CODEWITHSUNDEEP
c++qttypesqml
Stefan Monov
Stefan Monov

Reputation: 11732

Qt: How to check from C++ whether an object is of QML type Foo

Consider a QML type (defined in QML, not in C++):

Foo {
}

And assume I have a QQuickItem* item variable in C++. How do I check if the variable is of type Foo?

If Foo was a C++ type, I could do this:

qobject_cast<Foo*>(item) == nullptr

Since Foo is a QML type, one option would be

item->metaObject()->className().beginsWith("Foo")

(className() returns something like Foo_QMLTYPE_0)

But that seems unreliable and hacky.

Upvotes: 2

Views: 1015

Answers (2)

Daniel Chapman
Daniel Chapman

Reputation: 101

IMO you are part of the way there. Your best bet is using a combination of QQuickItem::inherits and QMetaObject::className.

This way you can figure out if it is actually a Foo or derived from Foo.

const QByteArray foo = QByteArrayLiteral("Foo");

if (item->metaObject()->className() == foo || item->inherits(foo)) {
    // It's a Foo
}

Upvotes: 0

dtech
dtech

Reputation: 49319

You could use a magic key property:

Foo {
  property bool isFoo : true
}

And then from C++:

if (obj->property("isFoo").toBool())

If the object doesn't have that property, then the condition should fail. This will also work for "derived" objects as the property will be "inherited", for example if something extends Foo but is not necessarily named FooSomething but rather SomethingFoo.

Naturally, there will be some overhead, which could get pronounced in scenarios where you need a lot of type checks on many instances. What I tend to do is have a single quint32 type property implemented efficiently on C++ level, then from QML this is explicitly set in objects where type information is necessary. This way the overhead is minimized, and it is also faster to access than using property(name).

Upvotes: 3

Related Questions