Creating instances of a class

The diagram below illustrates what happens, when a constructor is invoked to create an instance of a class. The rectangles in the diagrams represent environment frames. Environment frames are linked with one another making up (together with the global environent) the total environment.

The label `$<$environment 1$>$' refers to the inherited environment and `$<$environment 2$>$' to the new environment frame created by the activation of a closure (call of a function). In our example `$<$environment 2$>$' has been created by calling the constructor `make-account'.

• The first environment frame created in our example represents the top level environment containing the constructor `make-account' and the variable `acc1' used later on to store the object returned by `make-account'. An arrow pointing from the second rectangle back to this environment frame symbolizes the linkage.

• The second environment frame belongs to the constructor containing its attributes (balance) and methods (get-balance, deposit, self).

There are two closures of special interest in our example illustrated by the black bullets:

• the constructor
  taking one argument: `balance' and having three methods. There is a line drawn from this closure (two bullets) to the second environment labelled `$<$environment 2$>$'. This line is to be interpretet as a pointer to the closure's activation record.
• the lambda-abstraction `self'
  representing the object returned expecting 1 argument: `msg'. This closure has not been invoked yet and therefore has only the inherited environment consisting of the environment frames already discussed.

Georg P. Loczewski 2004-03-05

Impressum und Datenschutz