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.
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 unnamedlink 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.
The order of inclusion is not significant and cannot affect conflicts.
External classes may be included if the interface to the language permits this; external Fortran and C classes may not be included.
Immutable (see unnamedlink) and reference classes cannot be mixed into a single class during inclusion. In the case of arrays (see unnamedlink), there cannot be include paths from reference types to AVAL or from immutable types to AREF i.e. reference types cannot include an immutable array portion and immutable classes cannot include a reference array portion.
There must be no cycle of classes such that each class includes the next, ignoring the values of any type parameters.
class A is include B; ... class B is include C; ... class C is include A; ...
If SAME occurs in included code it is interpreted as the eventual type of the class (as late as possible). We make use of this fact every time we include a create routine that returns SAME
class FOO is create:SAME is return new; end; end; class SON_OF_FOO is include FOO; -- Since create returns SAME, we have create:SON_OF_FOO; end; class GRANDSON_OF_FOO is include SON_OF_FOO; -- Now we have create:GRANDSON_OF_FOO; end; a ::= #GRANDSON_OF_FOO; -- Calls GRANDSON_OF_FOO::create:SAME, -- which returns a GRANDSON_OF_FOO.
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
All overloaded features must be renamed at the same time - there is no way to specify them individually.
A public routine can be made private by either a private include or by renaming the individual routine to be private
class MANAGER is include EMPLOYEE id->private id; -- Renames both reader and writer routines of the attribute 'id'
In a private include, renaming a particular feature has the effect of making just that one feature public. For instance
class MANAGER is private include EMPLOYEE name->name; -- only 'name' is made public
Iterator names may only be renamed as iterator names.
It is an error if there are no appropriate methods to rename in the included class.
Both a reader and a writer method must exist if 'readonly' is used in a renaming clause.
class I_INTERVAL is private attr first, size:INT; finish:INT is return first + size - 1; end; finish(fin:INT) is size := fin - first + 1; end; ... class LINE_SEGMENT is include I_INTERVAL finish->readonly finish; -- makes private finish(fin:INT) -- and leaves public finish:INT;
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
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 unnamedlink. 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