Ruby setter method syntax method=(value) - Comparison to Java

问题

Hello dear stackoverflowers :)

I came from Java and have one doubt about the syntax of getters (if it's really just a syntax issue).

In java you would have a setter like

private void setName(value) {
    variableName = value;
}

who would take value as an argument and change the instance variable inside it.

In ruby, when I explicitly define a setter (due to constraint reasons), I need to use set_name=(value) or if I use the syntax set_name(value) would be the same? In other words, the = in the end of the method name does anything else or it's just syntax (like ! and ?).

Like this:

def set_name=(value)
    @name = value
end

Or this:

def set_name(value)
    @name = value
end

Thanks in advance for the attention.

Alex

回答1:

The trailing = in the method name identifies the method as a setter/mutator method. When you say this in Ruby:

o.p = v

You're really saying:

o.send(:p=, v)

so o.p = v is just a fancy way of calling the p= method in o. That's why things like this:

's'.pancakes = 11

gives you a NoMethodError exception that complains about 's' not having a pancakes= method: Strings don't (unfortunately) have pancakes= methods.

In your case, you wouldn't use set_name at all, you'd have a name= method:

def name=(value)
  @name = value
end

and possibly a name method as an accessor/getter:

def name
  @name
end

回答2:

The most idiomatic approach in Ruby is to do

def name=(value)
  @name = value
end

or better yet,

attr_writer :name

回答3:

I need to use set_name=(value) or if I use the syntax set_name(value) would be the same?

class Dog
  attr_reader :name

  def initialize(name)
    @name = name
  end

  def name1=(str)
    @name = str
  end

  def name2(str)
    @name = str
  end
end

d = Dog.new("Rover")
puts d.name  #=>Rover
d.name1 = "Ruthie"
puts d.name  #=>Ruthie
d.name2("John")
puts d.name  #=>John
d.name2 = "Roger"

--output:--
1.rb:23:in `<main>': undefined method `name2=' for #<Dog:0x00000100907030 @name="John"> (NoMethodError)

For setters, the name of the method includes the '=' sign. But ruby also allows you to use syntactic sugar with method names that end in '=':