What is the most efficient way to initialize a Cla

问题:

I would like to have a class and some attributes which you can either set during initialization or use its default value.

class Fruit
  attr_accessor :color, :type
  def initialize(color, type)
    @color=color ||= 'green'
    @type=type ||='pear'
  end
end

apple=Fruit.new(red, apple)

回答1:

The typical way to solve this problem is with a hash that has a default value. Ruby has a nice syntax for passing hash values, if the hash is the last parameter to a method.

class Fruit
  attr_accessor :color, :type

  def initialize(params = {})
    @color = params.fetch(:color, 'green')
    @type = params.fetch(:type, 'pear')
  end

  def to_s
    "#{color} #{type}"
  end
end

puts(Fruit.new)                                    # prints: green pear
puts(Fruit.new(:color => 'red', :type => 'grape')) # prints: red grape
puts(Fruit.new(:type => 'pomegranate')) # prints: green pomegranate

A good overview is here: http://deepfall.blogspot.com/2008/08/named-parameters-in-ruby.html

回答2:

Since Ruby 2.0 there is support of named or keyword parameters.

You may use:

class Fruit
  attr_reader      :color, :type

  def initialize(color: 'green', type: 'pear')
    @color = color
    @type = type
  end

  def to_s
    "#{color} #{type}"
  end
end

puts(Fruit.new)                                    # prints: green pear
puts(Fruit.new(:color => 'red', :type => 'grape')) # prints: red grape
puts(Fruit.new(:type => 'pomegranate')) # prints: green pomegranate

Some interesting notes on this topic:

• Keyword arguments in Ruby 2.0
• Ruby 2.0.0 by Example
• Named parameters in Ruby 2

回答3:

Brian's answer is excellent but I would like to suggest some modifications to make it mostly meta:

class Fruit

  # Now this is the only thing you have to touch when adding defaults or properties
  def set_defaults
    @color ||= 'green'
    @type  ||= 'pear'
  end

  def initialize(params = {})
    params.each { |key,value| instance_variable_set("@#{key}", value) }
    set_defaults
    instance_variables.each {|var| self.class.send(:attr_accessor, var.to_s.delete('@'))}
  end

  def to_s
    instance_variables.inject("") {|vars, var| vars += "#{var}: #{instance_variable_get(var)}; "}
  end

end

puts Fruit.new
puts Fruit.new :color => 'red', :type => 'grape'  
puts Fruit.new :type => 'pomegranate'
puts Fruit.new :cost => 20.21
puts Fruit.new :foo => "bar"


f = Fruit.new :potato => "salad"
puts "f.cost.nil? #{f.cost.nil?}"

Which outputs:

@color: green; @type: pear; 
@color: red; @type: grape; 
@color: green; @type: pomegranate; 
@color: green; @type: pear; @cost: 20.21; 
@color: green; @type: pear; @foo: bar; 
f.cost.nil? true

Of course this wouldn't be a perfect solution for everything but it gives you some ideas on making your code more dynamic.

回答4:

I'd do it like this:

class Fruit
  attr_accessor :color, :type

  def initialize(args={})
    options = {:color => 'green', :type => 'pear'}.merge(args)

    self.color = options[:color]
    self.type  = options[:type]
  end
end

apple = Fruit.new(:color => 'red', :type => 'apple')

This way, you never have to worry about missing arguments--or their order--and you'll always have your default values right there. .merge will of course overwrite the default values if they're present.

回答5:

More simple way:

class Fruit
  attr_accessor :color, :type
  def initialize(color = 'green', type = 'pear')
    @color = color
    @type = type
  end
  def to_s
    "#{color} #{type}"
  end
end


puts Fruit.new # prints: green pear
puts Fruit.new('red','apple') # prints: red apple
puts Fruit.new(nil,'pomegranate') # prints: green pomegranate

回答6:

I like vonconrad's answer but would have a separate defaults method. Maybe it's not efficient in terms of lines of code, but it's more intention-revealing and involves less cognitive overhead, and less cognitive overhead means more efficient dev onboarding.

class Fruit
  attr_accessor :color, :type

  def initialize(args={})
    options = defaults.merge(args)

    @color = options.fetch(:color)
    @type  = options.fetch(:type)
  end

  def defaults
    {
      color: 'green',
      type:  'pear'
    }
  end
end

apple = Fruit.new(:color => 'red', :type => 'apple')

回答7:

Even more tasty syntactic sugar:

class Fruit
  attr_accessor :color, :type
  def initialize *args
    @color, @type = args 
  end
end

pear = Fruit.new 'green', :pear