Dillo: lout::identity::IdentifiableObject Class Reference

Instances of classes, which are sub classes of this class, may be identified at run-time.

Testing the class

Since e.g. dw::Textblock is a sub class of IdentifiableObject, and implemented in the correct way (as described below), for any given IdentifiableObject the following test can be done:

isATextblock is true, when o is an instance of dw::Textblock, or of a sub class of dw::Textblock. Otherwise, isATextblock is false.

It is also possible to get the class identifier of an identity::IdentifiableObject, e.g.

would result in true, if o is an instance of dw::Textblock, but not an instance of a sub class of dw::Textblock.

Defining Sub Classes

Each direct or indirect sub class of IdentifiableObject must

• add a static int CLASS_ID with -1 as initial value, and
• call registerName (name, &CLASS_ID) in the constructor, where name should be unique, e.g. the fully qualified class name.

After this, class::CLASS_ID refers to a number, which denotes the class. (If this is still -1, since the class has not yet been instantiated, any test will fail, which is correct.)

Notes on implementation

If there are some classes like this:

and first, an instance of A, and then an instance of C is created, there will be the following calls of functions and constructors:

1. IdentifiableObject ();
2. registerName (“A”, &A::CLASS_ID);
3. IdentifiableObject ();
4. registerName (“B”, &B::CLASS_ID);
5. registerName (“C”, &C::CLASS_ID);

From this, the class hierarchy above can easily constructed, and stored in identity::IdentifiableObject::classesByName and in identity::IdentifiableObject::classesById. See the code for details.

N.b. Multiple inheritance is not supported, the construction of the tree would become confused.