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QMetaType Class Reference
[QtCore module]

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

 #include <QMetaType>

Note: All the functions in this class are thread-safe.

Public Types

  • enum Type { Void, Bool, Int, UInt, ..., User }

Static Public Members

  • void * construct ( int type, const void * copy = 0 )
  • void destroy ( int type, void * data )
  • bool isRegistered ( int type )
  • bool load ( QDataStream & stream, int type, void * data )
  • bool save ( QDataStream & stream, int type, const void * data )
  • int type ( const char * typeName )
  • const char * typeName ( int type )

Related Non-Members


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. Declare new types with Q_DECLARE_METATYPE() to make them available to QVariant and other template-based functions. Call qRegisterMetaType() to make type available to non-template based functions, such as the queued signal and slot connections.

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;

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.

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:

QMetaType::UInt3unsigned int
QMetaType::VoidStar128void *
QMetaType::ULong132unsigned long
QMetaType::UShort133unsigned short
QMetaType::UChar134unsigned char
QMetaType::QObjectStar136QObject *
QMetaType::QWidgetStar137QWidget *
QMetaType::User256Base value for user types

Additional types can be registered using Q_DECLARE_METATYPE().

See also type() and typeName().

Member Function Documentation

void * QMetaType::construct ( int type, const void * copy = 0 )   [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. Returns true if the object is loaded successfully; otherwise returns false.

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. Returns true if the object is saved successfully; otherwise returns false.

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:


Note: This function is thread-safe.

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

int qRegisterMetaType ()

This is an overloaded member function, provided for convenience.

Call this function to register the type T. T must be declared with Q_DECLARE_METATYPE(). Returns the meta type Id.


 int id = qRegisterMetaType<MyStruct>();

Note: To use the type T in QVariant, using Q_DECLARE_METATYPE() is sufficient. To use the type T in queued signal and slot connections, qRegisterMetaType<T>() must be called before the first connection is established.

Note: This function is thread-safe.

This function was introduced in Qt 4.2.


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.


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


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.

Adding a Q_DECLARE_METATYPE() makes the type known to all template based functions, including QVariant. Note that if you intend to use the type in queued signal and slot connections, you also have to call qRegisterMetaType() since such connections are resolved at runtime.

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

 struct MyStruct
     int i;


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

 namespace MyNamespace


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().


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