- Qt 6.9
- Qt Core
- C++ Classes
- QMetaMethod
- Qt 6.9.3 Reference Documentation

# QMetaMethod Class

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

Header: | `#include <QMetaMethod>`
CMake: | `find_package(Qt6 REQUIRED COMPONENTS Core)` `target_link_libraries(mytarget PRIVATE Qt6::Core)`
qmake: | `QT += core`

- List of all members, including inherited members
- Deprecated members

This class is equality-comparable.

## Public Types

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

## Public Functions

QMetaMethod::Access | access() const
`(since 6.5)` bool | invoke(QObject *obj, Args &&... arguments) const
`(since 6.5)` bool | invoke(QObject *obj, QTemplatedMetaMethodReturnArgument<ReturnArg> ret, Args &&... arguments) const
`(since 6.5)` bool | invoke(QObject *obj, Qt::ConnectionType type, Args &&... arguments) const
`(since 6.5)` bool | invoke(QObject *obj, Qt::ConnectionType type, QTemplatedMetaMethodReturnArgument<ReturnArg> ret, Args &&... arguments) const
`(since 6.5)` bool | invokeOnGadget(void *gadget, Args &&... arguments) const
`(since 6.5)` bool | invokeOnGadget(void *gadget, QTemplatedMetaMethodReturnArgument<ReturnArg> ret, Args &&... arguments) const
`(since 6.2)` bool | isConst() const
bool | isValid() const
int | methodIndex() const
QByteArray | methodSignature() const
QMetaMethod::MethodType | methodType() const
QByteArray | name() const
`(since 6.9)` QByteArrayView | nameView() const
int | parameterCount() const
`(since 6.0)` QMetaType | parameterMetaType(int index) const
QList<QByteArray> | parameterNames() const
int | parameterType(int index) const
`(since 6.0)` QByteArray | parameterTypeName(int index) const
QList<QByteArray> | parameterTypes() const
`(since 6.0)` int | relativeMethodIndex() const
`(since 6.0)` QMetaType | returnMetaType() 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 &lhs, const QMetaMethod &rhs)
bool | operator==(const QMetaMethod &lhs, const QMetaMethod &rhs)

## Macros

| Q_METAMETHOD_INVOKE_MAX_ARGS

## Detailed Description

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

### `[since 6.5]` template <typename... Args> bool QMetaMethod::invoke(QObject *obj, Args &&... arguments) const

### `[since 6.5]` template <typename ReturnArg, typename... Args> bool QMetaMethod::invoke(QObject *obj, QTemplatedMetaMethodReturnArgument<ReturnArg> ret, Args &&... arguments) const

### `[since 6.5]` template <typename... Args> bool QMetaMethod::invoke(QObject *obj, Qt::ConnectionType type, Args &&... arguments) const

### `[since 6.5]` template <typename ReturnArg, typename... Args> bool QMetaMethod::invoke(QObject *obj, Qt::ConnectionType type, QTemplatedMetaMethodReturnArgument<ReturnArg> ret, Args &&... arguments) 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.

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));

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.

This function was introduced in Qt 6.5.

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

### `[since 6.5]` template <typename... Args> bool QMetaMethod::invokeOnGadget(void *gadget, Args &&... arguments) const

### `[since 6.5]` template <typename ReturnArg, typename... Args> bool QMetaMethod::invokeOnGadget(void *gadget, QTemplatedMetaMethodReturnArgument<ReturnArg> ret, Args &&... arguments) 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.

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.

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.

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).

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]` 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.

Note: This method might erroneously return `false` for a const method if it belongs to a library compiled against an older version of Qt.

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.

Note: `moc` expands preprocessor macros, so it is necessary to surround the definition with `#ifndef` `Q_MOC_RUN`, as shown in the example above.

### 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).

Note: In Qt 7, this function will return a null pointer for constructors.

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

## Related Non-Members

### `[noexcept]` bool operator!=(const QMetaMethod &lhs, const QMetaMethod &rhs)

Returns `true` if method lhs is not equal to method rhs, otherwise returns `false`.

### `[noexcept]` bool operator==(const QMetaMethod &lhs, const QMetaMethod &rhs)

Returns `true` if method lhs is equal to method rhs, otherwise returns `false`.

## Macro Documentation

### Q_METAMETHOD_INVOKE_MAX_ARGS

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