Qt 5.6 | Qt Core | C++ Classes | QMetaMethod

Qt 5.6.2 Reference Documentation

# QMetaMethod Class

The QMetaMethod class provides meta-data about a member function. More...

Header: | #include <QMetaMethod>
qmake: | QT += core

- List of all members, including inherited members

## Public Types

enum | Access { Private, Protected, Public }
enum | MethodType { Method, Signal, Slot, Constructor }

## Public Functions

Access | access() const
bool | invoke(QObject *object, Qt::ConnectionType connectionType, QGenericReturnArgument returnValue, QGenericArgument val0 = QGenericArgument( Q_NULLPTR ), QGenericArgument val1 = QGenericArgument(), QGenericArgument val2 = QGenericArgument(), QGenericArgument val3 = QGenericArgument(), QGenericArgument val4 = QGenericArgument(), QGenericArgument val5 = QGenericArgument(), QGenericArgument val6 = QGenericArgument(), QGenericArgument val7 = QGenericArgument(), QGenericArgument val8 = QGenericArgument(), QGenericArgument val9 = QGenericArgument()) const
bool | invoke(QObject *object, QGenericReturnArgument returnValue, QGenericArgument val0 = QGenericArgument( Q_NULLPTR ), QGenericArgument val1 = QGenericArgument(), QGenericArgument val2 = QGenericArgument(), QGenericArgument val3 = QGenericArgument(), QGenericArgument val4 = QGenericArgument(), QGenericArgument val5 = QGenericArgument(), QGenericArgument val6 = QGenericArgument(), QGenericArgument val7 = QGenericArgument(), QGenericArgument val8 = QGenericArgument(), QGenericArgument val9 = QGenericArgument()) const
bool | invoke(QObject *object, Qt::ConnectionType connectionType, QGenericArgument val0 = QGenericArgument( Q_NULLPTR ), QGenericArgument val1 = QGenericArgument(), QGenericArgument val2 = QGenericArgument(), QGenericArgument val3 = QGenericArgument(), QGenericArgument val4 = QGenericArgument(), QGenericArgument val5 = QGenericArgument(), QGenericArgument val6 = QGenericArgument(), QGenericArgument val7 = QGenericArgument(), QGenericArgument val8 = QGenericArgument(), QGenericArgument val9 = QGenericArgument()) const
bool | invoke(QObject *object, QGenericArgument val0 = QGenericArgument( Q_NULLPTR ), QGenericArgument val1 = QGenericArgument(), QGenericArgument val2 = QGenericArgument(), QGenericArgument val3 = QGenericArgument(), QGenericArgument val4 = QGenericArgument(), QGenericArgument val5 = QGenericArgument(), QGenericArgument val6 = QGenericArgument(), QGenericArgument val7 = QGenericArgument(), QGenericArgument val8 = QGenericArgument(), QGenericArgument val9 = QGenericArgument()) const
bool | invokeOnGadget(void *gadget, QGenericReturnArgument returnValue, QGenericArgument val0 = QGenericArgument( Q_NULLPTR ), QGenericArgument val1 = QGenericArgument(), QGenericArgument val2 = QGenericArgument(), QGenericArgument val3 = QGenericArgument(), QGenericArgument val4 = QGenericArgument(), QGenericArgument val5 = QGenericArgument(), QGenericArgument val6 = QGenericArgument(), QGenericArgument val7 = QGenericArgument(), QGenericArgument val8 = QGenericArgument(), QGenericArgument val9 = QGenericArgument()) const
bool | invokeOnGadget(void *gadget, QGenericArgument val0 = QGenericArgument( Q_NULLPTR ), QGenericArgument val1 = QGenericArgument(), QGenericArgument val2 = QGenericArgument(), QGenericArgument val3 = QGenericArgument(), QGenericArgument val4 = QGenericArgument(), QGenericArgument val5 = QGenericArgument(), QGenericArgument val6 = QGenericArgument(), QGenericArgument val7 = QGenericArgument(), QGenericArgument val8 = QGenericArgument(), QGenericArgument val9 = QGenericArgument()) const
bool | isValid() const
int | methodIndex() const
QByteArray | methodSignature() const
MethodType | methodType() const
QByteArray | name() const
int | parameterCount() const
QList<QByteArray> | parameterNames() const
int | parameterType(int index) const
QList<QByteArray> | parameterTypes() const
int | returnType() const
int | revision() const
const char * | tag() const
const char * | typeName() const

## Static Public Members

QMetaMethod | fromSignal(PointerToMemberFunction signal)

## Related Non-Members

bool | operator!=(const QMetaMethod &m1, const QMetaMethod &m2)
bool | operator==(const QMetaMethod &m1, const QMetaMethod &m2)

## Macros

| Q_METAMETHOD_INVOKE_MAX_ARGS

## Detailed Description

The QMetaMethod class provides meta-data about a member function.

A QMetaMethod has a methodType(), a methodSignature(), a list of parameterTypes() and parameterNames(), a return typeName(), a tag(), and an access() specifier. You can use invoke() to invoke the method on an arbitrary QObject.

See also QMetaObject, QMetaEnum, QMetaProperty, and Qt's Property System.

## Member Type Documentation

### enum QMetaMethod::Access

This enum describes the access level of a method, following the conventions used in C++.

Constant | Value
--- | ---
`QMetaMethod::Private` | `0`
`QMetaMethod::Protected` | `1`
`QMetaMethod::Public` | `2`

### enum QMetaMethod::MethodType

Constant | Value | Description
--- | --- | ---
`QMetaMethod::Method` | `0` | The function is a plain member function.
`QMetaMethod::Signal` | `1` | The function is a signal.
`QMetaMethod::Slot` | `2` | The function is a slot.
`QMetaMethod::Constructor` | `3` | The function is a constructor.

## Member Function Documentation

### Access QMetaMethod::access() const

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

Note: Signals are always public, but you should regard that as an implementation detail. It is almost always a bad idea to emit a signal from outside its class.

See also methodType().

### `[static]` QMetaMethod QMetaMethod::fromSignal(PointerToMemberFunction signal)

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

Example:

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

This function was introduced in Qt 5.0.

### bool QMetaMethod::invoke(QObject *object, Qt::ConnectionType connectionType, QGenericReturnArgument returnValue, QGenericArgument val0 = QGenericArgument( Q_NULLPTR ), QGenericArgument val1 = QGenericArgument(), QGenericArgument val2 = QGenericArgument(), QGenericArgument val3 = QGenericArgument(), QGenericArgument val4 = QGenericArgument(), QGenericArgument val5 = QGenericArgument(), QGenericArgument val6 = QGenericArgument(), QGenericArgument val7 = QGenericArgument(), QGenericArgument val8 = QGenericArgument(), QGenericArgument val9 = QGenericArgument()) const

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.

The invocation can be either synchronous or asynchronous, depending on the connectionType:

- If connectionType is Qt::DirectConnection, the member will be invoked immediately.
- If connectionType is Qt::QueuedConnection, a QEvent will be posted and the member is invoked as soon as the application enters the main event loop.
- If connectionType is Qt::AutoConnection, the member is invoked synchronously if object lives in the same thread as the caller; otherwise it will invoke the member asynchronously.

The return value of this method call is placed in returnValue. If the invocation is asynchronous, the return value cannot be evaluated. You can pass up to ten arguments (val0, val1, val2, val3, val4, val5, val6, val7, val8, and val9) to this method call.

QGenericArgument and QGenericReturnArgument are internal helper classes. Because signals and slots can be dynamically invoked, you must enclose the arguments using the Q_ARG() and Q_RETURN_ARG() macros. Q_ARG() takes a type name and a const reference of that type; Q_RETURN_ARG() takes a type name and a non-const reference.

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 of types that are known to Qt's meta-object system, because Qt needs to copy the arguments to store them in an event behind the scenes. If you try to use a queued connection and get the error message

      QMetaMethod::invoke: Unable to handle unregistered datatype 'MyType'

call qRegisterMetaType() to register the data type before you call QMetaMethod::invoke().

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,
                    Q_RETURN_ARG(QString, retVal),
                    Q_ARG(QString, "sqrt"),
                    Q_ARG(int, 42),
                    Q_ARG(double, 9.7));

QMetaObject::normalizedSignature() is used here to ensure that the format of the signature is what invoke() expects. E.g. extra whitespace is removed.

If the "compute" slot does not take exactly one QString, one int and one double in the specified order, the call will fail.

Warning: this method will not test the validity of the arguments: object must be an instance of the class of the QMetaObject of which this QMetaMethod has been constructed with. The arguments must have the same type as the ones expected by the method, else, the behaviour is undefined.

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

### bool QMetaMethod::invoke(QObject *object, QGenericReturnArgument returnValue, QGenericArgument val0 = QGenericArgument( Q_NULLPTR ), QGenericArgument val1 = QGenericArgument(), QGenericArgument val2 = QGenericArgument(), QGenericArgument val3 = QGenericArgument(), QGenericArgument val4 = QGenericArgument(), QGenericArgument val5 = QGenericArgument(), QGenericArgument val6 = QGenericArgument(), QGenericArgument val7 = QGenericArgument(), QGenericArgument val8 = QGenericArgument(), QGenericArgument val9 = QGenericArgument()) const

This function overloads invoke().

This overload always invokes this method using the connection type Qt::AutoConnection.

### bool QMetaMethod::invoke(QObject *object, Qt::ConnectionType connectionType, QGenericArgument val0 = QGenericArgument( Q_NULLPTR ), QGenericArgument val1 = QGenericArgument(), QGenericArgument val2 = QGenericArgument(), QGenericArgument val3 = QGenericArgument(), QGenericArgument val4 = QGenericArgument(), QGenericArgument val5 = QGenericArgument(), QGenericArgument val6 = QGenericArgument(), QGenericArgument val7 = QGenericArgument(), QGenericArgument val8 = QGenericArgument(), QGenericArgument val9 = QGenericArgument()) const

This function overloads invoke().

This overload can be used if the return value of the member is of no interest.

### bool QMetaMethod::invoke(QObject *object, QGenericArgument val0 = QGenericArgument( Q_NULLPTR ), QGenericArgument val1 = QGenericArgument(), QGenericArgument val2 = QGenericArgument(), QGenericArgument val3 = QGenericArgument(), QGenericArgument val4 = QGenericArgument(), QGenericArgument val5 = QGenericArgument(), QGenericArgument val6 = QGenericArgument(), QGenericArgument val7 = QGenericArgument(), QGenericArgument val8 = QGenericArgument(), QGenericArgument val9 = QGenericArgument()) const

This function overloads invoke().

This overload invokes this method using the connection type Qt::AutoConnection and ignores return values.

### bool QMetaMethod::invokeOnGadget(void *gadget, QGenericReturnArgument returnValue, QGenericArgument val0 = QGenericArgument( Q_NULLPTR ), QGenericArgument val1 = QGenericArgument(), QGenericArgument val2 = QGenericArgument(), QGenericArgument val3 = QGenericArgument(), QGenericArgument val4 = QGenericArgument(), QGenericArgument val5 = QGenericArgument(), QGenericArgument val6 = QGenericArgument(), QGenericArgument val7 = QGenericArgument(), QGenericArgument val8 = QGenericArgument(), QGenericArgument val9 = QGenericArgument()) const

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.

The return value of this method call is placed in returnValue. You can pass up to ten arguments (val0, val1, val2, val3, val4, val5, val6, val7, val8, and val9) to this method call.

Warning: this method will not test the validity of the arguments: gadget must be an instance of the class of the QMetaObject of which this QMetaMethod has been constructed with. The arguments must have the same type as the ones expected by the method, else, the behavior is undefined.

This function was introduced in Qt 5.5.

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

### bool QMetaMethod::invokeOnGadget(void *gadget, QGenericArgument val0 = QGenericArgument( Q_NULLPTR ), QGenericArgument val1 = QGenericArgument(), QGenericArgument val2 = QGenericArgument(), QGenericArgument val3 = QGenericArgument(), QGenericArgument val4 = QGenericArgument(), QGenericArgument val5 = QGenericArgument(), QGenericArgument val6 = QGenericArgument(), QGenericArgument val7 = QGenericArgument(), QGenericArgument val8 = QGenericArgument(), QGenericArgument val9 = QGenericArgument()) const

This is an overloaded function.

This overload invokes this method for a gadget and ignores return values.

This function was introduced in Qt 5.5.

### bool QMetaMethod::isValid() const

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

This function was introduced in Qt 5.0.

### int QMetaMethod::methodIndex() const

Returns this method's index.

This function was introduced in Qt 4.6.

### QByteArray QMetaMethod::methodSignature() const

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

This function was introduced in Qt 5.0.

See also parameterTypes() and parameterNames().

### 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.

This function was introduced in Qt 5.0.

See also methodSignature() and parameterCount().

### int QMetaMethod::parameterCount() const

Returns the number of parameters of this method.

This function was introduced in Qt 5.0.

See also parameterType() and parameterNames().

### 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.

This function was introduced in Qt 5.0.

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

### QList<QByteArray> QMetaMethod::parameterTypes() const

Returns a list of parameter types.

See also parameterNames() and methodSignature().

### 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.

This function was introduced in Qt 5.0.

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

### int QMetaMethod::revision() const

Returns the method revision if one was specified by Q_REVISION, otherwise returns 0.

This function was introduced in Qt 5.1.

### 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.

Note: Since Qt 5.0, `moc` expands preprocessor macros, so it is necessary to surround the definition with `#ifndef` `Q_MOC_RUN`, as shown in the example above. This was not required in Qt 4. The code as shown above works with Qt 4 too.

### const char *QMetaMethod::typeName() const

Returns the return type name of this method.

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

## Related Non-Members

### bool operator!=(const QMetaMethod &m1, const QMetaMethod &m2)

This is an overloaded function.

Returns `true` if method m1 is not equal to method m2, otherwise returns `false`.

This function was introduced in Qt 5.0.

### bool operator==(const QMetaMethod &m1, const QMetaMethod &m2)

This is an overloaded function.

Returns `true` if method m1 is equal to method m2, otherwise returns `false`.

This function was introduced in Qt 5.0.

## Macro Documentation

### Q_METAMETHOD_INVOKE_MAX_ARGS

Equals maximum number of arguments available for execution of the method via QMetaMethod::invoke()