package overload;
+sub nil {}
+
sub OVERLOAD {
$package = shift;
my %arg = @_;
- my $hash = \%{$package . "::OVERLOAD"};
+ my ($sub, $fb);
+ $ {$package . "::OVERLOAD"}{dummy}++; # Register with magic by touching.
+ *{$package . "::()"} = \&nil; # Make it findable via fetchmethod.
for (keys %arg) {
- $hash->{$_} = $arg{$_};
+ if ($_ eq 'fallback') {
+ $fb = $arg{$_};
+ } else {
+ $sub = $arg{$_};
+ if (not ref $sub and $sub !~ /::/) {
+ $ {$package . "::(" . $_} = $sub;
+ $sub = \&nil;
+ }
+ #print STDERR "Setting `$ {'package'}::\cO$_' to \\&`$sub'.\n";
+ *{$package . "::(" . $_} = \&{ $sub };
+ }
}
+ ${$package . "::()"} = $fb; # Make it findable too (fallback only).
}
sub import {
sub unimport {
$package = (caller())[0];
- my $hash = \%{$package . "::OVERLOAD"};
+ ${$package . "::OVERLOAD"}{dummy}++; # Upgrade the table
shift;
for (@_) {
- delete $hash->{$_};
+ if ($_ eq 'fallback') {
+ undef $ {$package . "::()"};
+ } else {
+ delete $ {$package . "::"}{"(" . $_};
+ }
}
}
sub Overloaded {
- defined ($package = ref $_[0]) and defined %{$package . "::OVERLOAD"};
+ my $package = shift;
+ $package = ref $package if ref $package;
+ $package->can('()');
+}
+
+sub ov_method {
+ my $globref = shift;
+ return undef unless $globref;
+ my $sub = \&{*$globref};
+ return $sub if $sub ne \&nil;
+ return shift->can($ {*$globref});
}
sub OverloadedStringify {
- defined ($package = ref $_[0]) and
- defined %{$package . "::OVERLOAD"} and
- exists $ {$package . "::OVERLOAD"}{'""'} and
- defined &{$ {$package . "::OVERLOAD"}{'""'}};
+ my $package = shift;
+ $package = ref $package if ref $package;
+ #$package->can('(""')
+ ov_method mycan($package, '(""'), $package;
}
sub Method {
- defined ($package = ref $_[0]) and
- defined %{$package . "::OVERLOAD"} and
- $ {$package . "::OVERLOAD"}{$_[1]};
+ my $package = shift;
+ $package = ref $package if ref $package;
+ #my $meth = $package->can('(' . shift);
+ ov_method mycan($package, '(' . shift), $package;
+ #return $meth if $meth ne \&nil;
+ #return $ {*{$meth}};
}
sub AddrRef {
- $package = ref $_[0];
- bless $_[0], Overload::Fake; # Non-overloaded package
+ my $package = ref $_[0];
+ return "$_[0]" unless $package;
+ bless $_[0], overload::Fake; # Non-overloaded package
my $str = "$_[0]";
bless $_[0], $package; # Back
- $str;
+ $package . substr $str, index $str, '=';
}
sub StrVal {
- (OverloadedStringify) ?
- (AddrRef) :
+ (OverloadedStringify($_[0])) ?
+ (AddrRef(shift)) :
"$_[0]";
}
+sub mycan { # Real can would leave stubs.
+ my ($package, $meth) = @_;
+ return \*{$package . "::$meth"} if defined &{$package . "::$meth"};
+ my $p;
+ foreach $p (@{$package . "::ISA"}) {
+ my $out = mycan($p, $meth);
+ return $out if $out;
+ }
+ return undef;
+}
+
1;
__END__
=head1 NAME
-C<overload.pm> - Package for overloading perl operations
+overload - Package for overloading perl operations
=head1 SYNOPSIS
for the assignment form C<*=> of multiplication.
Arguments of this directive come in (key, value) pairs. Legal values
-are values legal inside a C<&{ ... }> call, so the name of a subroutine,
-a reference to a subroutine, or an anonymous subroutine will all work.
-Legal keys are listed below.
+are values legal inside a C<&{ ... }> call, so the name of a
+subroutine, a reference to a subroutine, or an anonymous subroutine
+will all work. Note that values specified as strings are
+interpreted as methods, not subroutines. Legal keys are listed below.
The subroutine C<add> will be called to execute C<$a+$b> if $a
is a reference to an object blessed into the package C<Number>, or if $a is
methods refer to methods triggered by an overloaded mathematical
operator.)
+Since overloading respects inheritance via the @ISA hierarchy, the
+above declaration would also trigger overloading of C<+> and C<*=> in
+all the packages which inherit from C<Number>.
+
=head2 Calling Conventions for Binary Operations
The functions specified in the C<use overload ...> directive are called
"&", "^", "|", "neg", "!", "~",
"C<neg>" stands for unary minus. If the method for C<neg> is not
-specified, it can be autogenerated using the method for subtraction.
+specified, it can be autogenerated using the method for
+subtraction. If the method for "C<!>" is not specified, it can be
+autogenerated using the methods for "C<bool>", or "C<\"\">", or "C<0+>".
=item * I<Increment and decrement>
"atan2", "cos", "sin", "exp", "abs", "log", "sqrt",
If C<abs> is unavailable, it can be autogenerated using methods
-for "<" or "<=>" combined with either unary minus or subtraction.
+for "E<lt>" or "E<lt>=E<gt>" combined with either unary minus or subtraction.
=item * I<Boolean, string and numeric conversion>
See L<"Fallback"> for an explanation of when a missing method can be autogenerated.
+=head2 Inheritance and overloading
+
+Inheritance interacts with overloading in two ways.
+
+=over
+
+=item Strings as values of C<use overload> directive
+
+If C<value> in
+
+ use overload key => value;
+
+is a string, it is interpreted as a method name.
+
+=item Overloading of an operation is inherited by derived classes
+
+Any class derived from an overloaded class is also overloaded. The
+set of overloaded methods is the union of overloaded methods of all
+the ancestors. If some method is overloaded in several ancestor, then
+which description will be used is decided by the usual inheritance
+rules:
+
+If C<A> inherits from C<B> and C<C> (in this order), C<B> overloads
+C<+> with C<\&D::plus_sub>, and C<C> overloads C<+> by C<"plus_meth">,
+then the subroutine C<D::plus_sub> will be called to implement
+operation C<+> for an object in package C<A>.
+
+=back
+
+Note that since the value of the C<fallback> key is not a subroutine,
+its inheritance is not governed by the above rules. In the current
+implementation, the value of C<fallback> in the first overloaded
+ancestor is used, but this is accidental and subject to change.
+
=head1 SPECIAL SYMBOLS FOR C<use overload>
Three keys are recognized by Perl that are not covered by the above
description.
-=head2 Last Resort
+=head2 Last Resort
C<"nomethod"> should be followed by a reference to a function of four
parameters. If defined, it is called when the overloading mechanism
=back
+B<Note.> C<"fallback"> inheritance via @ISA is not carved in stone
+yet, see L<"Inheritance and overloading">.
+
=head2 Copy Constructor
The value for C<"="> is a reference to a function with three
can be expressed in terms of subtraction.
+=item I<Negation>
+
+C<!> and C<not> can be expressed in terms of boolean conversion, or
+string or numerical conversion.
+
=item I<Concatenation>
can be expressed in terms of string conversion.
can be expressed in terms of its "spaceship" counterpart: either
C<E<lt>=E<gt>> or C<cmp>:
-
+
<, >, <=, >=, ==, != in terms of <=>
lt, gt, le, ge, eq, ne in terms of cmp
What follows is subject to change RSN.
-The table of methods for all operations is cached as magic in the
-symbol table hash for the package. The table is rechecked for changes due to
-C<use overload>, C<no overload>, and @ISA only during
-C<bless>ing; so if they are changed dynamically, you'll need an
-additional fake C<bless>ing to update the table.
-
-(Every SVish thing has a magic queue, and magic is an entry in that queue.
-This is how a single variable may participate in multiple forms of magic
-simultaneously. For instance, environment variables regularly have two
-forms at once: their %ENV magic and their taint magic.)
+The table of methods for all operations is cached in magic for the
+symbol table hash for the package. The cache is invalidated during
+processing of C<use overload>, C<no overload>, new function
+definitions, and changes in @ISA. However, this invalidation remains
+unprocessed until the next C<bless>ing into the package. Hence if you
+want to change overloading structure dynamically, you'll need an
+additional (fake) C<bless>ing to update the table.
+
+(Every SVish thing has a magic queue, and magic is an entry in that
+queue. This is how a single variable may participate in multiple
+forms of magic simultaneously. For instance, environment variables
+regularly have two forms at once: their %ENV magic and their taint
+magic. However, the magic which implements overloading is applied to
+the stashes, which are rarely used directly, thus should not slow down
+Perl.)
If an object belongs to a package using overload, it carries a special
flag. Thus the only speed penalty during arithmetic operations without
overloading is the checking of this flag.
-In fact, if C<use overload> is not present, there is almost no overhead for
-overloadable operations, so most programs should not suffer measurable
-performance penalties. A considerable effort was made to minimize the overhead
-when overload is used and the current operation is overloadable but
-the arguments in question do not belong to packages using overload. When
-in doubt, test your speed with C<use overload> and without it. So far there
-have been no reports of substantial speed degradation if Perl is compiled
-with optimization turned on.
-
-There is no size penalty for data if overload is not used.
+In fact, if C<use overload> is not present, there is almost no overhead
+for overloadable operations, so most programs should not suffer
+measurable performance penalties. A considerable effort was made to
+minimize the overhead when overload is used in some package, but the
+arguments in question do not belong to packages using overload. When
+in doubt, test your speed with C<use overload> and without it. So far
+there have been no reports of substantial speed degradation if Perl is
+compiled with optimization turned on.
+
+There is no size penalty for data if overload is not used. The only
+size penalty if overload is used in some package is that I<all> the
+packages acquire a magic during the next C<bless>ing into the
+package. This magic is three-words-long for packages without
+overloading, and carries the cache tabel if the package is overloaded.
Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is
carried out before any operation that can imply an assignment to the
=head1 AUTHOR
-Ilya Zakharevich <F<ilya@math.mps.ohio-state.edu>>.
+Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>.
=head1 DIAGNOSTICS
When Perl is run with the B<-Do> switch or its equivalent, overloading
induces diagnostic messages.
+Using the C<m> command of Perl debugger (see L<perldebug>) one can
+deduce which operations are overloaded (and which ancestor triggers
+this overloading). Say, if C<eq> is overloaded, then the method C<(eq>
+is shown by debugger. The method C<()> corresponds to the C<fallback>
+key (in fact a presence of this method shows that this package has
+overloading enabled, and it is what is used by the C<Overloaded>
+function).
+
=head1 BUGS
-Because it is used for overloading, the per-package associative array
-%OVERLOAD now has a special meaning in Perl.
+Because it is used for overloading, the per-package hash %OVERLOAD now
+has a special meaning in Perl. The symbol table is filled with names
+looking like line-noise.
-As shipped, mathemagical properties are not inherited via the @ISA tree.
+For the purpose of inheritance every overloaded package behaves as if
+C<fallback> is present (possibly undefined). This may create
+interesting effects if some package is not overloaded, but inherits
+from two overloaded packages.
This document is confusing.
https://perl5.git.perl.org / perl5.git / blobdiff