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Sather - A Language Manual - Chapter 4
Code Inclusion and Partial Classes


Object oriented languages usually support the derivation of new classes by inheriting from existing classes and modifying them. In Sather, the notion of inheritance is split into two separate concepts - type relations between classes and code relations between classes. In this chapter we will deal with the latter (and simpler) concept, that of reusing the code of one class in another. We refer to this as implementation inheritance or code inclusion.


4.1 Include Clauses

The re-use of code from one class in another class is defined by include clauses. These cause the incorporation of the implementation of the specified class, possibly undefining or renaming features with feature modifier clauses. The include clause may begin with the keyword 'private', in which case any unmodified included feature is made private.

include A a->b, c->, d->private d;
private include D e->readonly f;

Code inclusion permits the re-use of code from a parent concrete class in child concrete class . Including code is a purely syntactic operation in Sather. To help illustrate the following examples, we repeat the interface of EMPLOYEE.

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 ...

   highly_paid:BOOL is ...
end;

Routines that are redefined in the child class over-ride the corresponding routines in the included class. For instance suppose we define a new kind of EMPLOYEE - a MANAGER, who has a number of subordinates.

class MANAGER is
   include EMPLOYEE
      create->private oldcreate;
   -- Include employee code and rename create to 'oldcreate'

   readonly attr numsubordinates:INT; -- Public attribute

   create(aname:STR, aid:INT,awage:INT,nsubs:INT):SAME is
      -- Create a new manager with the name 'aname'
      -- and the id 'aid' and number of subordinates = 'nsubs'
      res ::= oldcreate(aname,aid,awage);
      res.numsubordinates := nsubs;
      return res;
   end;
end;

See A Running Example: Employees, section 2.9 for the EMPLOYEE definition. The create routine of the MANAGER class extends the EMPLOYEE create routine, which has been renamed to oldcreate (renaming is explained below) and is called by the new create routine.

Points to Note


4.1.1 Renaming

The include clause may selectively rename some of the included features. It is also possible to include a class and make all routines private, or some selectively public

class MANAGER is
   private include EMPLOYEE;
   -- All included features are made private

class MANAGER is
   private include EMPLOYEE  id->id;
   -- Makes the "id" routine public and others stay private

If no clause follows the '->' symbol, then the named features are not included in the class. This is equivalent to 'undefining' the routine or attribute.

class MANAGER is
   include EMPLOYEE id->;  -- Undefine the "id" routine
   attr id:MANAGER_ID;     -- This ' id' has a different type

Points to note


4.1.2 Multiple Inclusion

Sather permits inclusion of code from multiple source classes. The order of inclusion does not matter, but all conflicts between classes must be resolved by renaming. The example below shows a common idiom that is used in create routines to permit an including class to call the attribute initialization routines (by convention, this is frequently called 'init') of parent classes.

class PARENT1 is
   attr a:INT;

   create:SAME is
      return new.init;
   end;

   private init:SAME is
      a := 42;
      return self;
   end;
end;

In the above class, the attributes are initialized in the init routine. The use of such initialization routines is a good practice to avoid the problem of assigning attriutes to the "self" object in the create routine (which is void)

The other parent is similarly defined

class PARENT2 is
   attr c:INT;

   create:SAME is
      return new.init;
   end;

   private init:SAME is
      c := 72;
   end;
end;

In the child class, both parents are initialized by calling the initialization routines in the included classes

class DERIVED is
   include PARENT1 init-> PARENT_init;
   include PARENT2 init-> PARENT2_init; -- Rename init

   attr b:INT;

   create:SAME is
       -- a gets the value 42, b the value 99 and c the value 72
      return new.PARENT1_init.PARENT2_init.init
   end;

   private init:SAME is b := 99; return self;
end; -- class DERIVED


4.1.3 Resolving conflicts

Two methods which are included from different classes may not be able to coexist in the same interface. They are said to conflict with each other. For a full discussion of resolving conflicts, please see Conflicts during code inclusion, subsection 5.7.5. We have to first present the general overloading rule, before discussing when included signatures will conflict and what can then be done about it.

For now, we simply note that if we have signatures with the same name in two included classes, we can simply rename one of them away i.e.

class FOO is
   include BAR bar->;     -- eliminate this 'bar' routine
   include BAR2;          -- Use the 'bar' routine from BAR2


4.2 Partial Classes and Stub routines

Partial classes have no associated type and contain code that may only be included by other classes. Partial classes may not be instantiated: no routine calls from another class into a partial class are allowed, and no variables may be declared in another class of such a type.

A stub feature may only be present in a partial class. They have no body and are used to reserve a signature for redefinition by an including class. If code in a partial class contains calls to an unimplemented method, that method must be explicitly provided as a stub. The following class is a stub debugging class which checks on the value of a boolean and then prints out a debugging message (preceeding by the class name of 'self')

partial class DEBUG_MSG is
   stub debug:BOOL;

   debug_msg(msg:STR) is
   -- Prints out the type of "self" and a debugging message
      if not debug then
         -- Don't print out the message if the debug flag is false
         return;
      end;
      type_str:STR;
      -- Declared here since used in both branches of the if
      if ~void(self) then
         type_id:INT := SYS::tp(self);
         -- SYS::tp will not work if self is void!
         type_str:STR := SYS::str_for_tp(type_id);
      else
         type_str := "VOID SELF";
      end;
      #OUT + "Debug in class:" + type_str + " " + msg + "\n";
   end;
end;

This class can be used by some other class - for instance, a main routine that wants to print out all the arguments to main. The stub routine 'debug' must be filled in using either an attribute (a constant, in this case) or a routine.

class MAIN is
   include DEBUG_MSG;

   const debug:BOOL := true;           -- Fill in the stub.

   main(args:ARRAY{STR}) is
      loop
         arg:STR := args.elt!
         debug_msg("Argument:"+arg);    -- Print out the argument
      end;
   end;
end;

Points to note


4.2.1 Mixins: A Prompt Example

This code demonstrates the use of partial classes. Each MIXIN class provides a different way of prompting the user; each can be combined with COMPUTE to make a complete program. The stub in COMPUTE allows that class to be type checked without needing either mix-in class. Only COMPUTE_A and COMPUTE_B may actually be instantiated.

Now suppose that we have a 'COMPUTE' class that requires a prompt for some input data. It can leave the prompt routine as a stub, which will later be filled in by some prompt class

partial class COMPUTE is
   stub prompt_user:STR;

   main is
      res ::= prompt_user;
      -- Convert it to an integer and do something with it
      i:INT := res.cursor.get_int;
      #OUT + "I'm going to compute with this number, now:" + i + "\n";
      ...
   end;
end; -- partial class COMPUTE

We can now create different computation classes by mixing an arbitrary prompt style with the main computation partial class.

class COMPUTE_A is
   include COMPUTE;
   include PROMPT_STYLE_A;
end; -- class COMPUTE_A

class COMPUTE_B is
   include COMPUTE;
   include PROMPT_STYLE_B;
end; -- class COMPUTE_B


[back] [Abstract] [Copyright Notice] [Contents] [next]
Sather - A Language Manual
18 October 1999
B. Gomes, D. Stoutamire, B. Vaysman and H. Klawitter
Norbert Nemec nobbi@gnu.org