如何在 ruby 中拥有多个构造函数？

Question

I want to have more than one constructor in Ruby. I know that I have to use self methods, but the thing is that I don't know how to implement them. For example:

def initialize(n1 = 0,n2 = 0,n3 = 0)
  @num1 = n1
  @num2 = n2
  @num3 = n3
end

def self.MyClass(num1, num3)
...
end

def self.MyClass(num2,num3)
 ...
end

So in the first case what I want to do is to only give a value to num1 and num3, not to num2. And in the second case I want to give a value only to num2 and num3, but not to num1. How can I do that?

Answer 1

Couldn't you achieve what you want by using keyword arguments:

class MyClass
  attr_reader :num1, :num2, :num3
  def initialize(n1: nil, n2: nil, n3: nil)
    if n2.nil?
      puts 'num2 is nil'
    end
    if n3.nil?
      puts 'num3 is nil'
    end
    @num1 = n1 || 0
    @num2 = n2 || 0
    @num3 = n3 || 0
  end
end

MyClass.new(n1: 1, n2: 3)
# num3 is nil
# => <MyClass:0x0... @num1=1, @num2=3, @num3=0>

MyClass.new(n1: 4, n3: 1)
# num2 is nil
# => <MyClass:0x0... @num1=4, @num2=0, @num3=1>

Keyword arguments are available since ruby 2.0. Google or see for instance here for more information regarding keyword arguments.

If you want to stay close to the MyClass() syntax, you could set it up like this:

class MyClass
  attr_reader :num1, :num2, :num3
  def initialize(n1 = 0, n2 = 0, n3 = 0)
    @num1 = n1
    @num2 = n2
    @num3 = n3
  end
  def self.call(n1: nil, n2: nil, n3: nil)
    if n2.nil?
      puts 'num2 is nil'
    end
    if n3.nil?
      puts 'num3 is nil'
    end
    new n1, n2, n3
  end
end

MyClass.(n1: 1, n2: 3)
# num3 is nil
# => <MyClass:0x0... @num1=1, @num2=3, @num3=nil>
MyClass.(n1: 4, n3: 1)
# num2 is nil
# => <MyClass:0x0... @num1=4, @num2=nil, @num3=1>

Note the '.' after the class name.

Yet another alternative is to use def self.[](...) instead of def self.call(...) and make your calls use MyClass[...] (no dot after class).

Answer 2

Ruby semantics about constructors aside (and Jörg's comments about that are precisely correct, as usual), you have said that you want to create a class where you give a value to different attributes in different instances of the class, and not to all of them in any given instance.

You don't have to use "self methods" (Ruby calls them "class methods") to do that, and you certainly don't need "more than one constructor" to do that. Just give a default value of nil to each of them. Pretty similar to what you already have:

def initialize(n1 = nil, n2 = nil, n3 = nil)
  @num1 = n1
  @num2 = n2
  @num3 = n3
end

Now, if you don't set them when you initialize the instance, their value will be nil , and if you need to, you can test for that in your methods.

You haven't really explained the problem that you're trying to solve (you haven't said why you want to have some instances have some attributes and other instances have other ones), so I can't really tell you whether that's a solution to it. But that's the easiest way to do what you say you want to do. If you explain why you want to do it, maybe we can come up with something better.

Answer 3

To dynamic your class constructor, I think you need to dynamic attr_accessor of your instance variables too.
Because we need to know which instance variables are initialized then we should declare attr_accessor inside initialize block.
I already check this and it works well, you can give it a try here:

class MultipleConstructors
  def initialize(args={})
    args.each do |k,v|
      instance_variable_set("@#{k}", v)
      self.class.send(:attr_accessor, *args.keys)
    end
  end
end

And then you can get or set any instance variable after initialize constructor.

m = MultipleConstructors.new(a: 1, c: 4)
m2 = MultipleConstructors.new(d: 'blah', e: 100)
m3 = MultipleConstructors.new(f: true)

or you can initialize any constructor you want...
ref: https://apidock.com/ruby/Module/attr_accessor