Attribute Accessors

Data Structures and Algorithms with Object-Oriented Design Patterns in Ruby

Attribute Accessors

Program continues the definition of the Complex class. It defines two attribute accessors called real and image.

program56255
Program: Complex class real and imag attribute accessors.

The effect of the statement ``attr_accessor :symbol'' is equivalent to the following:

def symbol
    @symbol
end

def symbol=(value)
    @real = symbol
end

That is, the declaration of an attribute accessor has the effect of creating a pair of methos--one method that returns the current value of the corresponding attribute, the other method assigns a new value to the corresponding attribute.

Thus, Program defines two methods real and real=, that access the instance attribute @real and two methods imag and image=, that access the instance attribute @imag.

A method that accesses an instance but does not modify that instance is called an attribute reader . Therefore, real and imag are attribute readers.

The real= and imag= methods provide the means to modify the @real and @imag instance attributes, respectively. A method that modifies an instance attribute is called an attribute writer . Therefore, real= and image= are attribute writers.

Ruby provides the means to declare attribute readers and writers separately. The sequence of declarations

attr_reader :real
attr_writer :real

is equivalent to the declaration

attr_accessor : real

The dot operator is used to specify the object on which an accessor method is to be invoked. For example, the statement

c.real = 2

invokes the attribute writer method real=. The effect of this is to make the @real instance attribute refer to a Fixnum object that has the value 2.

Program defines four more methods of the Complex class: r, r=, theta and theta=.

program56299
Program: Complex class r, coder=, theta and theta= methods.

The r and theta methods are accessor methods that return the polar coordinates of the given complex number. Similarly, the r= and theta= methods are mutatormutator methods that handle the assignment of polar coordinate values to the given complex number.