QMetaObject and QMetaProperty - why 2 times the same object?

qwe3

Hi,

I would like to read a property from one object. I can do that, but I have to write:

const QMetaObject * metaObj = myObject->metaObject();

and in a loop:

QVariant property = metaObj->property(i).read(myObject);

Why I have to add myObject param in read() too ?

I use myObject here:
const QMetaObject * metaObj = myObject->metaObject();
and here:
QVariant property = metaObj->property(i).read(myObject);

This looks very bad, that I have to use the same word in two places.

Chris Kawa

QMetaObject is a meta object for a class, not an instance. myObject->metaObject() is basically the same as MyObjectClass::staticMetaObject. It's just a shortcut to look up the class of a given instance.

Properties on the other hand have values per object. When you have a meta object , which, again, is a class descriptor and not related to any particular instance of it, you can use it to read a property. The "location" of the property in given class is described in the meta object, but to get a value from a particular instance of that class you need to pass that instance.

So, if you want to avoid using the instance pointer twice you can do it like this:

QVariant property = MyObjectClass::staticMetaObject.property(i).read(myObject);

If you don't know the class of the object you have to use myObject->metaObject() so that it gives you the meta object of the right class.