1.2. Basic Concepts

Data structures in Sather are constructed from objects, each of which has a specific concrete type that determines the operations that may be performed on it. Abstract types specify a set of operations without providing an implementation and correspond to sets of concrete types. The implementation of concrete types is defined by textual units called classes; abstract types are specified by textual units called abstract classes. Sather programs consist of classes and abstract class specifications. Each Sather variable has a declared type which determines the types of objects it may hold.

Classes define the following features: attributes which make up the internal state of objects, shareds and constants which are shared by all objects of a type, and methods which may be either routines or iterators. Any features are by default public, but may be declared private to allow only the class in which it appears access to it. An attribute or shared may instead be declared readonly to allow only the class in which it appears to modify it. Accessor routines are automatically defined for reading or writing attributes, shareds, and constants. The set of non-private methods in a class defines the interface of the corresponding type. Method definitions consist of statements; for their construction expressions are used. There are special literal expressions for boolean, character, string, integer, and floating point objects.

Certain conditions are described as fatal errors. These conditions should never occur in correct programs and all implementations of Sather must be able to detect them. For efficiency reasons, however, implementations may provide the option of disabling checking for certain conditions.