内容

• 公共类型
• 公共函数
• 静态公共成员
• 相关非成员
• 宏
• 详细描述

QMetaType 类

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

头：	#include <QMetaType>
qmake:	QT += core

• 所有成员列表，包括继承成员
• 过时成员

注意： 此类的所有函数 thread-safe .

公共类型

enum	Type { Void, Bool, Int, UInt, ..., UnknownType }
enum	TypeFlag { NeedsConstruction, NeedsDestruction, MovableType, IsEnumeration, PointerToQObject }
flags	TypeFlags

公共函数

	QMetaType (const int typeId )
	~QMetaType ()
void *	construct (void * where , const void * copy = Q_NULLPTR) const
void *	create (const void * copy = Q_NULLPTR) const
void	destroy (void * data ) const
void	destruct (void * data ) const
TypeFlags	flags () const
bool	isRegistered () const
bool	isValid () const
const QMetaObject *	metaObject () const
int	sizeOf () const

静态公共成员

bool	compare (const void * lhs , const void * rhs , int typeId , int * result )
void *	construct (int type , void * where , const void * copy )
bool	convert (const void * from , int fromTypeId , void * to , int toTypeId )
void *	create (int type , const void * copy = Q_NULLPTR)
bool	debugStream (QDebug & dbg , const void * rhs , int typeId )
void	destroy (int type , void * data )
void	destruct (int type , void * where )
bool	equals (const void * lhs , const void * rhs , int typeId , int * result )
bool	hasRegisteredComparators ()
bool	hasRegisteredComparators (int typeId )
bool	hasRegisteredConverterFunction (int fromTypeId , int toTypeId )
bool	hasRegisteredConverterFunction ()
bool	hasRegisteredDebugStreamOperator ()
bool	hasRegisteredDebugStreamOperator (int typeId )
bool	isRegistered (int type )
bool	load (QDataStream & stream , int type , void * data )
const QMetaObject *	metaObjectForType (int type )
bool	registerComparators ()
bool	registerConverter ()
bool	registerConverter (MemberFunction function )
bool	registerConverter (MemberFunctionOk function )
bool	registerConverter (UnaryFunction function )
bool	registerDebugStreamOperator ()
bool	registerEqualsComparator ()
bool	save (QDataStream & stream , int type , const void * data )
int	sizeOf (int type )
int	type (const char * typeName )
int	type (const QByteArray & typeName )
TypeFlags	typeFlags (int type )
const char *	typeName (int typeId )

相关非成员

int	qMetaTypeId ()
int	qRegisterMetaType (const char * typeName )
int	qRegisterMetaType ()
void	qRegisterMetaTypeStreamOperators (const char * typeName )

宏

	Q_DECLARE_ASSOCIATIVE_CONTAINER_METATYPE ( Container )
	Q_DECLARE_METATYPE ( Type )
	Q_DECLARE_OPAQUE_POINTER ( PointerType )
	Q_DECLARE_SEQUENTIAL_CONTAINER_METATYPE ( Container )
	Q_DECLARE_SMART_POINTER_METATYPE ( SmartPointer )

详细描述

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

用作帮助程序的类能编组类型在 QVariant 和在队列信号及槽连接中。它将类型名称关联到类型，以便可以在运行时动态创建和销毁它。声明新类型采用 Q_DECLARE_METATYPE () 以使它们可用于 QVariant 和其它基于模板的函数。调用 qRegisterMetaType () 以使类型可用于基于非模板的函数，譬如：队列信号和槽连接。

可以注册具有公共默认构造函数、公共拷贝构造函数及公共析构函数的任何类或结构。

以下代码分配并销毁实例化的 MyClass :

int id = QMetaType::type("MyClass");
if (id != QMetaType::UnknownType) {
    void *myClassPtr = QMetaType::create(id);
    ...
    QMetaType::destroy(id, myClassPtr);
    myClassPtr = 0;
}
					

若想要流运算符 operator<<() and operator>>() 工作在 QVariant 对象存储自定义类型，自定义类型必须提供 operator<<() and operator>>() 运算符。

另请参阅 Q_DECLARE_METATYPE (), QVariant::setValue (), QVariant::value ()，和 QVariant::fromValue ().

成员类型文档编制

enum QMetaType:: Type

这些内置类型的支持通过 QMetaType :

常量	值	描述
QMetaType::Void	43	void
QMetaType::Bool	1	bool
QMetaType::Int	2	int
QMetaType::UInt	3	无符号 int
QMetaType::Double	6	double
QMetaType::QChar	7	QChar
QMetaType::QString	10	QString
QMetaType::QByteArray	12	QByteArray
QMetaType::VoidStar	31	void *
QMetaType::Long	32	long
QMetaType::LongLong	4	LongLong
QMetaType::Short	33	short
QMetaType::Char	34	char
QMetaType::ULong	35	unsigned long
QMetaType::ULongLong	5	ULongLong
QMetaType::UShort	36	unsigned short
QMetaType::SChar	40	signed char
QMetaType::UChar	37	unsigned char
QMetaType::Float	38	float
QMetaType::QObjectStar	39	QObject *
QMetaType::QVariant	41	QVariant
QMetaType::QCursor	74	QCursor
QMetaType::QDate	14	QDate
QMetaType::QSize	21	QSize
QMetaType::QTime	15	QTime
QMetaType::QVariantList	9	QVariantList
QMetaType::QPolygon	71	QPolygon
QMetaType::QPolygonF	86	QPolygonF
QMetaType::QColor	67	QColor
QMetaType::QSizeF	22	QSizeF
QMetaType::QRectF	20	QRectF
QMetaType::QLine	23	QLine
QMetaType::QTextLength	77	QTextLength
QMetaType::QStringList	11	QStringList
QMetaType::QVariantMap	8	QVariantMap
QMetaType::QVariantHash	28	QVariantHash
QMetaType::QIcon	69	QIcon
QMetaType::QPen	76	QPen
QMetaType::QLineF	24	QLineF
QMetaType::QTextFormat	78	QTextFormat
QMetaType::QRect	19	QRect
QMetaType::QPoint	25	QPoint
QMetaType::QUrl	17	QUrl
QMetaType::QRegExp	27	QRegExp
QMetaType::QRegularExpression	44	QRegularExpression
QMetaType::QDateTime	16	QDateTime
QMetaType::QPointF	26	QPointF
QMetaType::QPalette	68	QPalette
QMetaType::QFont	64	QFont
QMetaType::QBrush	66	QBrush
QMetaType::QRegion	72	QRegion
QMetaType::QBitArray	13	QBitArray
QMetaType::QImage	70	QImage
QMetaType::QKeySequence	75	QKeySequence
QMetaType::QSizePolicy	121	QSizePolicy
QMetaType::QPixmap	65	QPixmap
QMetaType::QLocale	18	QLocale
QMetaType::QBitmap	73	QBitmap
QMetaType::QMatrix	79	QMatrix
QMetaType::QTransform	80	QTransform
QMetaType::QMatrix4x4	81	QMatrix4x4
QMetaType::QVector2D	82	QVector2D
QMetaType::QVector3D	83	QVector3D
QMetaType::QVector4D	84	QVector4D
QMetaType::QQuaternion	85	QQuaternion
QMetaType::QEasingCurve	29	QEasingCurve
QMetaType::QJsonValue	45	QJsonValue
QMetaType::QJsonObject	46	QJsonObject
QMetaType::QJsonArray	47	QJsonArray
QMetaType::QJsonDocument	48	QJsonDocument
QMetaType::QModelIndex	42	QModelIndex
QMetaType::QPersistentModelIndex	50	QPersistentModelIndex (since 5.5)
QMetaType::QUuid	30	QUuid
QMetaType::QByteArrayList	49	QByteArrayList
QMetaType::User	1024	用于用户类型的基值
QMetaType::UnknownType	0	这是无效类型 ID。返回它从 QMetaType 为未注册类型

可以注册额外类型，使用 Q_DECLARE_METATYPE ().

另请参阅 type () 和 typeName ().

enum QMetaType:: TypeFlag
flags QMetaType:: TypeFlags

The enum describes attributes of a type supported by QMetaType .

常量	值	描述
QMetaType::NeedsConstruction	0x1	This type has non-trivial constructors. If the flag is not set instances can be safely initialized with memset to 0.
QMetaType::NeedsDestruction	0x2	This type has a non-trivial destructor. If the flag is not set calls to the destructor are not necessary before discarding objects.
QMetaType::MovableType	0x4	An instance of a type having this attribute can be safely moved by memcpy.
QMetaType::IsEnumeration	0x10	This type is an enumeration
QMetaType::PointerToQObject	0x8	This type is a pointer to a derived of QObject This type is a Q_GADGET and it's corresponding QMetaObject can be accessed with QMetaType::metaObject Since 5.5.

TypeFlags 类型是 typedef 对于 QFlags <TypeFlag>。它存储 TypeFlag 值的 OR 组合。

成员函数文档编制

QMetaType:: QMetaType (const int typeId )

构造 QMetaType object that contains all information about type typeId .

该函数在 Qt 5.0 引入。

QMetaType:: ~QMetaType ()

Destructs this object.

[static] bool QMetaType:: compare (const void * lhs , const void * rhs , int typeId , int * result )

比较对象在 lhs and rhs . Both objects need to be of type typeId . result is set to less than, equal to or greater than zero, if lhs is less than, equal to or greater than rhs 。返回 true , if the comparison succeeded, otherwise false .

该函数在 Qt 5.2 引入。

[static] void *QMetaType:: construct ( int type , void * where , const void * copy )

Constructs a value of the given type in the existing memory addressed by where , that is a copy of copy ，并返回 where 。若 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 that can store a value of type type , 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).

该函数在 Qt 5.0 引入。

另请参阅 destruct () 和 sizeOf ().

void *QMetaType:: construct ( void * where , const void * copy = Q_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 ，并返回 where 。若 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).

该函数在 Qt 5.0 引入。

[static] bool QMetaType:: convert (const void * from , int fromTypeId , void * to , int toTypeId )

Converts the object at from from fromTypeId to the preallocated space at to typed toTypeId 。返回 true ，若转换成功，否则 false。

该函数在 Qt 5.2 引入。

[static] void *QMetaType:: create ( int type , const void * copy = Q_NULLPTR)

Returns a copy of copy , assuming it is of type type 。若 copy is zero, creates a default constructed instance.

另请参阅 destroy (), isRegistered ()，和 Type .

void *QMetaType:: create (const void * copy = Q_NULLPTR) const

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

该函数在 Qt 5.0 引入。

另请参阅 QMetaType::destroy ().

[static] bool QMetaType:: debugStream ( QDebug & dbg , const void * rhs , int typeId )

Streams the object at rhs 类型 typeId to the debug stream dbg 。返回 true 当成功时，否则 false。

该函数在 Qt 5.2 引入。

[static] void QMetaType:: destroy ( int type , void * data )

销毁 data , assuming it is of the type 给定。

另请参阅 create (), isRegistered ()，和 Type .

void QMetaType:: destroy ( void * data ) const

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

该函数在 Qt 5.0 引入。

另请参阅 QMetaType::create ().

[static] void QMetaType:: destruct ( int type , void * where )

Destructs the value of the given type , located at where .

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

该函数在 Qt 5.0 引入。

另请参阅 construct ().

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.

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

该函数在 Qt 5.0 引入。

另请参阅 QMetaType::construct ().

[static] bool QMetaType:: equals (const void * lhs , const void * rhs , int typeId , int * result )

比较对象在 lhs and rhs . Both objects need to be of type typeId . result is set to zero, if lhs equals to rhs. Returns true , if the comparison succeeded, otherwise false .

该函数在 Qt 5.5 引入。

TypeFlags QMetaType:: flags () const

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

该函数在 Qt 5.0 引入。

另请参阅 QMetaType::TypeFlags and QMetaType::typeFlags ().

[static] bool QMetaType:: hasRegisteredComparators ()

返回 true , if the meta type system has registered comparators for type T.

该函数在 Qt 5.2 引入。

[static] bool QMetaType:: hasRegisteredComparators ( int typeId )

返回 true , if the meta type system has registered comparators for type id typeId .

该函数在 Qt 5.2 引入。

[static] bool QMetaType:: hasRegisteredConverterFunction ( int fromTypeId , int toTypeId )

返回 true , if the meta type system has a registered conversion from meta type id fromTypeId to toTypeId

该函数在 Qt 5.2 引入。

[static] bool QMetaType:: hasRegisteredConverterFunction ()

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

这是重载函数。

该函数在 Qt 5.2 引入。

[static] bool QMetaType:: hasRegisteredDebugStreamOperator ()

返回 true , if the meta type system has a registered debug stream operator for type T.

该函数在 Qt 5.2 引入。

[static] bool QMetaType:: hasRegisteredDebugStreamOperator ( int typeId )

返回 true , if the meta type system has a registered debug stream operator for type id typeId .

该函数在 Qt 5.2 引入。

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

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

另请参阅 type (), typeName ()，和 Type .

bool QMetaType:: isRegistered () const

返回 true 若此 QMetaType object contains valid information about a type, false otherwise.

该函数在 Qt 5.0 引入。

bool QMetaType:: isValid () const

返回 true 若此 QMetaType object contains valid information about a type, false otherwise.

该函数在 Qt 5.0 引入。

[static] bool QMetaType:: load ( QDataStream & stream , int type , void * data )

Reads the object of the specified type 从给定 stream into data 。返回 true if the object is loaded successfully; otherwise returns false .

The type must have been registered with qRegisterMetaType () 和 qRegisterMetaTypeStreamOperators () 事先。

Normally, you should not need to call this function directly. Instead, use QVariant 's operator>>() , which relies on load() to stream custom types.

另请参阅 save () 和 qRegisterMetaTypeStreamOperators ().

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 0 otherwise

该函数在 Qt 5.5 引入。

另请参阅 QMetaType::metaObjectForType () 和 QMetaType::flags ().

[static] const QMetaObject *QMetaType:: metaObjectForType ( int type )

返回 QMetaType::metaObject for type

该函数在 Qt 5.0 引入。

另请参阅 metaObject ().

[static] bool QMetaType:: registerComparators ()

Registers comparison operators for the user-registered type T. This requires T to have both an operator== and an operator<. Returns true if the registration succeeded, otherwise false.

该函数在 Qt 5.2 引入。

[static] 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.

该函数在 Qt 5.2 引入。

[static] bool QMetaType:: registerConverter ( MemberFunction function )

这是重载函数。

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.

该函数在 Qt 5.2 引入。

[static] bool QMetaType:: registerConverter ( MemberFunctionOk function )

这是重载函数。

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.

该函数在 Qt 5.2 引入。

[static] bool QMetaType:: registerConverter ( UnaryFunction function )

这是重载函数。

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.

该函数在 Qt 5.2 引入。

[static] bool QMetaType:: registerDebugStreamOperator ()

Registers the debug stream operator for the user-registered type T. This requires T to have an operator<<( QDebug dbg, T). Returns true if the registration succeeded, otherwise false.

[static] bool QMetaType:: registerEqualsComparator ()

Registers equals operator for the user-registered type T. This requires T to have an operator==. Returns true if the registration succeeded, otherwise false.

该函数在 Qt 5.5 引入。

[static] bool QMetaType:: save ( QDataStream & stream , int type , const void * data )

Writes the object pointed to by data with the ID type 到给定 stream 。返回 true if the object is saved successfully; otherwise returns false .

The type must have been registered with qRegisterMetaType () 和 qRegisterMetaTypeStreamOperators () 事先。

Normally, you should not need to call this function directly. Instead, use QVariant 's operator<<() , which relies on save() to stream custom types.

另请参阅 load () 和 qRegisterMetaTypeStreamOperators ().

[static] int QMetaType:: sizeOf ( int type )

Returns the size of the given type in bytes (i.e. sizeof(T), where T is the actual type identified by the type argument).

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

该函数在 Qt 5.0 引入。

另请参阅 construct ().

int 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.

该函数在 Qt 5.0 引入。

另请参阅 QMetaType::construct () and QMetaType::sizeOf().

[static] int QMetaType:: type (const char * typeName )

将句柄返回给类型调用 typeName ，或 QMetaType::UnknownType 若没有这种类型。

另请参阅 isRegistered (), typeName ()，和 Type .

[static] int QMetaType:: type (const QByteArray & typeName )

这是重载函数。

将句柄返回给类型调用 typeName ，或 0 若没有这种类型。

该函数在 Qt 5.5 引入。

另请参阅 isRegistered () 和 typeName ().

[static] TypeFlags QMetaType:: typeFlags ( int type )

返回标志为给定 type .

该函数在 Qt 5.0 引入。

另请参阅 QMetaType::TypeFlags .

[static] const char *QMetaType:: typeName ( int typeId )

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

另请参阅 type (), isRegistered ()，和 Type .

相关非成员

int qMetaTypeId ()

Returns the meta type id of type T at compile time. If the type was not declared with Q_DECLARE_METATYPE (), compilation will fail.

典型用法：

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

QMetaType::type () returns the same ID as qMetaTypeId(), but does a lookup at runtime based on the name of the type. QMetaType::type () is a bit slower, but compilation succeeds if a type is not registered.

注意： 此函数是 thread-safe .

该函数在 Qt 4.1 引入。

另请参阅 Q_DECLARE_METATYPE () 和 QMetaType::type ().

int qRegisterMetaType (const char * typeName )

注册类型名称 typeName 为类型 T 。返回的内部 ID 用于 QMetaType 。可以注册拥有公共默认构造函数、公共拷贝构造函数和公共析构函数的任何类 (或结构)。

此函数要求 T 是在函数被调用时的完全定义类型。对于指针类型，它还要求指向完全定义的类型。使用 Q_DECLARE_OPAQUE_POINTER () 能够注册指向转发声明类型的指针。

在已注册类型后，可以在运行时动态创建和销毁该类型的对象。

此范例注册类 MyClass :

qRegisterMetaType<MyClass>("MyClass");
					

此函数对注册 typedef 很有用，因此它们可以用于 QMetaProperty ，或在 QueuedConnections

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

警告： 此函数仅适用于注册别名 (typedef) 为每种其它用例 Q_DECLARE_METATYPE and qMetaTypeId () 应该使用以取而代之。

注意： 此函数是 thread-safe .

另请参阅 qRegisterMetaTypeStreamOperators (), isRegistered ()，和 Q_DECLARE_METATYPE ().

int qRegisterMetaType ()

调用此函数去注册类型 T . T 必须被声明采用 Q_DECLARE_METATYPE ()。返回元类型 ID。

范例：

int id = qRegisterMetaType<MyStruct>();
					

此函数要求 T 是在函数被调用时的完全定义类型。对于指针类型，它还要求指向完全定义的类型。使用 Q_DECLARE_OPAQUE_POINTER () 能够注册指向转发声明类型的指针。

在已注册类型后，可以在运行时动态创建和销毁该类型的对象。

To use the type T in QVariant ，使用 Q_DECLARE_METATYPE () is sufficient. To use the type T in queued signal and slot connections, qRegisterMetaType<T>() must be called before the first connection is established.

Also, to use type T 采用 QObject::property () API, qRegisterMetaType<T>() must be called before it is used, typically in the constructor of the class that uses T , or in the main() 函数。

注意： 此函数是 thread-safe .

该函数在 Qt 4.2 引入。

另请参阅 Q_DECLARE_METATYPE ().

void qRegisterMetaTypeStreamOperators (const char * typeName )

Registers the stream operators for the type T called typeName .

Afterward, the type can be streamed using QMetaType::load () 和 QMetaType::save (). These functions are used when streaming a QVariant .

qRegisterMetaTypeStreamOperators<MyClass>("MyClass");
					

The stream operators should have the following signatures:

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

注意： 此函数是 thread-safe .

另请参阅 qRegisterMetaType (), QMetaType::isRegistered ()，和 Q_DECLARE_METATYPE ().

宏文档编制

Q_DECLARE_ASSOCIATIVE_CONTAINER_METATYPE ( Container )

This macro makes the container Container 知道 QMetaType as an associative container. This makes it possible to put an instance of Container<T, U> into a QVariant , if T and U are themselves known to QMetaType .

Note that all of the Qt associative containers already have built-in support, and it is not necessary to use this macro with them. The std::map container also has built-in support.

This example shows a typical use of Q_DECLARE_ASSOCIATIVE_CONTAINER_METATYPE():

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

Q_DECLARE_METATYPE ( Type )

此宏使类型 Type 知道 QMetaType as long as it provides a public default constructor, a public copy constructor and a public destructor. It is needed to use the type Type as a custom type in QVariant .

This macro requires that Type is a fully defined type at the point where it is used. For pointer types, it also requires that the pointed to type is fully defined. Use in conjunction with Q_DECLARE_OPAQUE_POINTER () to register pointers to forward declared types.

Ideally, this macro should be placed below the declaration of the class or struct. If that is not possible, it can be put in a private header file which has to be included every time that type is used in a QVariant .

Adding a Q_DECLARE_METATYPE() makes the type known to all template based functions, including QVariant . Note that if you intend to use the type in queued signal and slot connections or in QObject 's property system, you also have to call qRegisterMetaType () since the names are resolved at runtime.

此范例展示 Q_DECLARE_METATYPE() 的典型用例：

struct MyStruct
{
    int i;
    ...
};
Q_DECLARE_METATYPE(MyStruct)
					

若 MyStruct is in a namespace, the Q_DECLARE_METATYPE() macro has to be outside the namespace:

namespace MyNamespace
{
    ...
}
Q_DECLARE_METATYPE(MyNamespace::MyStruct)
					

由于 MyStruct 现在知道 QMetaType ，它可以用于 QVariant :

MyStruct s;
QVariant var;
var.setValue(s); // copy s into the variant
...
// retrieve the value
MyStruct s2 = var.value<MyStruct>();
					

Some types are registered automatically and do not need this macro:

• Pointers to classes derived from QObject
• QList <T>, QVector <T>, QQueue <T>, QStack <T>, QSet <T> or QLinkedList <T> where T is a registered meta type
• QHash <T1, T2>, QMap <T1, T2> or QPair <T1, T2> where T1 and T2 are registered meta types
• QPointer <T>, QSharedPointer <T>, QWeakPointer <T>, where T is a class that derives from QObject
• Enumerations registered with Q_ENUM or Q_FLAG
• Classes that have a Q_GADGET macro

另请参阅 qRegisterMetaType ().

Q_DECLARE_OPAQUE_POINTER ( PointerType )

This macro enables pointers to forward-declared types ( PointerType ) to be registered with QMetaType using either Q_DECLARE_METATYPE () 或 qRegisterMetaType ().

该函数在 Qt 5.0 引入。

另请参阅 Q_DECLARE_METATYPE () 和 qRegisterMetaType ().

Q_DECLARE_SEQUENTIAL_CONTAINER_METATYPE ( Container )

This macro makes the container Container 知道 QMetaType as a sequential container. This makes it possible to put an instance of Container<T> into a QVariant , if T itself is known to QMetaType .

Note that all of the Qt sequential containers already have built-in support, and it is not necessary to use this macro with them. The std::vector and std::list containers also have built-in support.

This example shows a typical use of Q_DECLARE_SEQUENTIAL_CONTAINER_METATYPE():

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

Q_DECLARE_SMART_POINTER_METATYPE ( SmartPointer )

此宏使智能指针 SmartPointer 知道 QMetaType as a smart pointer. This makes it possible to put an instance of SmartPointer<T> into a QVariant , if T is a type which inherits QObject .

注意， QWeakPointer , QSharedPointer and QPointer already have built-in support, and it is not necessary to use this macro with them.

This example shows a typical use of Q_DECLARE_SMART_POINTER_METATYPE():

#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'.
    }
}