programming ruby - 5

1, module

 

If a third programwants to use these modules, it can simply load the two files (using the Ruby require statement, which we discuss on page 116) and reference the qualified names.

 

At a stroke, they pretty much eliminate the need for multiple inheritance, providing a facility called a mixin.

 

A module can’t have instances, because a module isn’t a class. However, you can include a module within a class definition. When this happens, all the module’s instance methods are suddenly available as methods in the class as well. They get mixed in. In fact, mixed-in modules effectively behave as superclasses.

 

The Ruby include statement simply makes a reference to a named module. If that module is in a separate file, you must use require (or its less commonly used cousin, load) to drag that file in before using include. Second, a Ruby include does not simply copy the module’s instance methods into the class. Instead, it makes a reference from the class to the included module.

 

Alternativly, the module could use a module-level hash, indexed by the current object ID, to store instance-specific data without using Ruby instance variables.

 

The answer is that Ruby looks first in the immediate class of an object, then in the mixins included into that class, and then in superclasses and their mixins. If a class has multiple modules mixed in, the last one included is searched first.

 

The load method includes the named Ruby source file every time the method is executed.

 

The more commonly used require method loads any given file only once.