Konstructor

Multiple constructors in a idiomatic Ruby way. Small, fast, thread-safe, doesn't depend on other gems.

Use konstructor keyword to declare constructors additional to the default one:

class SomeClass
  konstructor
  def create(val)
    @val = val
  end 
  
  attr_reader :val
end

obj0 = SomeClass.new
obj0.val # nil

obj1 = SomeClass.create(3)
obj1.val # 3

It's similar to having overloaded constructors in other languages. Works with Ruby 1.9.3 or higher.

Installation

Via Gemfile:

gem 'konstructor' 

and then execute bundle.

You can also install it without Bundler:

$ gem install konstructor

If you are a gem author or just wish to manually include konstructor keyword in your classes only when you need it, see Manual include page.

Usage

In its simplest form konstructor declaration creates a constructor from the next method.

  konstructor
  def create
  end
 
  konstructor
  def recreate
  end

When method names are given, it creates constructors from those methods without affecting the next method.

  konstructor :create, :recreate
 
  def not_a_constructor
  end
 
  def create
  end
 
  def recreate
  end

Declaration with method names can be placed anywhere in class definition.

  def create
  end
  konstructor :create
  
  konstructor
  def recreate
  end

Several declarations may be used, all declarations add up without overwriting each other.

  def create
  end
   
  konstructor :recreate
  konstructor :create
   
  def recreate
  end

In all above cases SomeClass will have the default constructor and two additional ones.

 obj0 = SomeClass.new
 obj1 = SomeClass.create
 obj2 = SomeClass.recreate

If you decide to remove the default Ruby constructor for some reason, you can achieve the effect by marking it private with Ruby method private_class_method:

 class SomeClass
   private_class_method :new
 end   

Same as default constructor

Additional constructors work exactly the same way as the default one.

You can pass blocks to them.

  konstructor
  def create(val)
    @val = yield val
  end
  #...

obj = SomeClass.create(3) { |v| v*3 }
obj.val # 9

You can override them in subclasses and call super.

class SomeClass
  konstructor
  def create(val)
    @val = val
  end
  
  attr_reader :val
end

class SomeSubclass < SomeClass
  def create(val1, val2)
    super(val1 * val2)
  end
end

obj = SomeSubclass.create(2, 3)
obj.val # 6

Once method is declared as konstructor in hierarchy, it is always a constructor.

There are certain limitations to what can be declared as konstructor, see Limitations page for details.

Reusing default constructor

Sometimes you don't want to completely replace the default constructor, instead you just want to add additional processing before/after it.

class Person
  def initialize(name)
    @name = name
    @word_count = name.split(' ').size 
  end

  konstructor
  def from_two_names(first_name, second_name)
    initialize(first_name + ' ' + second_name)
    @name_count = 2 
  end  
  
  attr_reader :name, :word_count, :name_count 
end

Person.new('John Doe')
Person.from_two_names('John', 'Doe')

Using with other gems

Konstructor doesn't affect other gems depending on metaprogramming, such as rake, thor, contracts, etc.

For instnace, this is how Konstructor works with Contracts:

class SomeClass
  konstructor
  Contract Num => SomeClass
  def create(some_number)
    @number = some_number
  end
end    

obj0 = SomeClass.create(3)
obj1 = SomeClass.create("three") # raises contract exception

If you stumble upon a metaprogramming gem that conflicts with Konstructor, please open an issue.

Details

The default Ruby constructor is a pair consisting of public class method new and a private instance method initialize. To create an additional one konstructor marks given instance method as private and defines a corresponding public class method with the same name.

Performance

Using konstructor declaration has no runtime perfomance penalty, since all work is done during class definition and then it's just standard Ruby instance creation.

Cost of konstructor declaration at initial load time is roughly the same as declaring 5 properties with attr_accessor.

  attr_accessor :one, :two, :three, :four, :five
  
  # following declaration takes the same time as above declaration
  konstructor
  def create
  end

See Benchmarks page for details.

Dependencies and requirements

Konstructor doesn't depend on other gems. Requires Ruby 1.9.3 or higher. Works with JRuby.

Thread safety

Konstructor is thread-safe in both CRuby and JRuby, see Thread safety page for details.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/snovity/konstructor. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.

License

The gem is available as open source under the terms of the MIT License.