Components of a constructor

• input arguments
  The input arguments passed to a constructor are used to initialize the attributes. In our example the balance of the bank acount is set to 0.
• definition of the attributes
  Normally all attributes are defined in A++ by performing an abstraction in the first of the two possible forms, i.e. using the key word `define'.

  In our example however the input argument itself is used as an attribute. This is possible due to `lexical scope', which, as was mentioned earlier, is coupled in A++ just like in Scheme with indefinite extent. This means it is not a variable on the runtime stack like in C and many other languages but it belongs to the environment of the lambda-abstraction, which is preserved as long as the lambda-abstraction is still in use.

• definition of the methods
  The methods are of course lambda-abstractions and must be given a name using `define'.
• definition of the special method `self'
  This method is a very special lambda-abstraction, because it it is returned by the constructor as the closure representing the entire object. As closure being defined and contained in the constructor it has access to everything in the constructor, i.e. all attributes and methods. This is why `self' can serve as a dispatcher, receiving the messages sent to the object, interpreting them and mapping them to calls to the corresponding methods.
• return value
  The return value of the constructor is the lambda-abstraction `self' as described above.

Georg P. Loczewski 2004-03-05

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