An Error Using Records

Watch Out for This Error

Possible Error with Arrays of Records

• Watch out for a subtle error!


• Consider this code:

        e: Employee;
        elist: EmployeeList;
    begin
        ...
        e := elist(i);
        e.salary := e.salary + raise;

        print(e.salary);
        print(elist(i).salary);
        ...
    

• What gets printed?
• In Ada?
• In Java?


• Have we changed the Employee in the array?
• In Ada?
• In Java?

Solution

• Solution 1:

        e: Employee;
        elist: EmployeeList;
    begin
        ...
        e := elist(i);
        e.salary := e.salary + raise;
        elist(i) := e;

        // Both show a raise
        print(e.salary);
        print(elist(i).salary);
        ...
    

• Solution 2:

        elist: EmployeeList;
    begin
        ...
        elist(i).salary := elist(i).salary + raise;

        print(elist(i).salary);
        ...
    

Same Error Occurs with Nested Records

• Consider the line example:

    l: Line := ( (1, 2), (3, 4) );
    p: Pair := l.p2;
begin
    p.x := 99;
    put(l.p2.x);   // What is printed
    

• Now in Java (assuming the required constructors):

    Line l = new Line(new Pair(1, 2), new Pair(3, 4));
    Pair p = l.p2;
begin
    p.x := 99;
    S.o.p(l.p2.x);   // What is printed
    

Underlying this Error: Reference vs Value Semantics

• What's in the array?
• In Java, the array is an array of Employee references
• In Ada, the array is an array of Employee values


• What happens when we assign an element from the array to and Employee variable?
• In Java: the variable is assigned a copy of the reference
• In Ada: the variable is assigned a copy of the Employee


• What happens when the variable is changed
• In Java: the object referenced by the variable is changed, changing the object in the array
• In Ada: the object contained in the variable is changed; the Employee in the array is unchanged