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- PPPPEEEERRRRLLLLBBBBOOOOTTTT((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLBBBBOOOOTTTT((((1111))))
-
-
-
- NNNNAAAAMMMMEEEE
- perlbot - Bag'o Object Tricks (the BOT)
-
- DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN
- The following collection of tricks and hints is intended to
- whet curious appetites about such things as the use of
- instance variables and the mechanics of object and class
- relationships. The reader is encouraged to consult relevant
- textbooks for discussion of Object Oriented definitions and
- methodology. This is not intended as a tutorial for
- object-oriented programming or as a comprehensive guide to
- Perl's object oriented features, nor should it be construed
- as a style guide.
-
- The Perl motto still holds: There's more than one way to do
- it.
-
- OOOOOOOO SSSSCCCCAAAALLLLIIIINNNNGGGG TTTTIIIIPPPPSSSS
- 1 Do not attempt to verify the type of $self. That'll
- break if the class is inherited, when the type of $self
- is valid but its package isn't what you expect. See
- rule 5.
-
- 2 If an object-oriented (OO) or indirect-object (IO)
- syntax was used, then the object is probably the
- correct type and there's no need to become paranoid
- about it. Perl isn't a paranoid language anyway. If
- people subvert the OO or IO syntax then they probably
- know what they're doing and you should let them do it.
- See rule 1.
-
- 3 Use the two-argument form of _b_l_e_s_s(). Let a subclass
- use your constructor. See the section on _I_N_H_E_R_I_T_I_N_G _A
- _C_O_N_S_T_R_U_C_T_O_R.
-
- 4 The subclass is allowed to know things about its
- immediate superclass, the superclass is allowed to know
- nothing about a subclass.
-
- 5 Don't be trigger happy with inheritance. A "using",
- "containing", or "delegation" relationship (some sort
- of aggregation, at least) is often more appropriate.
- See the section on _O_B_J_E_C_T _R_E_L_A_T_I_O_N_S_H_I_P_S, the section on
- _U_S_I_N_G _R_E_L_A_T_I_O_N_S_H_I_P _W_I_T_H _S_D_B_M, and the section on
- _D_E_L_E_G_A_T_I_O_N.
-
- 6 The object is the namespace. Make package globals
- accessible via the object. This will remove the guess
- work about the symbol's home package. See the section
- on _C_L_A_S_S _C_O_N_T_E_X_T _A_N_D _T_H_E _O_B_J_E_C_T.
-
-
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- Page 1 (printed 10/23/98)
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-
-
-
- 7 IO syntax is certainly less noisy, but it is also prone
- to ambiguities that can cause difficult-to-find bugs.
- Allow people to use the sure-thing OO syntax, even if
- you don't like it.
-
- 8 Do not use function-call syntax on a method. You're
- going to be bitten someday. Someone might move that
- method into a superclass and your code will be broken.
- On top of that you're feeding the paranoia in rule 2.
-
- 9 Don't assume you know the home package of a method.
- You're making it difficult for someone to override that
- method. See the section on _T_H_I_N_K_I_N_G _O_F _C_O_D_E _R_E_U_S_E.
-
- IIIINNNNSSSSTTTTAAAANNNNCCCCEEEE VVVVAAAARRRRIIIIAAAABBBBLLLLEEEESSSS
- An anonymous array or anonymous hash can be used to hold
- instance variables. Named parameters are also demonstrated.
-
- package Foo;
-
- sub new {
- my $type = shift;
- my %params = @_;
- my $self = {};
- $self->{'High'} = $params{'High'};
- $self->{'Low'} = $params{'Low'};
- bless $self, $type;
- }
-
- package Bar;
-
- sub new {
- my $type = shift;
- my %params = @_;
- my $self = [];
- $self->[0] = $params{'Left'};
- $self->[1] = $params{'Right'};
- bless $self, $type;
- }
-
- package main;
-
- $a = Foo->new( 'High' => 42, 'Low' => 11 );
- print "High=$a->{'High'}\n";
- print "Low=$a->{'Low'}\n";
-
- $b = Bar->new( 'Left' => 78, 'Right' => 40 );
- print "Left=$b->[0]\n";
- print "Right=$b->[1]\n";
-
-
- SSSSCCCCAAAALLLLAAAARRRR IIIINNNNSSSSTTTTAAAANNNNCCCCEEEE VVVVAAAARRRRIIIIAAAABBBBLLLLEEEESSSS
-
-
-
- PPPPaaaaggggeeee 2222 ((((pppprrrriiiinnnntttteeeedddd 11110000////22223333////99998888))))
-
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- PPPPEEEERRRRLLLLBBBBOOOOTTTT((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLBBBBOOOOTTTT((((1111))))
-
-
-
- An anonymous scalar can be used when only one instance
- variable is needed.
-
- package Foo;
-
- sub new {
- my $type = shift;
- my $self;
- $self = shift;
- bless \$self, $type;
- }
-
- package main;
-
- $a = Foo->new( 42 );
- print "a=$$a\n";
-
-
- IIIINNNNSSSSTTTTAAAANNNNCCCCEEEE VVVVAAAARRRRIIIIAAAABBBBLLLLEEEE IIIINNNNHHHHEEEERRRRIIIITTTTAAAANNNNCCCCEEEE
- This example demonstrates how one might inherit instance
- variables from a superclass for inclusion in the new class.
- This requires calling the superclass's constructor and
- adding one's own instance variables to the new object.
-
- package Bar;
-
- sub new {
- my $type = shift;
- my $self = {};
- $self->{'buz'} = 42;
- bless $self, $type;
- }
-
- package Foo;
- @ISA = qw( Bar );
-
- sub new {
- my $type = shift;
- my $self = Bar->new;
- $self->{'biz'} = 11;
- bless $self, $type;
- }
-
- package main;
-
- $a = Foo->new;
- print "buz = ", $a->{'buz'}, "\n";
- print "biz = ", $a->{'biz'}, "\n";
-
-
- OOOOBBBBJJJJEEEECCCCTTTT RRRREEEELLLLAAAATTTTIIIIOOOONNNNSSSSHHHHIIIIPPPPSSSS
- The following demonstrates how one might implement
-
-
-
- Page 3 (printed 10/23/98)
-
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- PPPPEEEERRRRLLLLBBBBOOOOTTTT((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLBBBBOOOOTTTT((((1111))))
-
-
-
- "containing" and "using" relationships between objects.
-
- package Bar;
-
- sub new {
- my $type = shift;
- my $self = {};
- $self->{'buz'} = 42;
- bless $self, $type;
- }
-
- package Foo;
-
- sub new {
- my $type = shift;
- my $self = {};
- $self->{'Bar'} = Bar->new;
- $self->{'biz'} = 11;
- bless $self, $type;
- }
-
- package main;
-
- $a = Foo->new;
- print "buz = ", $a->{'Bar'}->{'buz'}, "\n";
- print "biz = ", $a->{'biz'}, "\n";
-
-
- OOOOVVVVEEEERRRRRRRRIIIIDDDDIIIINNNNGGGG SSSSUUUUPPPPEEEERRRRCCCCLLLLAAAASSSSSSSS MMMMEEEETTTTHHHHOOOODDDDSSSS
- The following example demonstrates how to override a
- superclass method and then call the overridden method. The
- SSSSUUUUPPPPEEEERRRR pseudo-class allows the programmer to call an
- overridden superclass method without actually knowing where
- that method is defined.
-
- package Buz;
- sub goo { print "here's the goo\n" }
-
- package Bar; @ISA = qw( Buz );
- sub google { print "google here\n" }
-
- package Baz;
- sub mumble { print "mumbling\n" }
-
- package Foo;
- @ISA = qw( Bar Baz );
-
-
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- Page 4 (printed 10/23/98)
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-
-
-
- sub new {
- my $type = shift;
- bless [], $type;
- }
- sub grr { print "grumble\n" }
- sub goo {
- my $self = shift;
- $self->SUPER::goo();
- }
- sub mumble {
- my $self = shift;
- $self->SUPER::mumble();
- }
- sub google {
- my $self = shift;
- $self->SUPER::google();
- }
-
- package main;
-
- $foo = Foo->new;
- $foo->mumble;
- $foo->grr;
- $foo->goo;
- $foo->google;
-
-
- UUUUSSSSIIIINNNNGGGG RRRREEEELLLLAAAATTTTIIIIOOOONNNNSSSSHHHHIIIIPPPP WWWWIIIITTTTHHHH SSSSDDDDBBBBMMMM
- This example demonstrates an interface for the SDBM class.
- This creates a "using" relationship between the SDBM class
- and the new class Mydbm.
-
- package Mydbm;
-
- require SDBM_File;
- require Tie::Hash;
- @ISA = qw( Tie::Hash );
-
-
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-
-
-
- sub TIEHASH {
- my $type = shift;
- my $ref = SDBM_File->new(@_);
- bless {'dbm' => $ref}, $type;
- }
- sub FETCH {
- my $self = shift;
- my $ref = $self->{'dbm'};
- $ref->FETCH(@_);
- }
- sub STORE {
- my $self = shift;
- if (defined $_[0]){
- my $ref = $self->{'dbm'};
- $ref->STORE(@_);
- } else {
- die "Cannot STORE an undefined key in Mydbm\n";
- }
- }
-
- package main;
- use Fcntl qw( O_RDWR O_CREAT );
-
- tie %foo, "Mydbm", "Sdbm", O_RDWR|O_CREAT, 0640;
- $foo{'bar'} = 123;
- print "foo-bar = $foo{'bar'}\n";
-
- tie %bar, "Mydbm", "Sdbm2", O_RDWR|O_CREAT, 0640;
- $bar{'Cathy'} = 456;
- print "bar-Cathy = $bar{'Cathy'}\n";
-
-
- TTTTHHHHIIIINNNNKKKKIIIINNNNGGGG OOOOFFFF CCCCOOOODDDDEEEE RRRREEEEUUUUSSSSEEEE
- One strength of Object-Oriented languages is the ease with
- which old code can use new code. The following examples
- will demonstrate first how one can hinder code reuse and
- then how one can promote code reuse.
-
- This first example illustrates a class which uses a fully-
- qualified method call to access the "private" method _B_A_Z().
- The second example will show that it is impossible to
- override the _B_A_Z() method.
-
- package FOO;
-
-
-
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- Page 6 (printed 10/23/98)
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-
-
-
- sub new {
- my $type = shift;
- bless {}, $type;
- }
- sub bar {
- my $self = shift;
- $self->FOO::private::BAZ;
- }
-
- package FOO::private;
-
- sub BAZ {
- print "in BAZ\n";
- }
-
- package main;
-
- $a = FOO->new;
- $a->bar;
-
- Now we try to override the _B_A_Z() method. We would like
- _F_O_O::_b_a_r() to call _G_O_O_P::_B_A_Z(), but this cannot happen
- because _F_O_O::_b_a_r() explicitly calls _F_O_O::_p_r_i_v_a_t_e::_B_A_Z().
-
- package FOO;
-
- sub new {
- my $type = shift;
- bless {}, $type;
- }
- sub bar {
- my $self = shift;
- $self->FOO::private::BAZ;
- }
-
- package FOO::private;
-
- sub BAZ {
- print "in BAZ\n";
- }
-
- package GOOP;
- @ISA = qw( FOO );
- sub new {
- my $type = shift;
- bless {}, $type;
- }
-
- sub BAZ {
- print "in GOOP::BAZ\n";
- }
-
-
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- Page 7 (printed 10/23/98)
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-
-
-
- package main;
-
- $a = GOOP->new;
- $a->bar;
-
- To create reusable code we must modify class FOO, flattening
- class FOO::private. The next example shows a reusable class
- FOO which allows the method _G_O_O_P::_B_A_Z() to be used in place
- of _F_O_O::_B_A_Z().
-
- package FOO;
-
- sub new {
- my $type = shift;
- bless {}, $type;
- }
- sub bar {
- my $self = shift;
- $self->BAZ;
- }
-
- sub BAZ {
- print "in BAZ\n";
- }
-
- package GOOP;
- @ISA = qw( FOO );
-
- sub new {
- my $type = shift;
- bless {}, $type;
- }
- sub BAZ {
- print "in GOOP::BAZ\n";
- }
-
- package main;
-
- $a = GOOP->new;
- $a->bar;
-
-
- CCCCLLLLAAAASSSSSSSS CCCCOOOONNNNTTTTEEEEXXXXTTTT AAAANNNNDDDD TTTTHHHHEEEE OOOOBBBBJJJJEEEECCCCTTTT
- Use the object to solve package and class context problems.
- Everything a method needs should be available via the object
- or should be passed as a parameter to the method.
-
- A class will sometimes have static or global data to be used
- by the methods. A subclass may want to override that data
- and replace it with new data. When this happens the
- superclass may not know how to find the new copy of the
- data.
-
-
-
- Page 8 (printed 10/23/98)
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-
-
-
- This problem can be solved by using the object to define the
- context of the method. Let the method look in the object
- for a reference to the data. The alternative is to force
- the method to go hunting for the data ("Is it in my class,
- or in a subclass? Which subclass?"), and this can be
- inconvenient and will lead to hackery. It is better just to
- let the object tell the method where that data is located.
-
- package Bar;
-
- %fizzle = ( 'Password' => 'XYZZY' );
-
- sub new {
- my $type = shift;
- my $self = {};
- $self->{'fizzle'} = \%fizzle;
- bless $self, $type;
- }
-
- sub enter {
- my $self = shift;
-
- # Don't try to guess if we should use %Bar::fizzle
- # or %Foo::fizzle. The object already knows which
- # we should use, so just ask it.
- #
- my $fizzle = $self->{'fizzle'};
-
- print "The word is ", $fizzle->{'Password'}, "\n";
- }
-
- package Foo;
- @ISA = qw( Bar );
-
- %fizzle = ( 'Password' => 'Rumple' );
-
- sub new {
- my $type = shift;
- my $self = Bar->new;
- $self->{'fizzle'} = \%fizzle;
- bless $self, $type;
- }
-
- package main;
-
- $a = Bar->new;
- $b = Foo->new;
- $a->enter;
- $b->enter;
-
-
- IIIINNNNHHHHEEEERRRRIIIITTTTIIIINNNNGGGG AAAA CCCCOOOONNNNSSSSTTTTRRRRUUUUCCCCTTTTOOOORRRR
-
-
-
- PPPPaaaaggggeeee 9999 ((((pppprrrriiiinnnntttteeeedddd 11110000////22223333////99998888))))
-
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- PPPPEEEERRRRLLLLBBBBOOOOTTTT((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLBBBBOOOOTTTT((((1111))))
-
-
-
- An inheritable constructor should use the second form of
- _b_l_e_s_s() which allows blessing directly into a specified
- class. Notice in this example that the object will be a BAR
- not a FOO, even though the constructor is in class FOO.
-
- package FOO;
-
- sub new {
- my $type = shift;
- my $self = {};
- bless $self, $type;
- }
-
- sub baz {
- print "in FOO::baz()\n";
- }
-
- package BAR;
- @ISA = qw(FOO);
-
- sub baz {
- print "in BAR::baz()\n";
- }
-
- package main;
-
- $a = BAR->new;
- $a->baz;
-
-
- DDDDEEEELLLLEEEEGGGGAAAATTTTIIIIOOOONNNN
- Some classes, such as SDBM_File, cannot be effectively
- subclassed because they create foreign objects. Such a
- class can be extended with some sort of aggregation
- technique such as the "using" relationship mentioned earlier
- or by delegation.
-
- The following example demonstrates delegation using an
- _A_U_T_O_L_O_A_D() function to perform message-forwarding. This
- will allow the Mydbm object to behave exactly like an
- SDBM_File object. The Mydbm class could now extend the
- behavior by adding custom _F_E_T_C_H() and _S_T_O_R_E() methods, if
- this is desired.
-
- package Mydbm;
-
- require SDBM_File;
- require Tie::Hash;
- @ISA = qw(Tie::Hash);
-
-
-
-
-
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- Page 10 (printed 10/23/98)
-
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- PPPPEEEERRRRLLLLBBBBOOOOTTTT((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLBBBBOOOOTTTT((((1111))))
-
-
-
- sub TIEHASH {
- my $type = shift;
- my $ref = SDBM_File->new(@_);
- bless {'delegate' => $ref};
- }
-
- sub AUTOLOAD {
- my $self = shift;
-
- # The Perl interpreter places the name of the
- # message in a variable called $AUTOLOAD.
-
- # DESTROY messages should never be propagated.
- return if $AUTOLOAD =~ /::DESTROY$/;
-
- # Remove the package name.
- $AUTOLOAD =~ s/^Mydbm:://;
-
- # Pass the message to the delegate.
- $self->{'delegate'}->$AUTOLOAD(@_);
- }
-
- package main;
- use Fcntl qw( O_RDWR O_CREAT );
-
- tie %foo, "Mydbm", "adbm", O_RDWR|O_CREAT, 0640;
- $foo{'bar'} = 123;
- print "foo-bar = $foo{'bar'}\n";
-
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