初始化而不创建实例（Eiffel）

Question

This is a BON Diagram on the classes.

Notice how in the example, the attributes were not created into instances. At run time a uses take_off from b . I'm just wondering how take_off is being initialized in the B_747 class. In my assignment, I have a case where it's like b := ab c := ac where B_747 b and c each has the same attributes, but different value assigned to each. I don't know how to initialize the attributes without them being instantiated. It says I'm not allowed to use b.make ("abc", 123) .Can someone give me an example on how to implement this. Also, I'm not really sure what b:=ab really means.

Answer 1

The example illustrates something else, not object creation. Answering your first point, take_off need not be initialized. It is just a feature of class PLANE that is effected in B_747 . The slide says that even though the static type of a is PLANE where take_off is declared as deferred (ie without any specific implementation), when the variable is attached an object of type B_747 , it uses the feature implementation from B_747 .

Answering you last point, b := ab means that a result of a feature b , called on an object computed by an expression a , is attached to a variable b . The latter may be a local variable or an attribute of the current class. Expression a may be a function of the current class, an attribute of the current class, an argument of the current feature, a local variable of the current feature, etc. depending on the context. The feature b called on a may be a function of the class corresponding to the type of a or an attribute of that class.