QMetaType Class

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

• List of all members, including inherited members
• Obsolete members

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

Public Types

Public Functions

Static Public Members

Related Non-Members

Macros

Detailed Description

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 types 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 != QMetaType::UnknownType) {
    void *myClassPtr = QMetaType::create(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:

Additional types can be registered using Q_DECLARE_METATYPE().

See also type() and typeName().

enum QMetaType::TypeFlagflags QMetaType::TypeFlags

The enum describes attributes of a type supported by QMetaType.

The TypeFlags type is a typedef for QFlags<TypeFlag>. It stores an OR combination of TypeFlag values.

Member Function Documentation

[since 5.0] QMetaType::QMetaType(int typeId)

Constructs a QMetaType object that contains all information about type typeId.

This function was introduced in Qt 5.0.

[since 6.0] qsizetype QMetaType::alignOf() const

Returns the alignment of the type in bytes (i.e. alignof(T), where T is the actual type for which this QMetaType instance was constructed for).

This function is typically used together with construct() to perform low-level management of the memory used by a type.

This function was introduced in Qt 6.0.

See also QMetaType::construct() and QMetaType::sizeOf().

[static] bool QMetaType::canConvert(QMetaType fromType, QMetaType toType)

Returns true if QMetaType::convert can convert from fromType to toType.

The following conversions are supported by Qt:

Casting between primitive type (int, float, bool etc.) is supported.

Converting between pointers of types derived from QObject will also return true for this function if a qobject_cast from the type described by fromType to the type described by toType would succeed.

A cast from a sequential container will also return true for this function if the toType is QVariantList.

Similarly, a cast from an associative container will also return true for this function the toType is QVariantHash or QVariantMap.

See also convert(), QSequentialIterable, Q_DECLARE_SEQUENTIAL_CONTAINER_METATYPE(), QAssociativeIterable, and Q_DECLARE_ASSOCIATIVE_CONTAINER_METATYPE().

[static] bool QMetaType::canView(QMetaType fromType, QMetaType toType)

Returns true if QMetaType::view can create a mutable view of type toType on type fromType.

Converting between pointers of types derived from QObject will return true for this function if a qobject_cast from the type described by fromType to the type described by toType would succeed.

You can create a mutable view of type QSequentialIterable on any container registered with Q_DECLARE_SEQUENTIAL_CONTAINER_METATYPE().

Similarly you can create a mutable view of type QAssociativeIterable on any container registered with Q_DECLARE_ASSOCIATIVE_CONTAINER_METATYPE().

See also convert(), QSequentialIterable, Q_DECLARE_SEQUENTIAL_CONTAINER_METATYPE(), QAssociativeIterable, and Q_DECLARE_ASSOCIATIVE_CONTAINER_METATYPE().

[since 6.0] QPartialOrdering QMetaType::compare(const void *lhs, const void *rhs) const

Compares the objects at lhs and rhs for ordering.

Returns QPartialOrdering::Unordered if comparison is not supported or the values are unordered. Otherwise, returns QPartialOrdering::Less, QPartialOrdering::Equivalent or QPartialOrdering::Greater if lhs is less than, equivalent to or greater than rhs, respectively.

Both objects must be of the type described by this metatype. If either lhs or rhs is nullptr, the values are unordered. Comparison is only supported if the type's less than operator was visible to the metatype declaration.

If the type's equality operator was also visible, values will only compare equal if the equality operator says they are. In the absence of an equality operator, when neither value is less than the other, values are considered equal; if equality is also available and two such values are not equal, they are considered unordered, just as NaN (not a number) values of a floating point type lie outside its ordering.

Note: If no less than operator was visible to the metatype declaration, values are unordered even if an equality operator visible to the declaration considers them equal: compare() == 0 only agrees with equals() if the less than operator was visible.

This function was introduced in Qt 6.0.

See also equals() and isOrdered().

[since 5.0] void *QMetaType::construct(void *where, const void *copy = nullptr) const

Constructs a value of the type that this QMetaType instance was constructed for in the existing memory addressed by where, that is a copy of copy, and returns where. If copy is zero, the value is default constructed.

This is a low-level function for explicitly managing the memory used to store the type. Consider calling create() if you don't need this level of control (that is, use "new" rather than "placement new").

You must ensure that where points to a location where the new value can be stored and that where is suitably aligned. The type's size can be queried by calling sizeOf().

The rule of thumb for alignment is that a type is aligned to its natural boundary, which is the smallest power of 2 that is bigger than the type, unless that alignment is larger than the maximum useful alignment for the platform. For practical purposes, alignment larger than 2 * sizeof(void*) is only necessary for special hardware instructions (e.g., aligned SSE loads and stores on x86).

This function was introduced in Qt 5.0.

[static, since 5.2] bool QMetaType::convert(QMetaType fromType, const void *from, QMetaType toType, void *to)

Converts the object at from from fromType to the preallocated space at to typed toType. Returns true, if the conversion succeeded, otherwise false.

This function was introduced in Qt 5.2.

[since 5.0] void *QMetaType::create(const void *copy = nullptr) const

Returns a copy of copy, assuming it is of the type that this QMetaType instance was created for. If copy is nullptr, creates a default constructed instance.

This function was introduced in Qt 5.0.

See also QMetaType::destroy().

[since 5.2] bool QMetaType::debugStream(QDebug &dbg, const void *rhs)

Streams the object at rhs to the debug stream dbg. Returns true on success, otherwise false.

This function was introduced in Qt 5.2.

[since 5.0] void QMetaType::destroy(void *data) const

Destroys the data, assuming it is of the type that this QMetaType instance was created for.

This function was introduced in Qt 5.0.

See also QMetaType::create().

[since 5.0] void QMetaType::destruct(void *data) const

Destructs the value, located at data, assuming that it is of the type for which this QMetaType instance was constructed for.

Unlike destroy(), this function only invokes the type's destructor, it doesn't invoke the delete operator.

This function was introduced in Qt 5.0.

See also QMetaType::construct().

[since 6.0] bool QMetaType::equals(const void *lhs, const void *rhs) const

Compares the objects at lhs and rhs for equality.

Both objects must be of the type described by this metatype. Can only compare the two objects if a less than or equality operator for the type was visible to the metatype declaration. Otherwise, the metatype never considers values equal. When an equality operator was visible to the metatype declaration, it is authoritative; otherwise, if less than is visible, when neither value is less than the other, the two are considered equal. If values are unordered (see compare() for details) they are not equal.

Returns true if the two objects compare equal, otherwise false.

This function was introduced in Qt 6.0.

See also isEqualityComparable() and compare().

[since 5.0] QMetaType::TypeFlags QMetaType::flags() const

Returns flags of the type for which this QMetaType instance was constructed.

This function was introduced in Qt 5.0.

See also QMetaType::TypeFlags and QMetaType::flags().

[static] QMetaType QMetaType::fromName(QByteArrayView typeName)

Returns a QMetaType matching typeName. The returned object is not valid if the typeName is not known to QMetaType

[static, since 5.15] template <typename T> QMetaType QMetaType::fromType()

Returns the QMetaType corresponding to the type in the template parameter.

This function was introduced in Qt 5.15.

[static, since 5.2] bool QMetaType::hasRegisteredConverterFunction(QMetaType fromType, QMetaType toType)

Returns true, if the meta type system has a registered conversion from meta type id fromType to toType

This function was introduced in Qt 5.2.

[static, since 5.2] template <typename From, typename To> bool QMetaType::hasRegisteredConverterFunction()

Returns true, if the meta type system has a registered conversion from type From to type To.

This is an overloaded function.

This function was introduced in Qt 5.2.

[since 6.0] bool QMetaType::hasRegisteredDebugStreamOperator() const

Returns true, if the meta type system has a registered debug stream operator for this meta type.

This function was introduced in Qt 6.0.

[static, since 5.2] bool QMetaType::hasRegisteredMutableViewFunction(QMetaType fromType, QMetaType toType)

Returns true, if the meta type system has a registered mutable view on meta type id fromType of meta type id toType.

This function was introduced in Qt 5.2.

[static, since 6.0] template <typename From, typename To> bool QMetaType::hasRegisteredMutableViewFunction()

Returns true, if the meta type system has a registered mutable view on type From of type To.

This is an overloaded function.

This function was introduced in Qt 6.0.

[since 5.13] int QMetaType::id() const

Returns id type hold by this QMetatype instance.

This function was introduced in Qt 5.13.

bool QMetaType::isEqualityComparable() const

Returns true if a less than or equality operator for the type described by this metatype was visible to the metatype declaration, otherwise false.

See also equals() and isOrdered().

bool QMetaType::isOrdered() const

Returns true if a less than operator for the type described by this metatype was visible to the metatype declaration, otherwise false.

See also compare() and isEqualityComparable().

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

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

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

[since 5.0] bool QMetaType::isRegistered() const

Returns true if this QMetaType object contains valid information about a type, false otherwise.

This function was introduced in Qt 5.0.

[since 5.0] bool QMetaType::isValid() const

Returns true if this QMetaType object contains valid information about a type, false otherwise.

This function was introduced in Qt 5.0.

bool QMetaType::load(QDataStream &stream, void *data) const

Reads the object of this 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 Q_DECLARE_METATYPE() 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().

[since 5.5] const QMetaObject *QMetaType::metaObject() const

return a QMetaObject relative to this type.

If the type is a pointer type to a subclass of QObject, flags() contains QMetaType::PointerToQObject and this function returns the corresponding QMetaObject. This can be used to in combinaison with QMetaObject::construct to create QObject of this type.

If the type is a Q_GADGET, flags() contains QMetaType::IsGadget, and this function returns its QMetaObject. This can be used to retrieve QMetaMethod and QMetaProperty and use them on a pointer of this type. (given by QVariant::data for example)

If the type is an enumeration, flags() contains QMetaType::IsEnumeration, and this function returns the QMetaObject of the enclosing object if the enum was registered as a Q_ENUM or nullptr otherwise

This function was introduced in Qt 5.5.

See also QMetaType::flags().

[since 5.15] const char *QMetaType::name() const

Returns the type name associated with this QMetaType, or a null pointer if no matching type was found. The returned pointer must not be deleted.

This function was introduced in Qt 5.15.

See also typeName().

[static, since 5.2] template <typename From, typename To> bool QMetaType::registerConverter()

Registers the possibility of an implicit conversion from type From to type To in the meta type system. Returns true if the registration succeeded, otherwise false.

This function was introduced in Qt 5.2.

[static, since 5.2] template <typename MemberFunction, int> bool QMetaType::registerConverter(MemberFunction function)

This is an overloaded function.

Registers a method function like To From::function() const as converter from type From to type To in the meta type system. Returns true if the registration succeeded, otherwise false.

This function was introduced in Qt 5.2.

[static, since 5.2] template <typename MemberFunctionOk, char> bool QMetaType::registerConverter(MemberFunctionOk function)

This is an overloaded function.

Registers a method function like To From::function(bool *ok) const as converter from type From to type To in the meta type system. Returns true if the registration succeeded, otherwise false.

This function was introduced in Qt 5.2.

[static, since 5.2] template <typename UnaryFunction> bool QMetaType::registerConverter(UnaryFunction function)

This is an overloaded function.

Registers a unary function object function as converter from type From to type To in the meta type system. Returns true if the registration succeeded, otherwise false.

This function was introduced in Qt 5.2.

[static, since 6.0] template <typename MemberFunction, int> bool QMetaType::registerMutableView(MemberFunction function)

This is an overloaded function.

Registers a method function like To From::function() as mutable view of type To on type From in the meta type system. Returns true if the registration succeeded, otherwise false.

This function was introduced in Qt 6.0.

[static, since 6.0] template <typename MemberFunctionOk, char> bool QMetaType::registerMutableView(MemberFunctionOk function)

This is an overloaded function.

Registers a method function like To From::function(bool *ok) as mutable view of type To on type From in the meta type system. Returns true if the registration succeeded, otherwise false.

This function was introduced in Qt 6.0.

[static, since 6.0] template <typename UnaryFunction> bool QMetaType::registerMutableView(UnaryFunction function)

This is an overloaded function.

Registers a unary function object function as mutable view of type To on type From in the meta type system. Returns true if the registration succeeded, otherwise false.

This function was introduced in Qt 6.0.

bool QMetaType::save(QDataStream &stream, const void *data) const

Writes the object pointed to by data to the given stream. Returns true if the object is saved successfully; otherwise returns false.

The type must have been registered with Q_DECLARE_METATYPE() 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().

[since 5.0] qsizetype QMetaType::sizeOf() const

Returns the size of the type in bytes (i.e. sizeof(T), where T is the actual type for which this QMetaType instance was constructed for).

This function is typically used together with construct() to perform low-level management of the memory used by a type.

This function was introduced in Qt 5.0.

See also QMetaType::construct(), QMetaType::sizeOf(), and QMetaType::alignOf().

[static, since 6.0] bool QMetaType::view(QMetaType fromType, void *from, QMetaType toType, void *to)

Creates a mutable view on the object at from of fromType in the preallocated space at to typed toType. Returns true if the conversion succeeded, otherwise false.

This function was introduced in Qt 6.0.

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

qRegisterMetaType<MyClass>("MyClass");

typedef QString CustomString;
qRegisterMetaType<CustomString>("CustomString");

int id = qRegisterMetaType<MyStruct>();

#include <unordered_list>

Q_DECLARE_ASSOCIATIVE_CONTAINER_METATYPE(std::unordered_map)

void someFunc()
{
    std::unordered_map<int, bool> container;
    QVariant var = QVariant::fromValue(container);
    // ...
}

struct MyStruct
{
    int i;
    ...
};

Q_DECLARE_METATYPE(MyStruct)

namespace MyNamespace
{
    ...
}

Q_DECLARE_METATYPE(MyNamespace::MyStruct)

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

...

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

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

#include <deque>

Q_DECLARE_SEQUENTIAL_CONTAINER_METATYPE(std::deque)

void someFunc()
{
    std::deque<QFile*> container;
    QVariant var = QVariant::fromValue(container);
    // ...
}

#include <memory>

Q_DECLARE_SMART_POINTER_METATYPE(std::shared_ptr)

void someFunc()
{
    auto smart_ptr = std::make_shared<QFile>();
    QVariant var = QVariant::fromValue(smart_ptr);
    // ...
    if (var.canConvert<QObject*>()) {
        QObject *sp = var.value<QObject*>();
        qDebug() << sp->metaObject()->className(); // Prints 'QFile'.
    }
}