5.6. The typecase statement

It is sometimes necessary to bypass the abstraction and make use of information about the actual type of the object to perform a particular action. Given a variable of an abstract type, we might like to make use of the actual type of the object it refers to, in order to determine whether it either has a particular implementation or supports other abstractions.

The typecase statement provides us with the ability to make use of the actual type of an object held by a variable of an abstract type.
a:$OB := 5;
... some other code...

typecase a
when INT then                -- 'a' is of type INT in this branch
   #OUT + "Integer result: " + a;
when FLT then                -- 'a' is of type FLT in this branch
   #OUT + "Real result: " + a;
when $STR then               -- 'a' is $STR and supports '.str'
   #OUT + "Other printable result: " + a.str;
   #OUT + "Non printable result";

The typecase must act on a local variable or an argument of a method.On execution, each successive type specifier is tested for being a supertype of the type of the object held by the variable. The statement list following the first matching type specifier is executed and control passes to the statement following the typecase.

Points to note

Typecase Example

For instance, suppose we want to know the total number of subordinates in an array of general employees.
peter ::= #EMPLOYEE("Peter",1);      -- Name = "Peter", id = 1
paul  ::= #MANAGER("Paul",12,10);    -- id = 12,10 subordinates
mary  ::= #MANAGER("Mary",15,11);    -- id = 15,11 subordinates
employees: ARRAY{$EMPLOYEE} := |peter,paul,mary|;
totalsubs: INT := 0;
loop employee:$EMPLOYEE := employees.elt!; -- yields array elements
   typecase employee
   when MANAGER then
      totalsubs := totalsubs + employee.numsubordinates;
#OUT + "Number of subordinates: " + totalsubs + "\n";

Within each branch of the typecase, the variable has the type of that branch (or a more restrictive type, if the declared type of the variable is a subtype of the type of that branch).