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Abstract

This article presents a Super Constructor design pattern. It is equally suitable for C++ and Java.

The design pattern

This design pattern is applicable in cases where the constructor could return a null value or where you want to have a virtual constructor. To do this, the constructor needs to be renamed to a static method called for arguments sake: ctor. Here is the design pattern for null return values in action in the Java language:

class Foo
{
   int aProperty;

   private Foo(/* parameters */)
   {
      /* your code goes here */
   }

   public static Foo ctor(/* parameters */)
   {
      Foo returnValue = new Foo(/* arguments */);
      /* set properties here */
      returnValue.aProperty = 123;
      /* decide whether or not to return null here */
      if (/* code omitted */)
      {
         return returnValue;
      }
      else
      {
         return null;
      }
   }
}

The name ctor is used to remind the client that the method serves in the role of a constructor. The actual constructor is private so that it can only invoked indirectly by the ctor function. Here is the design pattern in action when you want to have a virtual constructor:

class Root
{
   private Root(/* parameters */)
   {
      /* your code goes here */
   }

   public static Root ctor(/* parameters */)
   {
      if (/* code omitted */)
      {
         return new Root(/* arguments */);
      }
      else if (/* code omitted */)
      {
         return new SubClass(/* arguments */);
      }
      else
      {
         return null;
      }
   }

   /* your code goes here */
}

class SubClass extends Root
{
   public SubClass(/* parameters */)
   {
      /* your code goes here */
   }

   /* your code goes here */
}

Here is an alternative implementation of the Super Constructor design pattern:

class Root
{
   public Root(/* parameters */)
   {
      /* your code goes here */
   }
}

class SubClass extends Root
{
   public static Root ctor(/* parameters */)
   {
      if (/* code omitted */)
      {
         return new Root(/* arguments */);
      }
      else if (/* code omitted */)
      {
         return new SubClass(/* arguments */);
      }
      else
      {
         return null;
      }
   }
}