VDM-10 Language Manual

Chapter 6. Expressions
new expression = ‘new’, name, ‘(’, [ expression list ], ‘)’ ;
Semantics: The new expression has the form:
✞
new classname(e1, e2, ..., en)
✡✝
✆
An object can be created (also called instantiated) from its class description using a new
expression. The effect of a new expression is that a ‘new’, unique object as described in class
classname is created. The value of the new expression is a reference to the new object.
If the new expression is invoked with no parameters, an object is created in which all instance
variables take their “default” values (i.e. the values defined by their initialisation conditions).
With parameters, the new expression represents a constructor (see Section 12.1) and creates
customised instances (i.e. where the instance variables may take values which are different
from their default values).
Examples: Suppose we have a class called Queue and that default instances of Queue are empty.
Suppose also that this class contains a constructor (which will also be called Queue) which
takes a single parameter which is a list of values representing an arbitrary starting queue.
Then we can create default instances of Queue in which the actual queue is empty using the
expression
✞
✡✝
new Queue()
and an instance of Queue in which the actual queue is, say, e1, e2, e3 using the expression
✞
✡✝
new Queue([e1, e2, e3])
Using the class Tree defined on page 28 we create new Tree instances to construct nodes:
✞
mk_node(new Tree(), x, new Tree())
✡✝
✆
✆
✆
6.14 The Self Expression (VDM++ and VDM-RT)
Syntax: expression = . . .
| self expression ;
self expression = ‘self’ ;
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