We will illustrate the points made above by using a simple example, which will be something of a running example to be extended in later chapters. We will start here by defining a class 'EMPLOYEE'. Please bear in mind that this example is used to illustrate various language features, not object-oriented design.
The class is composed of several attributes which hold the employee information. Various degrees of privacy are illustrated
class EMPLOYEE is private attr wage:INT; readonly attr name:STR; attr id:INT; const high_salary:INT := 40000; create(a_name:STR, a_id:INT, a_wage:INT):SAME is res ::= new; res.id := a_id; res.name := a_name; res.wage := a_wage; return(res); end; highly_paid:BOOL is return wage >= high_salary; end; end;
Note the use of the special type SAME as the return type of the create routine, which denotes the current class name. SAME changes to mean the including class when it is included, as will be explained in the next chapter on code inclusion.
The employee class may be exercised using the following main class.
class TESTEMP is main is john:EMPLOYEE := #EMPLOYEE("John",100,10000); peter:EMPLOYEE := #EMPLOYEE("Peter",3,10000); john.id := 100; -- Set the attr "id" in john to 100 #OUT + john.name + "\n"; -- Prints "John" #OUT + peter.id + "\n"; -- Prints "3" end; end;
Note that the following calls would be illegal:
#OUT + john.wage + "\n"; -- ILLEGAL! "wage" is private john.name := "martha"; -- ILLEGAL! "name" is readonly
A distinguished class must be specified when a Sather program is compiled (the default is to look for a class called MAIN). This class must define a routine named 'main'. When the program executes, an object of the specified type is created and 'main' is called on it.
To run the above example - type the code into a file emp.sa and then run the executable 'emp'
sacomp emp.sa -main TESTEMP -o emp
This generates the executable "emp", using the "main" routine in TESTEMP as its starting point. You can browse the resulting code by calling
sabrowse emp.sa -main TESTEMP