Modules¶↑

Modules serve two purposes in Ruby, namespacing and mix-in functionality.

A namespace can be used to organize code by package or functionality that separates common names from interference by other packages. For example, the IRB namespace provides functionality for irb that prevents a collision for the common name “Context”.

Mix-in functionality allows sharing common methods across multiple classes or modules. Ruby comes with theEnumerable mix-in module which provides many enumeration methods based on theeach method andComparable allows comparison of objects based on the<=> comparison method.

Note that there are many similarities between modules and classes. Besides the ability to mix-in a module, the description of modules below also applies to classes.

Module Definition¶↑

A module is created using themodule keyword:

moduleMyModule# ...end

A module may be reopened any number of times to add, change or remove functionality:

moduleMyModuledefmy_methodendendmoduleMyModulealiasmy_aliasmy_methodendmoduleMyModuleremove_method:my_methodend

Reopening modules (or classes) is a very powerful feature of Ruby, but it is best to only reopen modules you own. Reopening modules you do not own may lead to naming conflicts or difficult to diagnose bugs.

Nesting¶↑

Modules may be nested:

moduleOutermoduleInnerendend

Many packages create a single outermost module (or class) to provide a namespace for their functionality.

You may also define inner modules using:: provided the outer modules (or classes) are already defined:

moduleOuter::Inner::GrandChildend

Note that this will raise aNameError ifOuter andOuter::Inner are not already defined.

This style has the benefit of allowing the author to reduce the amount of indentation. Instead of 3 levels of indentation only one is necessary. However, the scope of constant lookup is different for creating a namespace using this syntax instead of the more verbose syntax.

Scope¶↑

self¶↑

self refers to the object that defines the current scope.self will change when entering a different method or when defining a new module.

Constants¶↑

Accessible constants are different depending on the module nesting (which syntax was used to define the module). In the following example the constantA::Z is accessible from B as A is part of the nesting:

moduleAZ =1moduleBpModule.nesting#=> [A::B, A]pZ#=> 1endend

However, if you use:: to defineA::B without nesting it insideA, aNameError exception will be raised because the nesting does not includeA:

moduleAZ =1endmoduleA::BpModule.nesting#=> [A::B]pZ#=> raises NameErrorend

If a constant is defined at the top-level you may preceded it with:: to reference it:

Z =0moduleAZ =1moduleBp::Z#=> 0endend

Methods¶↑

For method definition documentation see thesyntax documentation for methods.

Class methods may be called directly. (This is slightly confusing, but a method on a module is often called a “class method” instead of a “module method”. See alsoModule#module_function which can convert an instance method into a class method.)

When a class method references a constant, it uses the same rules as referencing it outside the method as the scope is the same.

Instance methods defined in a module are only callable when included. These methods have access to the constants defined when they were included through the ancestors list:

moduleAZ =1defzZendendincludeApself.class.ancestors#=> [Object, A, Kernel, BasicObject]pz#=> 1

Visibility¶↑

Ruby has three types of visibility. The default ispublic. A public method may be called from any other object.

The second visibility isprotected. When calling a protected method the sender must inherit the Class or Module which defines the method. Otherwise aNoMethodError will be raised.

Protected visibility is most frequently used to define== and other comparison methods where the author does not wish to expose an object’s state to any caller and would like to restrict it only to inherited classes.

Here is an example:

classAdefn(other)other.mendendclassB<Adefm1endprotected:mendclassC<Benda =A.newb =B.newc =C.newc.nb#=> 1 -- C is a subclass of Bb.nb#=> 1 -- m called on defining classa.nb# raises NoMethodError A is not a subclass of B

The third visibility isprivate. A private method may only be called from inside the owner class without a receiver, or with a literalself as a receiver. If a private method is called with a receiver other than a literalself, aNoMethodError will be raised.

classAdefwithoutmenddefwith_selfself.menddefwith_otherA.new.menddefwith_renamedcopy =selfcopy.menddefm1endprivate:menda =A.newa.without#=> 1a.with_self#=> 1a.with_other# NoMethodError (private method `m' called for #<A:0x0000559c287f27d0>)a.with_renamed# NoMethodError (private method `m' called for #<A:0x0000559c285f8330>)

alias andundef¶↑

You may also alias or undefine methods, but these operations are not restricted to modules or classes. See themiscellaneous syntax section for documentation.

Classes¶↑

Every class is also a module, but unlike modules a class may not be mixed-in to another module (or class). Like a module, a class can be used as a namespace. A class also inherits methods and constants from its superclass.

Defining a class¶↑

Use theclass keyword to create a class:

classMyClass# ...end

If you do not supply a superclass your new class will inherit fromObject. You may inherit from a different class using< followed by a class name:

classMySubclass<MyClass# ...end

There is a special classBasicObject which is designed as a blank class and includes a minimum of built-in methods. You can useBasicObject to create an independent inheritance structure. See theBasicObject documentation for further details.

Just like modules, classes can also be reopened. You can omit its superclass when you reopen a class. Specifying a different superclass than the previous definition will raise an error.

classCendclassD<Cend# OKclassD<Cend# OKclassDend# TypeError: superclass mismatch for class DclassD<Stringend

Inheritance¶↑

Any method defined on a class is callable from its subclass:

classAZ =1defzZendendclassB<AendpB.new.z#=> 1

The same is true for constants:

classAZ =1endclassB<AdefzZendendpB.new.z#=> 1

You can override the functionality of a superclass method by redefining the method:

classAdefm1endendclassB<Adefm2endendpB.new.m#=> 2

If you wish to invoke the superclass functionality from a method usesuper:

classAdefm1endendclassB<Adefm2+superendendpB.new.m#=> 3

When used without any argumentssuper uses the arguments given to the subclass method. To send no arguments to the superclass method usesuper(). To send specific arguments to the superclass method provide them manually likesuper(2).

super may be called as many times as you like in the subclass method.

Singleton Classes¶↑

The singleton class (also known as the metaclass or eigenclass) of an object is a class that holds methods for only that instance. You can access the singleton class of an object usingclass << object like this:

classCendclass<<C# self is the singleton class hereend

Most frequently you’ll see the singleton class accessed like this:

classCclass<<self# ...endend

This allows definition of methods and attributes on a class (or module) without needing to writedef self.my_method.

Since you can open the singleton class of any object this means that this code block:

o =Object.newdefo.my_method1+1end

is equivalent to this code block:

o =Object.newclass<<odefmy_method1+1endend

Both objects will have amy_method that returns2.