Minor Description
Java-style interfaces would be a nice feature for Clownfish. Here's a sketch of how it could be implemented. It's basically an adaption of the technique described by Bjarne Stroustrup in his paper Multiple Inheritance for C++.
If a subclass implements an interface, an additional instance variable pointing to an interface method table is created. A pointer to this instance variable represents an interface reference. The interface method table contains
- the offset of the pointer to the interface method table from the start of the object.
- a variable length array of method pointers to the implementations of the interface's methods.
- optionally a pointer to an Interface object for introspection.
Every subclass/interface combination needs a separate interface method table. Subclasses that don't override any of the interface methods of a parent class can reuse the parent class's table.
The memory layout would look like this:
object
+--------+
| head |
| vtable | itable
| ivars | +---------+
-->| itable +-->| offset |
| ivars | | method1 |
+--------+ | method2 |
| method3 |
| method4 |
| ... |
+---------+
Converting an object to an interface reference is done by returning a pointer to the itable struct member. Converting back to the object is done by subtracting the offset in the itable. Method invocation from an interface reference is done by looking up the method pointer and calling it with the original object.
In host languages, an interface is represented by a class with the interface's methods. Objects of this class contain the interface reference (address of the itable instance variable).