Qt Reference Documentation

- Qt 4.8
- ALL VERSIONS

- A
- A
- A
- Print

# QMetaMethod Class Reference

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

     #include <QMetaMethod>

- 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( 0 ), 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( 0 ), 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( 0 ), 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( 0 ), 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
int | methodIndex () const
MethodType | methodType () const
QList<QByteArray> | parameterNames () const
QList<QByteArray> | parameterTypes () const
const char * | signature () const
const char * | tag () const
const char * | typeName () const

## Detailed Description

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

A QMetaMethod has a methodType(), a signature(), 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.

A method will only be registered with the meta-object system if it is a slot, a signal, or declared with the Q_INVOKABLE macro. Constructors can also be registered with Q_INVOKABLE.

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).

Signals are always protected, meaning that you can only emit them from the class or from a subclass.

See also methodType().

### bool QMetaMethod::invoke ( QObject * object, Qt::ConnectionType connectionType, QGenericReturnArgument returnValue, QGenericArgument val0 = QGenericArgument( 0 ), 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 = 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( 0 ), 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( 0 ), 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( 0 ), 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.

### int QMetaMethod::methodIndex () const

Returns this method's index.

This function was introduced in Qt 4.6.

### MethodType QMetaMethod::methodType () const

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

See also access().

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

Returns a list of parameter names.

See also parameterTypes() and signature().

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

Returns a list of parameter types.

See also parameterNames() and signature().

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

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

See also parameterTypes() and parameterNames().

### 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:

     #define THISISTESTTAG // tag text
     ...
     private slots:
         THISISTESTTAG void testFunc();

and the information can be accessed by using:

     MainWindow win;
     win.show();

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

For the moment, `moc` doesn't support any special tags.

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

Returns the return type of this method, or an empty string if the return type is void.