Provided by: libclass-contract-perl_1.14-8_all 
NAME
Class::Contract - Design-by-Contract OO in Perl.
VERSION
This document describes version 1.10 of Class::Contract, released February 9, 2001.
SYNOPSIS
package ClassName
use Class::Contract;
contract {
inherits 'BaseClass';
invar { ... };
attr 'data1';
attr 'data2' => HASH;
class attr 'shared' => SCALAR;
ctor 'new';
method 'methodname';
pre { ... };
failmsg 'Error message';
post { ... };
failmsg 'Error message';
impl { ... };
method 'nextmethod';
impl { ... };
class method 'sharedmeth';
impl { ... };
# etc.
};
DESCRIPTION
Background
Design-by-contract is a software engineering technique in which each module of a software
system specifies explicitly what input (or data or arguments) it requires, and what output
(or information or results) it guarantees to produce in response.
These specifications form the "clauses" of a contract between a module and the client
software that uses it. If the client software abides by the input requirements, the module
guarantees to produce the correct output. Hence by verifying these clauses at each
interaction with a module, the overall behaviour of the system can be confidently
predicted.
Design-by-contract reinforces the benefits of modular design techniques by inserting
explicit compile-time or run-time checks on a contract. These checks are most often found
in object-oriented languages and are typically implemented as pre-conditions and post-
conditions on methods, and invariants on classes.
Note that these features differ from simple verification statements such as the C "assert"
statement. Conditions and invariants are properties of a class, and are inherited by
derived classes.
An additional capacity that is often provided in design-by-contract systems is the ability
to selectively disable checking in production code. This allows the contractual testing to
be carried out during implementation, without impinging on the performance of the final
system.
Adding design-by-contract to Perl
The Class::Contract module provides a framework for specifying methods and attributes for
a class (much like the existing class definition modules Class::Struct,
Class::MethodMaker, and Class::Generate). Class::Contract allows both per-object and per-
class methods and attributes to be defined. Attributes may be scalar-, array-, hash-, or
object-based.
Class::Contract differs from other class-specification modules (except Class::Generate) in
that it also provides the ability to specify invariant conditions on classes, and pre- and
post-conditions on methods and attributes. All of these clauses are fully inheritable, and
may be selectively disabled. It differs from all other modules in that it has a cleaner,
simpler specification syntax, and -- more importantly -- it enforces encapsulation of
object attributes, thereby ensuring that the class contract cannot be subverted.
Defining classes
Class::Contract provides an explicit syntax for defining the attributes, methods, and
constructors of a class. The class itself is defined using the "contract" subroutine.
"contract" takes a single argument -- a subroutine reference or a block. That block is
executed once and the results used to construct and install the various components of the
class in the current package:
package Queue;
contract {
# specification of class Queue attributes and methods here
};
Defining attributes
Attributes are defined within the "contract" block via the "attr" subroutine. Attributes
must be given a name, and may also be given a type: "SCALAR", "ARRAY", "HASH", or a class
name:
contract {
attr 'last'; # Scalar attribute (by default)
attr 'lest' => SCALAR; # Scalar attribute
attr 'list' => ARRAY; # Array attribute
attr 'lost' => HASH; # Hash attribute
attr 'lust' => MyClass; # Object attribute
};
For each attribute so declared, Class::Contract creates an accessor -- a method that
returns a reference to the attribute in question. Code using these accessors might look
like this:
${$obj->last}++;
push @{$obj->list}, $newitem;
print $obj->lost->{'marbles'};
$obj->lust->after('technology stocks');
Attributes are normally object-specific, but it is also possible to define attributes that
are shared by all objects of a class. Class objects are specified by prefixing the call to
"attr" with a call to the "class" subroutine:
class Queue;
contract {
class attr 'obj_count';
};
The accessor for this shared attribute can now be called either as an object method:
print ${$obj->obj_count};
or as a class method:
print ${Queue->obj_count};
In order to ensure that the clauses of a class' contract (see below) are honoured, both
class and object attributes are only accessible via their accessors, and those accessors
may only be called within methods belonging to the same class hierarchy. Objects are
implemented as "flyweight scalars" in order to ensure this strict encapsulation is
preserved.
Defining methods
Methods are defined in much the same way as attributes. The "method" subroutine is used to
specify the name of a method, then the "impl" subroutine is used to provide an
implementation for it:
contract {
attr list => ARRAY;
method 'next';
impl { shift @{self->list} };
method 'enqueue';
impl { push @{self->list}, $_[1] };
};
"impl" takes a block (or a reference to a subroutine), which is used as the implementation
of the method named by the preceding "method" call. Within that block, the subroutine
"self" may be used to return a reference to the object on which the method was called.
Unlike, regular OO Perl, the object reference is not passed as the method's first
argument. (Note: this change occurred in version 1.10)
Like attributes, methods normally belong to -- and are accessed via -- a specific object.
To define methods that belong to the entire class, the "class" qualifier is once again
used:
contract {
class attr 'obj_count';
class method 'inc_count';
impl { ${self->obj_count}++ };
};
Note that the "self" subroutine can still be used -- within a class method it returns the
appropriate class name, rather than an object reference.
Defining constructors
Class::Contract requires constructors to be explicitly defined using the "ctor"
subroutine:
contract {
ctor 'new';
impl { @{self->list} = ( $_[0] ) }
};
Note that the implementation section of a constructor doesn't specify code to build or
bless the new object. That is taken care of automatically (in order to ensure the correct
"flyweight" implementation of the object).
Instead, the constructor implementation is invoked after the object has been created and
blessed into the class. Hence the implementation only needs to initialize the various
attributes of the "self" object. In addition, the return value of the implementation is
ignored: constructor calls always return a reference to the newly created object.
Any attribute that is not initialized by a constructor is automatically "default
initialized". By default, scalar attributes remain "undef", array and hash attributes are
initialized to an empty array or hash, and object attributes are initialized by having
their "new" constructor called (with no arguments). This is the only reasonable default
for object attributes, but it is usually advisable to initialize them explicitly in the
constructor.
It is also possible to define a "class constructor", which may be used to initialize class
attributes:
contract {
class attr 'obj_count';
class ctor;
impl { ${self->obj_count} = 0 };
};
The class constructor is invoked at the very end of the call to "contract" in which the
class is defined.
Note too that the class constructor does not require a name. It may, however, be given
one, so that it can be explicitly called again (as a class method) later in the program:
class MyClass;
contract {
class attr 'obj_count';
class ctor 'reset';
impl { ${self->obj_count} = 0 };
};
# and later...
MyClass->reset;
Defining destructors
Destructors are also explicitly defined under Class::Contract, using the "dtor"
subroutine:
contract {
dtor;
impl { print STDLOG "Another object died\n" }
};
As with the constructor, the implementation section of a destructor doesn't specify code
to clean up the "flyweight" implementation of the object. Class::Contract takes care of
that automatically.
Instead, the implementation is invoked before the object is deallocated, and may be used
to clean up any of the internal structure of the object (for example to break reference
cycles).
It is also possible to define a "class destructor", which may be used to clean up class
attributes:
contract {
class attr 'obj_count';
class dtor;
impl { print STDLOG "Total was ${self->obj_count}\n" };
};
The class destructor is invoked from an "END" block within Class::Contract (although the
implementation itself is a closure, so it executes in the namespace of the original
class).
Constraining class elements
As described so far, Class::Contract doesn't provide any features that differ greatly from
those of any other class definition module. But Class::Contract does have one significant
difference: it allows the class designer to specify "clauses" that implement and enforce a
contract on the class's interface.
Contract clauses are specified as labelled blocks of code, associated with a particular
class, method, or attribute definition.
Class invariants
Classes may be given invariants: clauses than must be satisfied at the end of any method
call that is invoked from outside the class itself. For example, to specify that a class's
object count attribute must never fall below zero:
contract {
invar { ${self->obj_count} >= 0 };
};
The block following "invar" is treated as if it were a class method that is automatically
invoked after every other method invocation. If the method returns false, "croak" is
invoked with the error message: 'Class invariant at %s failed' (where the '%s' is replaced
by the file and line number at which the invariant was defined).
This error message can be customized, using the "failmsg" subroutine:
contract {
invar { ${self->obj_count} >= 0 };
failmsg 'Anti-objects detected by invariant at %s';
};
Once again, the '%s' is replaced by the appropriate file name and line number. A "failmsg"
can be specified after other types of clause too (see below).
A class may have as many invariants as it requires, and they may be specified anywhere
throughout the the body of the "contract".
Attribute and method pre- and post-conditions
Pre- and post-conditions on methods and attributes are specified using the "pre" and
"post" subroutines respectively.
For attributes, pre-conditions are called before the attribute's accessor is invoked, and
post-conditions are called after the reference returned by the accessor is no longer
accessible. This is achieved by having the accessor return a tied scalar whose "DESTROY"
method invokes the post-condition.
Method pre-conditions are tested before their method's implementation is invoked; post-
conditions are tested after the implementation finishes (but before the method's result is
returned). Constructors are (by definition) class methods and may have pre- and post-
conditions, just like any other method.
Both types of condition clause receive the same argument list as the accessor or method
implementation that they constrain. Both are expected to return a false value if they
fail:
contract {
class attr 'obj_count';
post { ${&value} > 0 };
failmsg 'Anti-objects detected by %s';
method 'inc_count';
post { ${self->obj_count} < 1000000 };
failmsg 'Too many objects!';
impl { ${self->obj_count}++ };
};
Note that within the pre- and post-conditions of an attribute, the special "value"
subroutine returns a reference to the attribute itself, so that conditions can check
properties of the attribute they guard.
Methods and attributes may have as many distinct pre- and post-conditions as they require,
specified in any convenient order.
Checking state changes.
Post-conditions and invariants can access the previous state of an object or the class,
via the "old" subroutine. Within any post-condition or invariant, this subroutine returns
a reference to a copy of the object or class state, as it was just before the current
method or accessor was called.
For example, an "append" method might use "old" to verify the appropriate change in size
of an object:
contract {
method 'append';
post { @{self->queue} == @{old->queue} + @_ }
impl { push @{self->queue}, @_ };
};
Note that the implementation's return value is also available in the method's
post-condition(s) and the class's invariants, through the subroutine "value". In the above
example, the implementation of "append" returns the new size of the queue (i.e. what
"push" returns), so the post-condition could also be written:
contract {
method 'append';
post { ${&value} == @{old->queue} + @_ }
impl { push @{self->queue}, @_ };
};
Note that "value" will return a reference to a scalar or to an array, depending on the
context in which the method was originally called.
Clause control
Any type of clause may be declared optional:
contract {
optional invar { @{self->list} > 0 };
failmsg 'Empty queue detected at %s after call';
};
By default, optional clauses are still checked every time a method or accessor is invoked,
but they may also be switched off (and back on) at run-time, using the "check" method:
local $_ = 'Queue'; # Specify in $_ which class to disable
check my %contract => 0; # Disable optional checks for class Queue
This (de)activation is restricted to the scope of the hash that is passed as the first
argument to "check". In addition, the change only affects the class whose name is held in
the variable $_ at the time "check" is called. This makes it easy to (de)activate checks
for a series of classes:
check %contract => 0 for qw(Queue PriorityQueue DEQueue); # Turn off
check %contract => 1 for qw(Stack PriorityStack Heap); # Turn on
The special value '__ALL__' may also be used as a (pseudo-)class name:
check %contract => 0 for __ALL__;
This enables or disables checking on every class defined using Class::Contract. But note
that only clauses that were originally declared "optional" are affected by calls to
"check". Non-optional clauses are always checked.
Optional clauses are typically universally disabled in production code, so Class::Contract
provides a short-cut for this. If the module is imported with the single argument
'production', optional clauses are universally and irrevocably deactivated. In fact, the
"optional" subroutine is replaced by:
sub Class::Contract::optional {}
so that optional clauses impose no run-time overhead at all.
In production code, contract checking ought to be disabled completely, and the requisite
code optimized away. To do that, simply change:
use Class::Contract;
to
use Class::Contract::Production;
Inheritance
The semantics of class inheritance for Class::Contract classes differ in several respects
from those of normal object-oriented Perl.
To begin with, classes defined using Class::Contract have a static inheritance hierarchy.
The inheritance relationships of contracted classes are defined using the "inherits"
subroutine within the class's "contract" block:
package PriorityQueue;
contract {
inherits qw( Queue OrderedContainer );
};
That means that ancestor classes are fixed at compile-time (rather than being determined
at run-time by the @ISA array). Note that multiple inheritance is supported.
Method implementations are only inherited if they are not explicitly provided. As with
normal OO Perl, a method's implementation is inherited from the left-most ancestral class
that provides a method of the same name (though with Class::Contract, this is determined
at compile-time).
Constructors are a special case, however. Their "constructive" behaviour is always
specific to the current class, and hence involves no inheritance under any circumstances.
However, the "initialising" behaviour specified by a constructor's "impl" block is
inherited. In fact, the implementations of all base class constructors are called
automatically by the derived class constructor (in left-most, depth-first order), and
passed the same argument list as the invoked constructor. This behaviour is much more like
that of other OO programming languages (for example, Eiffel or C++).
Methods in a base class can also be declared as being abstract:
contract {
abstract method 'remove';
post { ${self->count} == ${old->count}-1 };
};
Abstract methods act like placeholders in an inheritance hierarchy. Specifically, they
have no implementation, existing only to reserve the name of a method and to associate
pre- and post-conditions with it.
An abstract method cannot be directly called (although its associated conditions may be).
If such a method is ever invoked, it immediately calls "croak". Therefore, the presence of
an abstract method in a base class requires the derived class to redefine that method, if
the derived class is to be usable. To ensure this, any constructor built by
Class::Contract will refuse to create objects belonging to classes with abstract methods.
Methods in a base class can also be declared as being private:
contract {
private method 'remove';
impl { pop @{self->queue} };
};
Private methods may only be invoked by the class or one of its descendants.
Inheritance and condition checking
Attribute accessors and object methods inherit all post-conditions of every ancestral
accessor or method of the same name. Objects and classes also inherit all invariants from
any ancestor classes. That is, methods accumulate all the post- and invariant checks that
their ancestors performed, as well as any new ones they define for themselves, and must
satisfy all of them in order to execute successfully.
Pre-conditions are handled slightly differently. The principles of design-by-contract
programming state that pre-conditions in derived classes can be no stronger than those in
base classes (and may well be weaker). In other words, a derived class must handle every
case that its base class handled, but may choose to handle other cases as well, by being
less demanding regarding its pre-conditions.
Meyers suggests an efficient way to achieve this relaxation of constraints without the
need for detailed logical analysis of pre-conditions. His solution is to allow a derived
class method or accessor to run if either the pre-conditions it inherits are satisfied or
its own pre-conditions are satisfied. This is precisely the semantics that Class::Contract
uses when checking pre-conditions in derived classes.
A complete example
The following code implements a PriorityStack class, in which elements pushed onto the
stack "sink" until they encounter an element with lower priority. Note the use of "old"
to check that object state has changed correctly, and the use of explicit dispatch (e.g.
"self->Stack::pop") to invoke inherited methods from the derived-class methods that
redefine them.
package PriorityStack;
use Class::Contract;
contract {
# Reuse existing implementation...
inherits 'Stack';
# Name the constructor (nothing special to do, so no implementation)
ctor 'new';
method 'push';
# Check that data to be added is okay...
pre { defined $_[0] };
failmsg 'Cannot push an undefined value';
pre { $_[1] > 0 };
failmsg 'Priority must be greater than zero';
# Check that push increases stack depth appropriately...
post { self->count == old->count+1 };
# Check that the right thing was left on top...
post { old->top->{'priority'} <= self->top->{'priority'} };
# Implementation reuses inherited methods: pop any higher
# priority entries, push the new entry, then re-bury it...
impl {
my ($newval, $priority) = @_[0,1];
my @betters;
unshift @betters, self->Stack::pop
while self->count
&& self->Stack::top->{'priority'} > $priority;
self->Stack::push( {'val'=>$newval, priority=>$priority} );
self->Stack::push( $_ ) foreach @betters;
};
method 'pop';
# Check that pop decreases stack depth appropriately...
post { self->count == old->count-1 };
# Reuse inherited method...
impl {
return unless self->count;
return self->Stack::pop->{'val'};
};
method 'top';
post { old->count == self->count }
impl {
return unless self->count;
return self->Stack::top->{'val'};
};
};
FUTURE WORK
Future work on Class::Contract will concentrate on three areas:
1. Improving the attribute accessor mechanism
Lvalue subroutines will be introduced in perl version 5.6. They will allow a return
value to be treated as an alias for the (scalar) argument of a "return" statement.
This will make it possible to write subroutines whose return value may be assigned to
(like the built-in "pos" and "substr" functions).
In the absence of this feature, Class::Contract accessors of all types return a
reference to their attribute, which then requires an explicit dereference:
${self->value} = $newval;
${self->access_count}++;
When this feature is available, accessors for scalar attributes will be able to return
the actual attribute itself as an lvalue. The above code would then become cleaner:
self->value = $newval;
self->access_count++;
2. Providing better software engineering tools.
Contracts make the consequences of inheritance harder to predict, since they
significantly increase the amount of ancestral behaviour (i.e. contract clauses) that
a class inherits.
Languages such as Eiffel provide useful tools to help the software engineer make sense
of this extra information. In particular, Eiffel provides two alternate ways of
inspecting a particular class -- flat form and short form.
"Flattening" a class produces an equivalent class definition without any inheritance.
That is, the class is modified by making explicit all the attributes, methods,
conditions, and invariants it inherits from other classes. This allows the designer to
see every feature a class possesses in one location.
"Shortening" a class, takes the existing class definition and removes all
implementation aspects of it -- that is, those that have no bearing on its public
interface. A shortened representation of a class therefore has all attribute
specifications and method implementations removed. Note that the two processes can be
concatenated: shortening a flattened class produces an explicit listing of its
complete public interface. Such a representation can be profitably used as a basis for
documenting the class.
It is envisaged that Class::Contract will eventually provide a mechanism to produce
equivalent class representations in Perl.
3. Offering better facilities for retrofitting contracts.
At present, adding contractual clauses to an existing class requires a major
restructuring of the original code. Clearly, if design-by-contract is to gain
popularity with Perl programmers, this transition cost must be minimized.
It is as yet unclear how this might be accomplished, but one possibility would be to
allow the implementation of certain parts of a Class::Contract class (perhaps even the
underlying object implementation itself) to be user-defined.
AUTHOR
Damian Conway (damian@conway.org)
MAINTAINER
C. Garrett Goebel (ggoebel@cpan.org)
BUGS
There are undoubtedly serious bugs lurking somewhere in code this funky :-) Bug reports
and other feedback are most welcome.
COPYRIGHT
Copyright (c) 1997-2000, Damian Conway. All Rights Reserved. This module is free
software. It may be used, redistributed and/or modified under the terms of the Perl
Artistic License
(see http://www.perl.com/perl/misc/Artistic.html)
Copyright (c) 2000-2001, C. Garrett Goebel. All Rights Reserved. This module is free
software. It may be used, redistributed and/or modified under the terms of the Perl
Artistic License
(see http://www.perl.com/perl/misc/Artistic.html)