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)