

See also isRegistered(), typeName(), and Type.

const char * QMetaType::typeName ( int type )   [static]

Returns the type name associated with the given type, or 0 if no matching type was found. The returned pointer must not be deleted.

See also type(), isRegistered(), and Type.

Related Non-Members

int qMetaTypeId ()

Returns the meta type id of type T at compile time. If the type was not declared with Q_DECLARE_METATYPE(), compilation will fail.

Typical usage:

    int id = qMetaTypeId<QString>();    // id is now QMetaType::QString
    id = qMetaTypeId<MyStruct>();       // compile error if MyStruct not declared

QMetaType::type() returns the same ID as qMetaTypeId(), but does a lookup at runtime based on the name of the type. QMetaType::type() is a bit slower, but compilation succeeds if a type is not registered.

Note: This function is thread-safe.

This function was introduced in Qt 4.1.

See also Q_DECLARE_METATYPE() and QMetaType::type().

int qRegisterMetaType ( const char * typeName )

Registers the type name typeName to the type T. Returns the internal ID used by QMetaType. Any class or struct that has a public constructor, a public copy constructor, and a public destructor can be registered.

After a type has been registered, you can create and destroy objects of that type dynamically at run-time.

This example registers the class MyClass:

    qRegisterMetaType<MyClass>("MyClass");

Note: This function is thread-safe.

See also qRegisterMetaTypeStreamOperators(), QMetaType::isRegistered(), and Q_DECLARE_METATYPE().

void qRegisterMetaTypeStreamOperators ( const char * typeName )

Registers the stream operators for the type T called typeName.

Afterward, the type can be streamed using QMetaType::load() and QMetaType::save(). These functions are used when streaming a QVariant.

    qRegisterMetaTypeStreamOperator<MyClass>("MyClass");

The stream operators should have the following signatures:

    QDataStream &operator<<(QDataStream &out, const MyClass &myObj);
    QDataStream &operator>>(QDataStream &in, MyClass &myObj);

Note: This function is thread-safe.

See also qRegisterMetaType(), QMetaType::isRegistered(), and Q_DECLARE_METATYPE().

Macro Documentation

Q_DECLARE_METATYPE ( Type )

This macro makes the type Type known to QMetaType. It is needed to use the type Type as a custom type in QVariant.

Ideally, this macro should be placed below the declaration of the class or struct. If that is not possible, it can be put in a private header file which has to be included every time that type is used in a QVariant.

Q_DECLARE_METATYPE() doesn't actually register the type; you must still use qRegisterMetaType() for that, particularly if you intend to use the type in signal and slot connections.

This example shows a typical use case of Q_DECLARE_METATYPE():

    struct MyStruct
    {
        int i;
        ...
    };

    Q_DECLARE_METATYPE(MyStruct)

If MyStruct is in a namespace, the Q_DECLARE_METATYPE() macro has to be outside the namespace:

    namespace MyNamespace
    {
        ...
    }

    Q_DECLARE_METATYPE(MyNamespace::MyStruct)

Since MyStruct is now known to QMetaType, it can be used in QVariant:

    MyStruct s;
    QVariant var;
    var.setValue(s); // copy s into the variant

    ...

    // retrieve the value
    MyStruct s2 = var.value<MyStruct>();

See also qRegisterMetaType().