QMetaMethod Class

Member Function Documentation

Invokes this method on the object object. Returns true if the member could be invoked. Returns false if there is no such member or the parameters did not match.

For the overloads with a QTemplatedMetaMethodReturnArgument parameter, the return value of the member function call is placed in ret. For the overloads without such a member, the return value of the called function (if any) will be discarded. QTemplatedMetaMethodReturnArgument is an internal type you should not use directly. Instead, use the qReturnArg() function.

The overloads with a Qt::ConnectionType type parameter allow explicitly selecting whether the invocation will be synchronous or not:

• If type is Qt::DirectConnection, the member will be invoked immediately in the current thread.
• If type is Qt::QueuedConnection, a QEvent will be sent and the member is invoked as soon as the application enters the event loop in the thread the obj was created in or was moved to.
• If type is Qt::BlockingQueuedConnection, the method will be invoked in the same way as for Qt::QueuedConnection, except that the current thread will block until the event is delivered. Using this connection type to communicate between objects in the same thread will lead to deadlocks.
• If type is Qt::AutoConnection, the member is invoked synchronously if obj lives in the same thread as the caller; otherwise it will invoke the member asynchronously. This is the behavior of the overloads that do not have the type parameter.

To asynchronously invoke the animateClick() slot on a QPushButton:

int methodIndex = pushButton->metaObject()->indexOfMethod("animateClick()");
QMetaMethod method = metaObject->method(methodIndex);
method.invoke(pushButton, Qt::QueuedConnection);

With asynchronous method invocations, the parameters must be copyable types, because Qt needs to copy the arguments to store them in an event behind the scenes. Since Qt 6.5, this function automatically registers the types being used; however, as a side-effect, it is not possible to make calls using types that are only forward-declared. Additionally, it is not possible to make asynchronous calls that use references to non-const-qualified types as parameters either.

To synchronously invoke the compute(QString, int, double) slot on some arbitrary object obj retrieve its return value:

QString retVal;
QByteArray normalizedSignature = QMetaObject::normalizedSignature("compute(QString, int, double)");
int methodIndex = obj->metaObject()->indexOfMethod(normalizedSignature);
QMetaMethod method = obj->metaObject()->method(methodIndex);
method.invoke(obj, Qt::DirectConnection, qReturnArg(retVal),
              QString("sqrt"), 42, 9.7);

If the "compute" slot does not take exactly one QString, one int, and one double in the specified order, the call will fail. Note how it was necessary to be explicit about the type of the QString, as the character literal is not exactly the right type to match. If the method instead took a QByteArray, a qint64, and a long double, the call would need to be written as:

QString retVal;
QByteArray normalizedSignature = QMetaObject::normalizedSignature("compute(QByteArray, qint64, long double)");
int methodIndex = obj->metaObject()->indexOfMethod(normalizedSignature);
QMetaMethod method = obj->metaObject()->method(methodIndex);
method.invoke(obj, Qt::DirectConnection, qReturnArg(retVal),
              QByteArray("sqrt"), qint64(42), 9.7L);

The same call can be executed using the Q_ARG() and Q_RETURN_ARG() macros, as in:

QString retVal;
QByteArray normalizedSignature = QMetaObject::normalizedSignature("compute(QString, int, double)");
int methodIndex = obj->metaObject()->indexOfMethod(normalizedSignature);
QMetaMethod method = obj->metaObject()->method(methodIndex);
method.invoke(obj,
              Qt::DirectConnection,
              Q_RETURN_ARG(QString, retVal),
              Q_ARG(QString, "sqrt"),
              Q_ARG(int, 42),
              Q_ARG(double, 9.7));

This function was introduced in Qt 6.5.

See also Q_ARG(), Q_RETURN_ARG(), qRegisterMetaType(), and QMetaObject::invokeMethod().

Invokes this method on a Q_GADGET. Returns true if the member could be invoked. Returns false if there is no such member or the parameters did not match.

The pointer gadget must point to an instance of the gadget class.

The invocation is always synchronous.

For the overload with a QTemplatedMetaMethodReturnArgument parameter, the return value of the member function call is placed in ret. For the overload without it, the return value of the called function (if any) will be discarded. QTemplatedMetaMethodReturnArgument is an internal type you should not use directly. Instead, use the qReturnArg() function.

This function was introduced in Qt 6.5.

See also Q_ARG(), Q_RETURN_ARG(), qRegisterMetaType(), and QMetaObject::invokeMethod().

QMetaMethod::Access QMetaMethod::access() const

Returns the access specification of this method (private, protected, or public).

See also methodType().

[static] template <typename PointerToMemberFunction> QMetaMethod QMetaMethod::fromSignal(PointerToMemberFunction signal)

Returns the meta-method that corresponds to the given signal, or an invalid QMetaMethod if signal is nullptr or not a signal of the class.

Example:

QMetaMethod destroyedSignal = QMetaMethod::fromSignal(&QObject::destroyed);

[since 6.2] bool QMetaMethod::isConst() const

Returns whether the method is const qualified.

This function was introduced in Qt 6.2.

bool QMetaMethod::isValid() const

Returns true if this method is valid (can be introspected and invoked), otherwise returns false.

int QMetaMethod::methodIndex() const

Returns this method's index.

QByteArray QMetaMethod::methodSignature() const

Returns the signature of this method (e.g., setValue(double)).

See also parameterTypes() and parameterNames().

QMetaMethod::MethodType QMetaMethod::methodType() const

Returns the type of this method (signal, slot, or method).

See also access().

QByteArray QMetaMethod::name() const

Returns the name of this method.

See also methodSignature() and parameterCount().

[since 6.9] QByteArrayView QMetaMethod::nameView() const

Returns the name of this method. The returned QByteArrayView is valid as long as the meta-object of the class to which the method belongs is valid.

This function was introduced in Qt 6.9.

See also name.

int QMetaMethod::parameterCount() const

Returns the number of parameters of this method.

See also parameterType() and parameterNames().

[since 6.0] QMetaType QMetaMethod::parameterMetaType(int index) const

Returns the metatype of the parameter at the given index.

If the index is smaller than zero or larger than parameterCount(), an invalid QMetaType is returned.

This function was introduced in Qt 6.0.

See also parameterCount(), returnMetaType(), and QMetaType.

QList<QByteArray> QMetaMethod::parameterNames() const

Returns a list of parameter names.

See also parameterTypes() and methodSignature().

int QMetaMethod::parameterType(int index) const

Returns the type of the parameter at the given index.

The return value is one of the types that are registered with QMetaType, or QMetaType::UnknownType if the type is not registered.

See also parameterCount(), parameterMetaType(), returnType(), and QMetaType.

[since 6.0] QByteArray QMetaMethod::parameterTypeName(int index) const

Returns the name of the type at position index If there is no parameter at index, returns an empty QByteArray

This function was introduced in Qt 6.0.

See also parameterNames().

QList<QByteArray> QMetaMethod::parameterTypes() const

Returns a list of parameter types.

See also parameterNames() and methodSignature().

[since 6.0] int QMetaMethod::relativeMethodIndex() const

Returns this method's local index inside.

This function was introduced in Qt 6.0.

[since 6.0] QMetaType QMetaMethod::returnMetaType() const

Returns the return type of this method.

This function was introduced in Qt 6.0.

See also parameterMetaType(), QMetaType, and typeName().

int QMetaMethod::returnType() const

Returns the return type of this method.

The return value is one of the types that are registered with QMetaType, or QMetaType::UnknownType if the type is not registered.

See also parameterType(), QMetaType, typeName(), and returnMetaType().

int QMetaMethod::revision() const

Returns the method revision if one was specified by Q_REVISION, otherwise returns 0. Since Qt 6.0, non-zero values are encoded and can be decoded using QTypeRevision::fromEncodedVersion().

const char *QMetaMethod::tag() const

Returns the tag associated with this method.

Tags are special macros recognized by moc that make it possible to add extra information about a method.

Tag information can be added in the following way in the function declaration:

    // In the class MainWindow declaration
    #ifndef Q_MOC_RUN
    // define the tag text as empty, so the compiler doesn't see it
    #  define MY_CUSTOM_TAG
    #endif
    ...
    private slots:
        MY_CUSTOM_TAG void testFunc();

and the information can be accessed by using:

    MainWindow win;
    win.show();

    int functionIndex = win.metaObject()->indexOfSlot("testFunc()");
    QMetaMethod mm = win.metaObject()->method(functionIndex);
    qDebug() << mm.tag(); // prints MY_CUSTOM_TAG

For the moment, moc will extract and record all tags, but it will not handle any of them specially. You can use the tags to annotate your methods differently, and treat them according to the specific needs of your application.

const char *QMetaMethod::typeName() const

Returns the return type name of this method. If this method is a constructor, this function returns an empty string (constructors have no return types).

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