VDM-10 Language Manual

Chapter 14. Top-level Specification (VDM++ and VDM-RT)
| instance variable definitions
| synchronization definitions
| thread definitions ;
Semantics: Each class description has the following parts:
• A class header with the class name and an optional inheritance clause.
• An optional class body.
• A class tail.
The class name as given in the class header is the defining occurrence of the name of the
class. A class name is globally visible, i.e. visible in all other classes in the specification.
The class name in the class header must be the same as the class name in the class tail.
Furthermore, defining class names must be unique throughout the specification.
The (optional) class body may consist of:
• A set of value definitions (constants).
• A set of type definitions.
• A set of function definitions.
• A set of instance variable definitions describing the internal state of an object instantiated
from the class. State invariant expressions are encouraged but are not mandatory.
• A set of operation definitions that can act on the internal state.
• A set of the synchronization definitions, specified either in terms of permission predicates
or using mutex constraints.
• A set of thread definitions that describe the thread of control for active objects.
• A set of traces that are used to indicate the sequences of operation calls for which test
cases are desired to be produced automatically.
In general, all constructs defined within a class must have a unique name, e.g. it is not allowed
to define an operation and a type with the same name. However, it is possible to overload
function and operation names (i.e. it is possible to have two or more functions with the same
name and two or more operations with the same name) subject to the restriction that the
types of their input parameters should not overlap. That is, it should be possible using static
type checking alone to determine uniquely and unambiguously which function/operation
definition corresponds to each function/operation call. Note that this applies not only to
functions and operations defined in the local interface of a class but also to those inherited
from superclasses. Thus, for example, in a design involving multiple inheritance a class C
may inherit a function from a class A and a function with the same name from a class B and
all calls involving this function name must be resolvable in class C.
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