Konstructor

Konstructor is a small gem that gives you multiple constructors in Ruby.

Use konstructor keyword to define constructors additional to the defaul 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.

Installation

Via Gemfile:

gem 'konstructor' 

and then execute bundle.

You can also install it without Bundler:

$ gem install konstructor

If you wish to manually include Konstructor in your classes only when you need it, see Manual include page.

Usage

In simplest form konstructor 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_constructor
 end

 def create
 end

 def recreate
 end

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

  def create
  end
  konstructor :create

  konstructor
  def recreate
  end

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

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

Same as default constructor

Additional constructors work exactly the same way as built-in Ruby constructor.

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 a marked as konstructor in hierarchy, it is always a konstructor.

Methods inherited from superclasses can't become konstructors in subclasses. To achieve the effect, define a new method, mark it as konstructor and call the inherited one.

Reserved names

Using reserved method names new and initialize for additional constructor declaration will raise an error:

konstructor
def initialize # raises Konstructor::ReservedNameError
end

or

konstructor
def new # raises Konstructor::ReservedNameError
end

Defining konstructors in Modules

Modules can't have konstructors. Use ActiveSupport::Concern and define konstructor in included block.

Using with other gems

Konstructor doesn't affect other gems, including those that depend on metaprogramming, such as rake, thor, contracts, etc.

For instnace, this is how Konstructor works with contracts gem:

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

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

Removing default constructor

You can effectively remove default Ruby constructor by marking it as private:

class SomeClass
  private_class_method :new
end   

Performance

Konstructor does all its work when class is being defined. Once class has been defined, it's just standard Ruby instance creation. Therefore, there is no runtime performance penalty.

Konstructor doesn't depend on other gems.

Thread safety

Konstructor is thread safe.

Details

Ruby constructor is a pair consisting of public factory method defined on a class and a private instance method. Therefore, to achieve its goal konstructor marks instance method as private and defines a corresponding public class method with the same name.

You can check if certain instance method name has been declared as constructor or is a default constructor by running.

Konstructor.is?(SomeClass, :initialize) # true
Konstructor.is?(SomeClass, :create) # true
Konstructor.is?(SomeClass, :recreate) # true
Konstructor.is?(SomeClass, :something_else) # false

It will return true even if no such constructor has been defined yet. Like:

class SomeClass
  konstructor :create
end

Konstructor body may be supplied in subclasses.

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.