QMetaType Class

See also https://(), https://(), https://(), https://()

These are the built-in types supported by https://:

Additional types can be registered using https://() or by calling https://().

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flags QMetaType::TypeFlags

The enum describes attributes of a type supported by https://.

Note that the Needs flags may be set but the meta type may not have a publicly-accessible constructor of the relevant type or a publicly-accessible destructor.

The TypeFlags type is a typedef for https://<TypeFlag>. It stores an OR combination of TypeFlag values.

Constructs a https:// object that contains all information about type typeId.

Constructs a default, invalid, https:// object.

This function was introduced in Qt 6.0.

Returns the alignment of the type in bytes (i.e. alignof(T), where T is the actual type for which this https:// instance was constructed for).

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

This function was introduced in Qt 6.0.

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Returns true if https:// can convert from fromType to toType.

The following conversions are supported by Qt:

Casting between primitive type (int, float, bool etc.) is supported.

Converting between pointers of types derived from https:// will also return true for this function if a https:// from the type described by fromType to the type described by toType would succeed.

A cast from a sequential container will also return true for this function if the toType is https://.

Similarly, a cast from an associative container will also return true for this function the toType is https:// or https://.

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Returns true if https:// can create a mutable view of type toType on type fromType.

Converting between pointers of types derived from https:// will return true for this function if a https:// from the type described by fromType to the type described by toType would succeed.

You can create a mutable view of type https:// on any container registered with https://().

Similarly you can create a mutable view of type https:// on any container registered with https://().

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Compares the objects at lhs and rhs for ordering.

Returns https:// if comparison is not supported or the values are unordered. Otherwise, returns https://, https:// or https:// if lhs is less than, equivalent to or greater than rhs, respectively.

Both objects must be of the type described by this metatype. If either lhs or rhs is nullptr, the values are unordered. Comparison is only supported if the type's less than operator was visible to the metatype declaration.

If the type's equality operator was also visible, values will only compare equal if the equality operator says they are. In the absence of an equality operator, when neither value is less than the other, values are considered equal; if equality is also available and two such values are not equal, they are considered unordered, just as NaN (not a number) values of a floating point type lie outside its ordering.

This function was introduced in Qt 6.0.

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Constructs a value of the type that this https:// instance was constructed for in the existing memory addressed by where, that is a copy of copy, and returns where. If 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 https://().

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).

Converts the object at from from fromType to the preallocated space at to typed toType. Returns true, if the conversion succeeded, otherwise false.

Both from and to have to be valid pointers.

Returns a copy of copy, assuming it is of the type that this https:// instance was created for. If copy is nullptr, creates a default constructed instance.

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Streams the object at rhs to the debug stream dbg. Returns true on success, otherwise false.

Destroys the data, assuming it is of the type that this https:// instance was created for.

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Destructs the value, located at data, assuming that it is of the type for which this https:// instance was constructed for.

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

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Compares the objects at lhs and rhs for equality.

Both objects must be of the type described by this metatype. Can only compare the two objects if a less than or equality operator for the type was visible to the metatype declaration. Otherwise, the metatype never considers values equal. When an equality operator was visible to the metatype declaration, it is authoritative; otherwise, if less than is visible, when neither value is less than the other, the two are considered equal. If values are unordered (see https://() for details) they are not equal.

Returns true if the two objects compare equal, otherwise false.

This function was introduced in Qt 6.0.

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Returns flags of the type for which this https:// instance was constructed. To inspect specific type traits, prefer using one of the "is-" functions rather than the flags directly.

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Returns a https:// matching typeName. The returned object is not valid if the typeName is not known to https://

Returns the https:// corresponding to the type in the template parameter.

Returns true, if the meta type system has a registered conversion from meta type id fromType to toType

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

This is an overloaded function.

Returns true, if the meta type system has registered data stream operators for this meta type.

This function was introduced in Qt 6.1.

Returns true, if the meta type system has a registered debug stream operator for this meta type.

This function was introduced in Qt 6.0.

Returns true, if the meta type system has a registered mutable view on meta type id fromType of meta type id toType.

Returns true, if the meta type system has a registered mutable view on type From of type To.

This is an overloaded function.

This function was introduced in Qt 6.0.

Returns id type held by this QMetatype instance.

Returns true if this type can be copy-constructed. If it can be, then construct() and create() can be used with a copy parameter that is not null.

This function was introduced in Qt 6.5.

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Returns true if this type can be default-constructed. If it can be, then construct() and create() can be used with a copy parameter that is null.

This function was introduced in Qt 6.5.

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Returns true if this type can be destroyed. If it can be, then destroy() and destruct() can be called.

This function was introduced in Qt 6.5.

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Returns true if a less than or equality operator for the type described by this metatype was visible to the metatype declaration, otherwise false.

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Returns true if this type can be move-constructed. https:// currently does not have an API to make use of this trait.

This function was introduced in Qt 6.5.

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Returns true if a less than operator for the type described by this metatype was visible to the metatype declaration, otherwise false.

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

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Returns true if this https:// object has been registered with the Qt global metatype registry. Registration allows the type to be found by its name (using https://()) or by its ID (using the constructor).

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Returns true if this https:// object contains valid information about a type, false otherwise.

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Reads the object of this type from the given stream into data. Returns true if the object is loaded successfully; otherwise returns false.

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

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Returns a https:// relative to this type.

If the type is a pointer type to a subclass of https://, https://() contains https:// and this function returns the corresponding https://. This can be used in combination with QMetaObject::newInstance() to create QObjects of this type.

If the type is a https://, https://() contains https://. If the type is a pointer to a https://, https://() contains https://. In both cases, this function returns its https://. This can be used to retrieve https:// and https:// and use them on a pointer of this type for example, as given by https://().

If the type is an enumeration, https://() contains https://. In this case, this function returns the https:// of the enclosing object if the enum was registered as a https:// or nullptr otherwise.

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Returns the type name associated with this https://, or a null pointer if no matching type was found. The returned pointer must not be deleted.

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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.

class Counter {
  int number = 0;
public:
  int value() const { return number; }
  operator int() const { return value(); }
  void increment() {++number;}
};
QMetaType::registerConverter<Counter, int>();

This is an overloaded function.

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.

struct Coordinates {
  int x;
  int y;
  int z;

  QString toString() const { return u"[x: %1; y: %2, z: %3]"_s.arg(QString::number(x),
    QString::number(y),
    QString::number(z)); }
};
QMetaType::registerConverter<Coordinates, QString>(&Coordinates::toString);

This is an overloaded function.

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.

The ok pointer can be used by the function to indicate whether the conversion succeeded.

struct BigNumber {
    long long l;

    int toInt(bool *ok = nullptr) const {
      const bool canConvertSafely = l < std::numeric_limits<int>::max();
      if (ok)
        *ok = canConvertSafely;
      return l;
    }
};
QMetaType::registerConverter<BigNumber, int>(&BigNumber::toInt);

This is an overloaded function.

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.

function must take an instance of type From and return an instance of To. It can be a function pointer, a lambda or a functor object. Since Qt 6.5, the function can also return an instance of std::optional<To> to be able to indicate failed conversions.

QMetaType::registerConverter<CustomStringType, QString>([](const CustomStringType &str) {
    return QString::fromUtf8(str.data());
});
QMetaType::registerConverter<QJsonValue, QPointF>(
          [](const QJsonValue &value) -> std::optional<QPointF> {
    const auto object = value.toObject();
    if (!object.contains("x") || !object.contains("y"))
        return std::nullopt;  // The conversion fails if the required properties are missing
    return QPointF{object["x"].toDouble(), object["y"].toDouble()};
});

This is an overloaded function.

Registers a method function like To From::function() as mutable view of type To on type From in the meta type system. Returns true if the registration succeeded, otherwise false.

This function was introduced in Qt 6.0.

This is an overloaded function.

Registers a unary function object function as mutable view of type To on type From in the meta type system. Returns true if the registration succeeded, otherwise false.

This function was introduced in Qt 6.0.

Registers this https:// with the type registry so it can be found by name, using https://().

This function was introduced in Qt 6.5.

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Writes the object pointed to by data to the given stream. Returns true if the object is saved successfully; otherwise returns false.

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

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Returns the size of the type in bytes (i.e. sizeof(T), where T is the actual type for which this https:// instance was constructed for).

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

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If this metatype represents an enumeration, this method returns a metatype of a numeric class of the same signedness and size as the enums underlying type. If it represents a https:// type, it returns https://. In all other cases an invalid https:// is returned.

This function was introduced in Qt 6.6.

Creates a mutable view on the object at from of fromType in the preallocated space at to typed toType. Returns true if the conversion succeeded, otherwise false.

This function was introduced in Qt 6.0.

Returns the hash value for the type, using seed to seed the calculation.

This function was introduced in Qt 6.4.

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

Typical usage:

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.

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Call this function to register the type T. Returns the meta type Id.

Example:

int id = qRegisterMetaType<MyStruct>();

This function requires that T is a fully defined type at the point where the function is called. For pointer types, it also requires that the pointed to type is fully defined. Use https://() to be able to register pointers to forward declared types.

To use the type T in https://, https://, or with the https://() API, registration is not necessary.

To use the type T in queued signal and slot connections, qRegisterMetaType<T>() must be called before the first connection is established. That is typically done in the constructor of the class that uses T, or in the main() function.

After a type has been registered, it can be found by its name using https://().

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Registers the meta type meta and returns its type Id.

This function requires that T is a fully defined type at the point where the function is called. For pointer types, it also requires that the pointed to type is fully defined. Use https://() to be able to register pointers to forward declared types.

To use the type T in https://, https://, or with the https://() API, registration is not necessary.

To use the type T in queued signal and slot connections, qRegisterMetaType<T>() must be called before the first connection is established. That is typically done in the constructor of the class that uses T, or in the main() function.

After a type has been registered, it can be found by its name using https://().

This function was introduced in Qt 6.5.

This is an overloaded function.

Returns true if the https:// a represents a different type than the https:// b, otherwise returns false.

Writes the https:// m to the stream d, and returns the stream.

This function was introduced in Qt 6.5.

This is an overloaded function.

Returns true if the https:// a represents the same type as the https:// b, otherwise returns false.

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

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);
    // ...
}

This macro makes the type Type known to https:// 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 https://.

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 https://() 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 https://.

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

This example shows a typical use case of Q_DECLARE_METATYPE():

struct MyStruct
{
    int i;
    ...
};

Q_DECLARE_METATYPE(MyStruct)

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

namespace MyNamespace
{
    ...
}

Q_DECLARE_METATYPE(MyNamespace::MyStruct)

Since MyStruct is now known to https://, it can be used in https://:

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 https://

• https://<T>, https://<T>, https://<T> or https://<T> where T is a registered meta type

• https://<T1, T2>, https://<T1, T2> or https://<T1, T2> where T1 and T2 are registered meta types

• https://<T>, https://<T>, https://<T>, where T is a class that derives from https://

• Enumerations registered with https:// or https://

• Classes that have a https:// macro

The stream operators should have the following signatures:

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

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This macro enables pointers to forward-declared types (PointerType) to be registered with https:// using either https://() or https://().

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

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);
    // ...
}

This macro makes the smart pointer SmartPointer known to https:// as a smart pointer. This makes it possible to put an instance of SmartPointer<T> into a https://, if T is a type which inherits https://.

Note that the https://, https:// and https:// 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'.
    }
}

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

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

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

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

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

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

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

[static] bool QMetaType::hasRegisteredDebugStreamOperator()▲

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

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

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

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

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

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

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

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

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

int qRegisterMetaType(const char *typeName)▲

qRegisterMetaType<MyClass>("MyClass");

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