Class::Struct::FIELDS - Combine Class::Struct, base and fields

NAME

Class::Struct::FIELDS - Combine Class::Struct, base and fields

SYNOPSIS

(This page documents Class::Struct::FIELDS v.1.1.)

    use Class::Struct::FIELDS;
            # declare struct, based on fields, explicit class name:
    struct (CLASS_NAME => { ELEMENT_NAME => ELEMENT_TYPE, ... });
    use Class::Struct::FIELDS;
            # declare struct, based on fields, explicit class name
            # with inheritance:
    struct (CLASS_NAME => [qw(BASE_CLASSES ...)],
            { ELEMENT_NAME => ELEMENT_TYPE, ... });
    package CLASS_NAME;
    use Class::Struct::FIELDS;
            # declare struct, based on fields, implicit class name:
    struct (ELEMENT_NAME => ELEMENT_TYPE, ...);
    package CLASS_NAME;
    use Class::Struct::FIELDS;
            # declare struct, based on fields, implicit class name
            # with inheritance:
    struct ([qw(BASE_CLASSES ...)], ELEMENT_NAME => ELEMENT_TYPE, ...);
    package MyObj;
    use Class::Struct::FIELDS;
            # declare struct with four types of elements:
    struct (s => '$', a => '@', h => '%', x => '&', c => 'My_Other_Class');
    $obj = new MyObj;               # constructor
                                    # scalar type accessor:
    $element_value = $obj->s;           # element value
    $obj->s ('new value');              # assign to element
                                    # array type accessor:
    $ary_ref = $obj->a;                 # reference to whole array
    $ary_element_value = $obj->a->[2];  # array element value
    $ary_element_value = $obj->a (2);   # same thing
    $obj->a->[2] = 'new value';         # assign to array element
    $obj->a (2, 'newer value');         # same thing
                                    # hash type accessor:
    $hash_ref = $obj->h;                # reference to whole hash
    $hash_element_value = $obj->h->{x}; # hash element value
    $hash_element_value = $obj->h (x);  # same thing
    $obj->h->{x} = 'new value';         # assign to hash element
    $obj->h (x, 'newer value');         # same thing
                                    # code type accessor:
    $code_ref = $obj->x;                # reference to code
    $obj->x->(...);                     # call code
    $obj->x (sub {...});                # assign to element
                                    # regexp type accessor:
    $regexp = $obj->r;                  # reference to code
    $string =~ m/$obj->r/;              # match regexp
    $obj->r (qr/ ... /);                # assign to element
                                    # class type accessor:
    $element_value = $obj->c;            # object reference
    $obj->c->method (...);               # call method of object
    $obj->c (My_Other_Class::->new);     # assign a new object

DESCRIPTION

Class::Struct::FIELDS exports a single function, struct. Given a list of element names and types, and optionally a class name and/or an array reference of base classes, struct creates a Perl 5 class that implements a ``struct-like'' data structure with inheritance.

The new class is given a constructor method, new, for creating struct objects.

Each element in the struct data has an accessor method, which is used to assign to the element and to fetch its value. The default accessor can be overridden by declaring a sub of the same name in the package. (See Example 2.)

Each element's type can be scalar, array, hash, code or class.

Differences from Class::Struct base and fields

Class::Struct::FIELDS is a combination of Class::Struct, base and fields.

Unlike Class::Struct, inheritance is explicitly supported, and there is better support for user overrides of constructed accessor methods. One result is that you may no longer use the array ([]) notation for indicating internal representation. Also, Class::Struct::FIELDS relies on fields for internal representation.

Also, Class::Struct::FIELDS supports code and regular expression elements. (Class::Struct handles code and regular expressions as scalars.)

Lastly, Class::Struct::FIELDS passes it's import list, if any, from the call to use Class::Struct::FIELDS ... to struct so that you may create new packages at compile-time.

Unlike fields, each element has a data type, and is automatically created at first access.

Calling use Class::Struct::FIELDS

You may call use Class::Struct::FIELDS just as with any module library:

    use Class::Struct::FIELDS;
    struct Bob => [];

However, if you try my Dog $spot syntax with this example:

    use Class::Struct::FIELDS;
    struct Bob => [];
    my Bob $bob = Bob::->new;

you will get a compile-time error:

    No such class Bob at <filename> line <number>, near "my Bob"
    Bareword "Bob::" refers to nonexistent package at <filename> line
    <number>.

since the compiler has not seen your class declarations yet until after the call to struct, by which time it has already seen your my declarations. Oops, too late. Instead, create the package for Bob during compilation:

    use Class::Struct::FIELDS qw(Bob);
    my Bob $bob = Bob::->new;

This compiles without error as import for Class::Struct::FIELDS calls struct for you if you have any arguments in the use statement. A more interesting example is:

    use Class::Struct::FIELDS Bob => { a => '$' };
    use Class::Struct::FIELDS Fred => [qw(Bob)];
    my Bob $bob = Bob::->new;
    my Fred $fred = Fred::->new;

The struct subroutine

The struct subroutine has three forms of parameter-list:

    struct (CLASS_NAME => { ELEMENT_LIST });
    struct (CLASS_NAME, ELEMENT_LIST);
    struct (ELEMENT_LIST);

The first form explicitly identifies the name of the class being created. The second form is equivalent. The second form assumes the current package name as the class name. The second and third forms are distinguished by the parity of the argument list: an odd number of arguments is taken to be of the second form.

Optionally, you may specify base classes with an array reference as the first non-class-name argument:

    struct (CLASS_NAME => [qw(BASE_CLASSES ...)], { ELEMENT_LIST });
    struct (CLASS_NAME => [qw(BASE_CLASSES ...)], ELEMENT_LIST);
    struct ([qw(BASE_CLASSES ...)], { ELEMENT_LIST });
    struct ([qw(BASE_CLASSES ...)], ELEMENT_LIST);

(Since there is no ambiguity between CLASS_NAME and ELEMENT_LIST with the interposing array reference, you may always make ELEMENT_LIST a list or a hash reference with this form.)

The class created by struct may be either a subclass or superclass of other classes. See the base manpage and the fields manpage for details.

The ELEMENT_LIST has the form

    NAME => TYPE, ...

Each name-type pair declares one element of the struct. Each element name will be usually be defined as an accessor method of the same name as the field, unless a method by that name is explicitly defined (called a ``user override'') by the caller prior to the use statement for Class::Struct::FIELDS. (See Replacing member access methods with user overrides.)

struct returns the name of the newly-constructed package.

Element Types and Accessor Methods

The five element types -- scalar, array, hash, code and class -- are represented by strings -- $, @, %, &, / and a class name.

The accessor method provided by struct for an element depends on the declared type of the element.

Scalar ($, \$ or *$)
The element is a scalar, and by default is initialized to undef (but see Initializing with new).

The accessor's argument, if any, is assigned to the element.

If the element type is $, the value of the element (after assignment) is returned. If the element type is \$ or *$, a reference to the element is returned.

Array (@, \@ or *@)
The element is an array, initialized by default to ().

With no argument, the accessor returns a reference to the element's whole array (whether or not the element was specified as @, \@ or *@).

With one or two arguments, the first argument is an index specifying one element of the array; the second argument, if present, is assigned to the array element. If the element type is @, the accessor returns the array element value. If the element type is \@ or *@, a reference to the array element is returned.

Hash (%, \% or *%)
The element is a hash, initialized by default to ().

With no argument, the accessor returns a reference to the element's whole hash (whether or not the element was specified as %, \% or *%).

With one or two arguments, the first argument is a key specifying one element of the hash; the second argument, if present, is assigned to the hash element. If the element type is %, the accessor returns the hash element value. If the element type is \% or *%, a reference to the hash element is returned.

Code (&, \& or *&)
The element is code, and by default is initialized to undef (but see Initializing with new).

The accessor's argument, if any, is assigned to the element.

If the element type is &, the value of the element (after assignment) is returned. If the element type is \& or *&, a reference to the element is returned. (It is unclear of what value this facility is. XXX)

Regexp (/, \/ or */)
If the element type is /, the value of the element (after assignment) is returned. If the element type is \/ or */, a reference to the element is returned. (It is unclear of what value this facility is. XXX)

Regular expressions really are special in that you create them with special syntax, not with a call to a constructor:

  $obj->r (qr/^$/); # fine
  $obj->r (Regexp->new); # WRONG

Class (Class_Name, \Class_Name or *Class_Name)
The element's value must be a reference blessed to the named class or to one of its subclasses. The element is initialized to the result of calling the new constructor of the named class.

The accessor's argument, if any, is assigned to the element. The accessor will croak if this is not an appropriate object reference.

If the element type does not start with a \ or *, the accessor returns the element value (after assignment). If the element type starts with a \ or *, a reference to the element itself is returned.

The class is automatically required for you so that, for example, you can safely write:

    struct MyObj {io => 'IO::Scalar'};

and access io immediately. The same applies for nested structs:

    BEGIN {
      struct Alice { when => '$' };
      struct Bob { who => 'Alice' };
    }
    my Bob $b = Bob::->new;
    $b->who->when ('what');

Note, however, the BEGIN block so that this example can use the my Dog $spot syntax for my Bob $b. Also, no actual import happens for the caller -- the automatic use is only for convenience in auto-constructing members, not magic. Another way to do this is:

    { package Bob; use Class::Struct::FIELDS; struct }
    my Bob $b = Bob::->new;

And of course the best way to do this is simply:

    use Class::Struct::FIELDS qw(Bob);
    my Bob $b = Bob::->new;

What about globs (*) and other funny types?
At present, Class::Struct::FIELDS does not support special notation for other intrinsic types. Use a scalar to hold a reference to globs and other unusual specimens, or wrap them in a class such as IO::Handle (globs). XXX

Initializing with new

struct always creates a constructor called new. That constructor may take a list of initializers for the various elements of the new struct.

Each initializer is a pair of values: element name => value. The initializer value for a scalar element is just a scalar value. The initializer for an array element is an array reference. The initializer for a hash is a hash reference. The initializer for code is a code reference.

The initializer for a class element is also a hash reference, and the contents of that hash are passed to the element's own constructor.

new tries to be as clever as possible in deducing what type of object to construct. All of these are valid:

    use Class::Struct::FIELDS qw(Bob);
    my Bob $b = Bob::->new; # good style
    my Bob $b2 = $b->new; # works fine
    my Bob $b3 = &Bob::new; # if you insist
    my Bob $b4 = Bob::new (apple => 3, banana => 'four'); # WRONG!

The last case doesn't behave as hoped for: new tries to construct an object of package apple (and hopefully fails, unless you actually have a package named apple), not an object of package Bob.

See Example 3 below for an example of initialization.

Initializing with init

You may also use init as a constructor to assign initial values to new objects. (In fact, this is the preferred method.) struct will see to it that you have a ready object to work with, and pass you any arguments used in the call to new:

  sub init {
    my MyObj $self = shift;
    @self->a->[0..3] = (a..d);
    return $self;
  }

It is essential that you return an object from init, as this is returned to the caller of new. You may return a different object if you wish, but this would be rather uncommon.

First, new arranges for any constructor argument list to be processed first before calling init.

Second, new arranges to call init for base classes, calling them in bottom-up order, before calling init. This is so that ancestors may construct an object before descendents.

There is no corresponding facility for DESTROY. XXX

Replacing member access methods with user overrides

You might want to create custom access methods, or user overrides. The most straight forward way to do this and still retain string and warnings is:

    use strict;
    use warnings;
    sub Bob::ff ($;$$); # declare sub so Class::Struct::FIELDS can see
    use Class::Struct::FIELDS Bob => { ff => '$' };
    sub Bob::ff ($;$$) {
      my Bob $self = shift;
      &some_other_sub (@_);
    }

If you do not declare the user override prior to the use statement, a warning is issued if the warning flag (-w) is set.

Notice that we changed the default sub signature for ff from ($;$) to ($;$$). Normally, this might generate a warning if we redefine the sub, but declaring the sub ahead of time keeps strict and warnings happy. You might prefer this construction:

    { package Bob; }
    sub Bob::ff ($;$$) {
      my Bob $self = shift;
      &some_other_sub (@_);
    }
    use Class::Struct::FIELDS Bob => { ff => '$' };

You might still want the advantages of the the constructed accessor methods, even with user overrides (for example, checking that an assigned value is the right type or package). Class::Struct::FIELDS constructs the accessor with a special name, so that you may use it yourself in the user override. That special name is the regular field name prepended by a double underscore, __. You can access these so:

    use strict;
    use warnings;
    sub Bob::ff ($;$); # declare sub so Class::Struct::FIELDS can see
    sub Bob::gg ($;$); # declare sub so Class::Struct::FIELDS can see
    use Class::Struct::FIELDS Bob => { ff => '$', gg => '$' };
    # This example is identical to having no user override.
    sub Bob::ff ($;$) {
      my Bob $self = shift;
      $self->__ff (@_);
    }
    # This example illustrates a workaround for v5.6.0.
    sub Bob::gg ($;$) {
      # This silliness is due to a bug in 5.6.0: it thinks you can't
      # fiddle with @_ if you've given it a prototype.  XXX
      my @args = @_;
      $args[1] *= 2 if @args == 2 and defined $args[1];
      @_ = @args;
      goto &Bob::__gg;
    }

Private fields

Fields starting with a leading underscore, _, are private: they are still valid fields, but Class::Struct::FIELDS does not create subroutines to access them. Instead, you should access them the usual way for hash members:

    $self->{_private_key}; # ok
    $self->_private_key; # Compilation error

See the fields manpage for more details.

Extra magic: auto-stringify

If there exists a subroutine named as_string at the time you invoke struct (or, equivalently, during the call to use), then Class::Struct::FIELDS will glue that into auto-stringification with overload for you.

EXAMPLES

Example 1
Giving a struct element a class type that is also a struct is how structs are nested. Here, timeval represents a time (seconds and microseconds), and rusage has two elements, each of which is of type timeval. 
    use Class::Struct::FIELDS;
    struct (rusage => {
      ru_utime => timeval,  # seconds
      ru_stime => timeval,  # microseconds
    });
    struct (timeval => {
      tv_secs  => '$',
      tv_usecs => '$',
    });
        # create an object:
    my $t = new rusage;
        # $t->ru_utime and $t->ru_stime are objects of type timeval.
        # set $t->ru_utime to 100.0 sec and $t->ru_stime to 5.0 sec.
    $t->ru_utime->tv_secs (100);
    $t->ru_utime->tv_usecs (0);
    $t->ru_stime->tv_secs (5);
    $t->ru_stime->tv_usecs (0);

Example 2
An accessor function can be redefined in order to provide additional checking of values, etc. Here, we want the count element always to be nonnegative, so we redefine the count accessor accordingly. 
    package MyObj;
    use Class::Struct::FIELDS;
    # declare the struct
    struct (MyObj => {count => '$', stuff => '%'});
    # override the default accessor method for 'count'
    sub count {
      my MyObj $self = shift;
      if (@_) {
        die 'count must be nonnegative' if $_[0] < 0;
        $self->{count} = shift;
        warn "Too many args to count" if @_;
      }
      return $self->{count};
    }
    package main;
    $x = new MyObj;
    print "\$x->count (5) = ", $x->count (5), "\n";
                            # prints '$x->count (5) = 5'
    print "\$x->count = ", $x->count, "\n";
                            # prints '$x->count = 5'
    print "\$x->count (-5) = ", $x->count (-5), "\n";
                            # dies due to negative argument!

Example 3
The constructor of a generated class can be passed a list of element=>value pairs, with which to initialize the struct. If no initializer is specified for a particular element, its default initialization is performed instead. Initializers for non-existent elements are silently ignored.

Note that the initializer for a nested struct is specified as an anonymous hash of initializers, which is passed on to the nested struct's constructor.

    use Class::Struct::FIELDS;
    struct Breed =>
    {
      name  => '$',
      cross => '$',
    };
    struct Cat =>
    {
      name     => '$',
      kittens  => '@',
      markings => '%',
      breed    => 'Breed',
    };
    my $cat = Cat->new
      (name     => 'Socks',
       kittens  => ['Monica', 'Kenneth'],
       markings => { socks => 1, blaze => "white" },
       breed    => { name => 'short-hair', cross => 1 });
    print "Once a cat called ", $cat->name, "\n";
    print "(which was a ", $cat->breed->name, ")\n";
    print "had two kittens: ", join(' and ', @{$cat->kittens}), "\n";

Example 4
Class::Struct::FIELDS has a very elegant idiom for creating inheritance trees: 
    use Class::Struct::FIELDS Fred => [];
    use Class::Struct::FIELDS Barney => [qw(Fred)];
    use Class::Struct::FIELDS Wilma => [qw(Barney)],
      { aa => '@',
        bb => 'IO::Scalar' };

That's all the code it takes!

EXPORTS

Class::Struct::FIELDS export struct for backwards-compatibility with Class::Struct.

DIAGNOSTICS

The following are diagnostics generated by Class::Struct::Fields. Items marked ``(W)'' are non-fatal (invoke Carp::carp); those marked ``(F)'' are fatal (invoke Carp::croak).

'struct' usage error
(F) The caller failed to read the documentation for Class::Struct::FIELDS and follow the advice therein.

Accessor '%s' exists in package '%s' hides method in base class
(W) There is already a subroutine, with the name of one of the accessors, located in a base class of the given package. You should consider renaming the field with the given name.

Method '%s' exists in package '%s' overrides accessor
(W) There is already a subroutine, with the name of one of the accessors, located in the given package. You may have intended this, however, if defining your own custom accessors.

Method 'new' already exists in package '%s'
(W) There is already a 'new' subroutine located in the given package. As long as the caveats for defining your own new are followed, this warning is harmless; otherwise your objects may not be properly initialized.

Initializer for '%s' must be %s reference
(F) At runtime, the caller tried to assign the wrong type of argument to the element. An example which triggers this message: 
    use Class::Struct::FIELDS Bob => { ary => '@' };
    my $b = Bob::->new;
    $b->ary ({hash => 'reference'}); # croaks

The last statement will croak with the message, ``Initializer for 'ary' must be ARRAY reference''.

Initializer for '%s' must be %s object
(F) At runtime, the caller tried to assign the wrong class of argument to the element. An example which triggers this message: 
    use Class::Struct::FIELDS Bob => { mary => 'Mary' };
    use Class::Struct::FIELDS qw(Fred); # NOT inherit from Mary
    my $b = Bob::->new;
    $b->ary (Fred::->new); # croaks

The last statement will croak with the message, ``Initializer for 'aa' must be Mary object''.

BUGS AND CAVEATS

Please see the TODO list.

GIANT MAN-EATING HOLE: due to bugs in lvalue subs in 5.6.0 (try running some under the debugger), I had to disable the obvious syntax:

    use Class::Struct::FIELDS Bob => { s => '$' };
    my Bob $b = Bob::->new;
    $b->s = 3;

and provide the clumsier:

    use Class::Struct::FIELDS Bob => { s => '$' };
    my Bob $b = Bob::->new;
    $b->s (3);

Some of these constructs work fine as long as you don't try to debug the generated code.

CREDITS

Dean Roehrich, Jim Miner <jfm@winternet.com> and Dr. Damian Conway <damian@conway.org> wrote the original Class::Struct which inspired this module and provided much of its documentation.

AUTHOR

B. K. Oxley (binkley) <binkley@bigfoot.com>

Copyright (c) 2000 B. K. Oxley (binkley). All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

SEE ALSO

the Class::Contract manpage
Class::Contract is an extension module by Damian Conway for writing in a design-by-contract object-oriented style. It has many of the features of Class::Struct::FIELDS, and many more besides.

the Class::Struct manpage
Class::Struct is a standard module for creating simple, uninherited data structures.

the base manpage
base is a standard pragma for establishing IS-A relationships with base classes at compile time.

the fields manpage
fields is a standard pragma for imbuing your class with efficient pseudo-hashes for data members.

the overload manpage
overload is a standard pragma for overloading Perl syntax with your own subroutines.

 Class::Struct::FIELDS - Combine Class::Struct, base and fields