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overload(3p)     Perl Programmers Reference Guide    overload(3p)


NAME
       overload - Package for overloading perl operations

SYNOPSIS
           package SomeThing;

           use overload
               '+' => \&myadd,
               '-' => \&mysub;
               # etc
           ...

           package main;
           $a = new SomeThing 57;
           $b=5+$a;
           ...
           if (overload::Overloaded $b) {...}
           ...
           $strval = overload::StrVal $b;

DESCRIPTION
       Declaration of overloaded functions

       The compilation directive

           package Number;
           use overload
               "+" => \&add,
               "*=" => "muas";

       declares function Number::add() for addition, and method
       muas() in the "class" "Number" (or one of its base
       classes) for the assignment form "*=" of multiplication.

       Arguments of this directive come in (key, value) pairs.
       Legal values are values legal inside a "&{ ... }" 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 sub-
       routines.  Legal keys are listed below.

       The subroutine "add" will be called to execute "$a+$b" if
       $a is a reference to an object blessed into the package
       "Number", or if $a is not an object from a package with
       defined mathemagic addition, but $b is a reference to a
       "Number".  It can also be called in other situations, like
       "$a+=7", or "$a++".  See "MAGIC AUTOGENERATION".  (Math-
       emagical methods refer to methods triggered by an over-
       loaded mathematical operator.)

       Since overloading respects inheritance via the @ISA hier-
       archy, the above declaration would also trigger overload-
       ing of "+" and "*=" in all the packages which inherit from
       "Number".



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overload(3p)     Perl Programmers Reference Guide    overload(3p)


       Calling Conventions for Binary Operations

       The functions specified in the "use overload ..." direc-
       tive are called with three (in one particular case with
       four, see "Last Resort") arguments.  If the corresponding
       operation is binary, then the first two arguments are the
       two arguments of the operation.  However, due to general
       object calling conventions, the first argument should
       always be an object in the package, so in the situation of
       "7+$a", the order of the arguments is interchanged.  It
       probably does not matter when implementing the addition
       method, but whether the arguments are reversed is vital to
       the subtraction method.  The method can query this infor-
       mation by examining the third argument, which can take
       three different values:

       FALSE  the order of arguments is as in the current opera-
              tion.

       TRUE   the arguments are reversed.

       "undef"
              the current operation is an assignment variant (as
              in "$a+=7"), but the usual function is called
              instead.  This additional information can be used
              to generate some optimizations.  Compare "Calling
              Conventions for Mutators".

       Calling Conventions for Unary Operations

       Unary operation are considered binary operations with the
       second argument being "undef".  Thus the functions that
       overloads "{"++"}" is called with arguments
       "($a,undef,'')" when $a++ is executed.

       Calling Conventions for Mutators

       Two types of mutators have different calling conventions:

       "++" and "--"
           The routines which implement these operators are
           expected to actually mutate their arguments.  So,
           assuming that $obj is a reference to a number,

             sub incr { my $n = $ {$_[0]}; ++$n; $_[0] = bless \$n}

           is an appropriate implementation of overloaded "++".
           Note that

             sub incr { ++$ {$_[0]} ; shift }

           is OK if used with preincrement and with postincre-
           ment. (In the case of postincrement a copying will be
           performed, see "Copy Constructor".)



perl v5.8.5                 2002-11-06                          2





overload(3p)     Perl Programmers Reference Guide    overload(3p)


       "x=" and other assignment versions
           There is nothing special about these methods.  They
           may change the value of their arguments, and may leave
           it as is.  The result is going to be assigned to the
           value in the left-hand-side if different from this
           value.

           This allows for the same method to be used as over-
           loaded "+=" and "+".  Note that this is allowed, but
           not recommended, since by the semantic of "Fallback"
           Perl will call the method for "+" anyway, if "+=" is
           not overloaded.

       Warning.  Due to the presence of assignment versions of
       operations, routines which may be called in assignment
       context may create self-referential structures.  Currently
       Perl will not free self-referential structures until
       cycles are "explicitly" broken.  You may get problems when
       traversing your structures too.

       Say,

         use overload '+' => sub { bless [ \$_[0], \$_[1] ] };

       is asking for trouble, since for code "$obj += $foo" the
       subroutine is called as "$obj = add($obj, $foo, undef)",
       or "$obj = [\$obj, \$foo]".  If using such a subroutine is
       an important optimization, one can overload "+=" explic-
       itly by a non-"optimized" version, or switch to non-opti-
       mized version if "not defined $_[2]" (see "Calling Conven-
       tions for Binary Operations").

       Even if no explicit assignment-variants of operators are
       present in the script, they may be generated by the opti-
       mizer.  Say, ",$obj," or ',' . $obj . ',' may be both
       optimized to

         my $tmp = ',' . $obj;    $tmp .= ',';

       Overloadable Operations

       The following symbols can be specified in "use overload"
       directive:

       * Arithmetic operations
                "+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=",
                "**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=",

            For these operations a substituted non-assignment
            variant can be called if the assignment variant is
            not available.  Methods for operations "+", "-",
            "+=", and "-=" can be called to automatically gener-
            ate increment and decrement methods.  The operation
            "-" can be used to autogenerate missing methods for



perl v5.8.5                 2002-11-06                          3





overload(3p)     Perl Programmers Reference Guide    overload(3p)


            unary minus or "abs".

            See "MAGIC AUTOGENERATION", "Calling Conventions for
            Mutators" and "Calling Conventions for Binary Opera-
            tions") for details of these substitutions.

       * Comparison operations
                "<",  "<=", ">",  ">=", "==", "!=", "<=>",
                "lt", "le", "gt", "ge", "eq", "ne", "cmp",

            If the corresponding "spaceship" variant is avail-
            able, it can be used to substitute for the missing
            operation.  During "sort"ing arrays, "cmp" is used to
            compare values subject to "use overload".

       * Bit operations
                "&", "^", "|", "neg", "!", "~",

            "neg" stands for unary minus.  If the method for
            "neg" is not specified, it can be autogenerated using
            the method for subtraction. If the method for "!" is
            not specified, it can be autogenerated using the
            methods for "bool", or "", or "0+".

       * Increment and decrement
                "++", "--",

            If undefined, addition and subtraction methods can be
            used instead.  These operations are called both in
            prefix and postfix form.

       * Transcendental functions
                "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", "int"

            If "abs" is unavailable, it can be autogenerated
            using methods for "<" or "<=>" combined with either
            unary minus or subtraction.

            Note that traditionally the Perl function int rounds
            to 0, thus for floating-point-like types one should
            follow the same semantic.  If "int" is unavailable,
            it can be autogenerated using the overloading of
            "0+".

       * Boolean, string and numeric conversion
                'bool', '""', '0+',

            If one or two of these operations are not overloaded,
            the remaining ones can be used instead.  "bool" is
            used in the flow control operators (like "while") and
            for the ternary "?:" operation.  These functions can
            return any arbitrary Perl value.  If the correspond-
            ing operation for this value is overloaded too, that
            operation will be called again with this value.



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overload(3p)     Perl Programmers Reference Guide    overload(3p)


            As a special case if the overload returns the object
            itself then it will be used directly. An overloaded
            conversion returning the object is probably a bug,
            because you're likely to get something that looks
            like "YourPackage=HASH(0x8172b34)".

       * Iteration
                "<>"

            If not overloaded, the argument will be converted to
            a filehandle or glob (which may require a stringifi-
            cation).  The same overloading happens both for the
            read-filehandle syntax "<$var>" and globbing syntax
            "<${var}>".

            BUGS Even in list context, the iterator is currently
            called only once and with scalar context.

       * Dereferencing
                '${}', '@{}', '%{}', '&{}', '*{}'.

            If not overloaded, the argument will be dereferenced
            as is, thus should be of correct type.  These func-
            tions should return a reference of correct type, or
            another object with overloaded dereferencing.

            As a special case if the overload returns the object
            itself then it will be used directly (provided it is
            the correct type).

            The dereference operators must be specified explic-
            itly they will not be passed to "nomethod".

       * Special
                "nomethod", "fallback", "=",

            see "SPECIAL SYMBOLS FOR "use overload"".

       See "Fallback" for an explanation of when a missing method
       can be autogenerated.

       A computer-readable form of the above table is available
       in the hash %overload::ops, with values being space-sepa-
       rated lists of names:













perl v5.8.5                 2002-11-06                          5





overload(3p)     Perl Programmers Reference Guide    overload(3p)


        with_assign      => '+ - * / % ** << >> x .',
        assign           => '+= -= *= /= %= **= <<= >>= x= .=',
        num_comparison   => '< <= > >= == !=',
        '3way_comparison'=> '<=> cmp',
        str_comparison   => 'lt le gt ge eq ne',
        binary           => '& | ^',
        unary            => 'neg ! ~',
        mutators         => '++ --',
        func             => 'atan2 cos sin exp abs log sqrt',
        conversion       => 'bool "" 0+',
        iterators        => '<>',
        dereferencing    => '${} @{} %{} &{} *{}',
        special          => 'nomethod fallback ='

       Inheritance and overloading

       Inheritance interacts with overloading in two ways.

       Strings as values of "use overload" directive
           If "value" in

             use overload key => value;

           is a string, it is interpreted as a method name.

       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 "A" inherits from "B" and "C" (in this order), "B"
           overloads "+" with "\&D::plus_sub", and "C" overloads
           "+" by "plus_meth", then the subroutine "D::plus_sub"
           will be called to implement operation "+" for an
           object in package "A".

       Note that since the value of the "fallback" key is not a
       subroutine, its inheritance is not governed by the above
       rules.  In the current implementation, the value of "fall-
       back" in the first overloaded ancestor is used, but this
       is accidental and subject to change.

SPECIAL SYMBOLS FOR "use overload"
       Three keys are recognized by Perl that are not covered by
       the above description.

       Last Resort

       "nomethod" should be followed by a reference to a function
       of four parameters.  If defined, it is called when the



perl v5.8.5                 2002-11-06                          6





overload(3p)     Perl Programmers Reference Guide    overload(3p)


       overloading mechanism cannot find a method for some opera-
       tion.  The first three arguments of this function coincide
       with the arguments for the corresponding method if it were
       found, the fourth argument is the symbol corresponding to
       the missing method.  If several methods are tried, the
       last one is used.  Say, "1-$a" can be equivalent to

               &nomethodMethod($a,1,1,"-")

       if the pair "nomethod" => "nomethodMethod" was specified
       in the "use overload" directive.

       The "nomethod" mechanism is not used for the dereference
       operators ( ${} @{} %{} &{} *{} ).

       If some operation cannot be resolved, and there is no
       function assigned to "nomethod", then an exception will be
       raised via die()-- unless "fallback" was specified as a
       key in "use overload" directive.

       Fallback

       The key "fallback" governs what to do if a method for a
       particular operation is not found.  Three different cases
       are possible depending on the value of "fallback":

       * "undef"       Perl tries to use a substituted method
                       (see "MAGIC AUTOGENERATION").  If this
                       fails, it then tries to calls "nomethod"
                       value; if missing, an exception will be
                       raised.

       * TRUE          The same as for the "undef" value, but no
                       exception is raised.  Instead, it silently
                       reverts to what it would have done were
                       there no "use overload" present.

       * defined, but FALSE
                       No autogeneration is tried.  Perl tries to
                       call "nomethod" value, and if this is
                       missing, raises an exception.

       Note. "fallback" inheritance via @ISA is not carved in
       stone yet, see "Inheritance and overloading".

       Copy Constructor

       The value for "=" is a reference to a function with three
       arguments, i.e., it looks like the other values in "use
       overload". However, it does not overload the Perl assign-
       ment operator. This would go against Camel hair.

       This operation is called in the situations when a mutator
       is applied to a reference that shares its object with some



perl v5.8.5                 2002-11-06                          7





overload(3p)     Perl Programmers Reference Guide    overload(3p)


       other reference, such as

               $a=$b;
               ++$a;

       To make this change $a and not change $b, a copy of $$a is
       made, and $a is assigned a reference to this new object.
       This operation is done during execution of the "++$a", and
       not during the assignment, (so before the increment $$a
       coincides with $$b).  This is only done if "++" is
       expressed via a method for '++' or '+=' (or "nomethod").
       Note that if this operation is expressed via '+' a nonmu-
       tator, i.e., as in

               $a=$b;
               $a=$a+1;

       then $a does not reference a new copy of $$a, since $$a
       does not appear as lvalue when the above code is executed.

       If the copy constructor is required during the execution
       of some mutator, but a method for '=' was not specified,
       it can be autogenerated as a string copy if the object is
       a plain scalar.

       Example
            The actually executed code for

                    $a=$b;
                    Something else which does not modify $a or $b....
                    ++$a;

            may be

                    $a=$b;
                    Something else which does not modify $a or $b....
                    $a = $a->clone(undef,"");
                    $a->incr(undef,"");

            if $b was mathemagical, and '++' was overloaded with
            "\&incr", '=' was overloaded with "\&clone".

       Same behaviour is triggered by "$b = $a++", which is con-
       sider a synonym for "$b = $a; ++$a".

MAGIC AUTOGENERATION
       If a method for an operation is not found, and the value
       for  "fallback" is TRUE or undefined, Perl tries to auto-
       generate a substitute method for the missing operation
       based on the defined operations.  Autogenerated method
       substitutions are possible for the following operations:

       Assignment forms of arithmetic operations
                       "$a+=$b" can use the method for "+" if the



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overload(3p)     Perl Programmers Reference Guide    overload(3p)


                       method for "+=" is not defined.

       Conversion operations
                       String, numeric, and boolean conversion
                       are calculated in terms of one another if
                       not all of them are defined.

       Increment and decrement
                       The "++$a" operation can be expressed in
                       terms of "$a+=1" or "$a+1", and "$a--" in
                       terms of "$a-=1" and "$a-1".

       "abs($a)"       can be expressed in terms of "$a<0" and
                       "-$a" (or "0-$a").

       Unary minus     can be expressed in terms of subtraction.

       Negation        "!" and "not" can be expressed in terms of
                       boolean conversion, or string or numerical
                       conversion.

       Concatenation   can be expressed in terms of string con-
                       version.

       Comparison operations
                       can be expressed in terms of its "space-
                       ship" counterpart: either "<=>" or "cmp":

                           <, >, <=, >=, ==, !=        in terms of <=>
                           lt, gt, le, ge, eq, ne      in terms of cmp

       Iterator
                           <>                          in terms of builtin operations

       Dereferencing
                           ${} @{} %{} &{} *{}         in terms of builtin operations

       Copy operator   can be expressed in terms of an assignment
                       to the dereferenced value, if this value
                       is a scalar and not a reference.

Losing overloading
       The restriction for the comparison operation is that even
       if, for example, `"cmp"' should return a blessed refer-
       ence, the autogenerated `"lt"' function will produce only
       a standard logical value based on the numerical value of
       the result of `"cmp"'.  In particular, a working numeric
       conversion is needed in this case (possibly expressed in
       terms of other conversions).

       Similarly, ".="  and "x=" operators lose their mathemagi-
       cal properties if the string conversion substitution is
       applied.




perl v5.8.5                 2002-11-06                          9





overload(3p)     Perl Programmers Reference Guide    overload(3p)


       When you chop() a mathemagical object it is promoted to a
       string and its mathemagical properties are lost.  The same
       can happen with other operations as well.

Run-time Overloading
       Since all "use" directives are executed at compile-time,
       the only way to change overloading during run-time is to

           eval 'use overload "+" => \&addmethod';

       You can also use

           eval 'no overload "+", "--", "<="';

       though the use of these constructs during run-time is
       questionable.

Public functions
       Package "overload.pm" provides the following public func-
       tions:

       overload::StrVal(arg)
            Gives string value of "arg" as in absence of
            stringify overloading.

       overload::Overloaded(arg)
            Returns true if "arg" is subject to overloading of
            some operations.

       overload::Method(obj,op)
            Returns "undef" or a reference to the method that
            implements "op".

Overloading constants
       For some application Perl parser mangles constants too
       much.  It is possible to hook into this process via over-
       load::constant() and overload::remove_constant() func-
       tions.

       These functions take a hash as an argument.  The recog-
       nized keys of this hash are

       integer to overload integer constants,

       float   to overload floating point constants,

       binary  to overload octal and hexadecimal constants,

       q       to overload "q"-quoted strings, constant pieces of
               "qq"- and "qx"-quoted strings and here-documents,

       qr      to overload constant pieces of regular expres-
               sions.




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overload(3p)     Perl Programmers Reference Guide    overload(3p)


       The corresponding values are references to functions which
       take three arguments: the first one is the initial string
       form of the constant, the second one is how Perl inter-
       prets this constant, the third one is how the constant is
       used.  Note that the initial string form does not contain
       string delimiters, and has backslashes in backslash-delim-
       iter combinations stripped (thus the value of delimiter is
       not relevant for processing of this string).  The return
       value of this function is how this constant is going to be
       interpreted by Perl.  The third argument is undefined
       unless for overloaded "q"- and "qr"- constants, it is "q"
       in single-quote context (comes from strings, regular
       expressions, and single-quote HERE documents), it is "tr"
       for arguments of "tr"/"y" operators, it is "s" for right-
       hand side of "s"-operator, and it is "qq" otherwise.

       Since an expression "ab$cd,," is just a shortcut for 'ab'
       . $cd . ',,', it is expected that overloaded constant
       strings are equipped with reasonable overloaded catenation
       operator, otherwise absurd results will result.  Simi-
       larly, negative numbers are considered as negations of
       positive constants.

       Note that it is probably meaningless to call the functions
       overload::constant() and overload::remove_constant() from
       anywhere but import() and unimport() methods.  From these
       methods they may be called as

               sub import {
                 shift;
                 return unless @_;
                 die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant';
                 overload::constant integer => sub {Math::BigInt->new(shift)};
               }

       BUGS Currently overloaded-ness of constants does not prop-
       agate into "eval '...'".

IMPLEMENTATION
       What follows is subject to change RSN.

       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 "use overload", "no over-
       load", new function definitions, and changes in @ISA. How-
       ever, this invalidation remains unprocessed until the next
       "bless"ing into the package. Hence if you want to change
       overloading structure dynamically, you'll need an addi-
       tional (fake) "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



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overload(3p)     Perl Programmers Reference Guide    overload(3p)


       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 car-
       ries a special flag.  Thus the only speed penalty during
       arithmetic operations without overloading is the checking
       of this flag.

       In fact, if "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 "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 all the packages acquire a magic during the next
       "bless"ing into the package. This magic is three-words-
       long for packages without overloading, and carries the
       cache table if the package is overloaded.

       Copying ("$a=$b") is shallow; however, a one-level-deep
       copying is carried out before any operation that can imply
       an assignment to the object $a (or $b) refers to, like
       "$a++".  You can override this behavior by defining your
       own copy constructor (see "Copy Constructor").

       It is expected that arguments to methods that are not
       explicitly supposed to be changed are constant (but this
       is not enforced).

Metaphor clash
       One may wonder why the semantic of overloaded "=" is so
       counter intuitive.  If it looks counter intuitive to you,
       you are subject to a metaphor clash.

       Here is a Perl object metaphor:

         object is a reference to blessed data

       and an arithmetic metaphor:

         object is a thing by itself.

       The main problem of overloading "=" is the fact that these
       metaphors imply different actions on the assignment "$a =
       $b" if $a and $b are objects.  Perl-think implies that $a



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overload(3p)     Perl Programmers Reference Guide    overload(3p)


       becomes a reference to whatever $b was referencing.
       Arithmetic-think implies that the value of "object" $a is
       changed to become the value of the object $b, preserving
       the fact that $a and $b are separate entities.

       The difference is not relevant in the absence of mutators.
       After a Perl-way assignment an operation which mutates the
       data referenced by $a would change the data referenced by
       $b too.  Effectively, after "$a = $b" values of $a and $b
       become indistinguishable.

       On the other hand, anyone who has used algebraic notation
       knows the expressive power of the arithmetic metaphor.
       Overloading works hard to enable this metaphor while pre-
       serving the Perlian way as far as possible.  Since it is
       not possible to freely mix two contradicting metaphors,
       overloading allows the arithmetic way to write things as
       far as all the mutators are called via overloaded access
       only.  The way it is done is described in "Copy Construc-
       tor".

       If some mutator methods are directly applied to the over-
       loaded values, one may need to explicitly unlink other
       values which references the same value:

           $a = new Data 23;
           ...
           $b = $a;            # $b is "linked" to $a
           ...
           $a = $a->clone;     # Unlink $b from $a
           $a->increment_by(4);

       Note that overloaded access makes this transparent:

           $a = new Data 23;
           $b = $a;            # $b is "linked" to $a
           $a += 4;            # would unlink $b automagically

       However, it would not make

           $a = new Data 23;
           $a = 4;             # Now $a is a plain 4, not 'Data'

       preserve "objectness" of $a.  But Perl has a way to make
       assignments to an object do whatever you want.  It is just
       not the overload, but tie()ing interface (see "tie" in
       perlfunc).  Adding a FETCH() method which returns the
       object itself, and STORE() method which changes the value
       of the object, one can reproduce the arithmetic metaphor
       in its completeness, at least for variables which were
       tie()d from the start.

       (Note that a workaround for a bug may be needed, see
       "BUGS".)



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overload(3p)     Perl Programmers Reference Guide    overload(3p)


Cookbook
       Please add examples to what follows!

       Two-face scalars

       Put this in two_face.pm in your Perl library directory:

         package two_face;             # Scalars with separate string and
                                       # numeric values.
         sub new { my $p = shift; bless [@_], $p }
         use overload '""' => \&str, '0+' => \&num, fallback => 1;
         sub num {shift->[1]}
         sub str {shift->[0]}

       Use it as follows:

         require two_face;
         my $seven = new two_face ("vii", 7);
         printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1;
         print "seven contains `i'\n" if $seven =~ /i/;

       (The second line creates a scalar which has both a string
       value, and a numeric value.)  This prints:

         seven=vii, seven=7, eight=8
         seven contains `i'

       Two-face references

       Suppose you want to create an object which is accessible
       as both an array reference and a hash reference, similar
       to the pseudo-hash builtin Perl type.  Let's make it bet-
       ter than a pseudo-hash by allowing index 0 to be treated
       as a normal element.

         package two_refs;
         use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} };
         sub new {
           my $p = shift;
           bless \ [@_], $p;
         }
         sub gethash {
           my %h;
           my $self = shift;
           tie %h, ref $self, $self;
           \%h;
         }










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overload(3p)     Perl Programmers Reference Guide    overload(3p)


         sub TIEHASH { my $p = shift; bless \ shift, $p }
         my %fields;
         my $i = 0;
         $fields{$_} = $i++ foreach qw{zero one two three};
         sub STORE {
           my $self = ${shift()};
           my $key = $fields{shift()};
           defined $key or die "Out of band access";
           $$self->[$key] = shift;
         }
         sub FETCH {
           my $self = ${shift()};
           my $key = $fields{shift()};
           defined $key or die "Out of band access";
           $$self->[$key];
         }

       Now one can access an object using both the array and hash
       syntax:

         my $bar = new two_refs 3,4,5,6;
         $bar->[2] = 11;
         $bar->{two} == 11 or die 'bad hash fetch';

       Note several important features of this example.  First of
       all, the actual type of $bar is a scalar reference, and we
       do not overload the scalar dereference.  Thus we can get
       the actual non-overloaded contents of $bar by just using
       $$bar (what we do in functions which overload derefer-
       ence).  Similarly, the object returned by the TIEHASH()
       method is a scalar reference.

       Second, we create a new tied hash each time the hash syn-
       tax is used.  This allows us not to worry about a possi-
       bility of a reference loop, which would lead to a memory
       leak.

       Both these problems can be cured.  Say, if we want to
       overload hash dereference on a reference to an object
       which is implemented as a hash itself, the only problem
       one has to circumvent is how to access this actual hash
       (as opposed to the virtual hash exhibited by the over-
       loaded dereference operator).  Here is one possible fetch-
       ing routine:

         sub access_hash {
           my ($self, $key) = (shift, shift);
           my $class = ref $self;
           bless $self, 'overload::dummy'; # Disable overloading of %{}
           my $out = $self->{$key};
           bless $self, $class;        # Restore overloading
           $out;
         }




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overload(3p)     Perl Programmers Reference Guide    overload(3p)


       To remove creation of the tied hash on each access, one
       may an extra level of indirection which allows a non-cir-
       cular structure of references:

         package two_refs1;
         use overload '%{}' => sub { ${shift()}->[1] },
                      '@{}' => sub { ${shift()}->[0] };
         sub new {
           my $p = shift;
           my $a = [@_];
           my %h;
           tie %h, $p, $a;
           bless \ [$a, \%h], $p;
         }
         sub gethash {
           my %h;
           my $self = shift;
           tie %h, ref $self, $self;
           \%h;
         }

         sub TIEHASH { my $p = shift; bless \ shift, $p }
         my %fields;
         my $i = 0;
         $fields{$_} = $i++ foreach qw{zero one two three};
         sub STORE {
           my $a = ${shift()};
           my $key = $fields{shift()};
           defined $key or die "Out of band access";
           $a->[$key] = shift;
         }
         sub FETCH {
           my $a = ${shift()};
           my $key = $fields{shift()};
           defined $key or die "Out of band access";
           $a->[$key];
         }

       Now if $baz is overloaded like this, then $baz is a refer-
       ence to a reference to the intermediate array, which keeps
       a reference to an actual array, and the access hash.  The
       tie()ing object for the access hash is a reference to a
       reference to the actual array, so

       o   There are no loops of references.

       o   Both "objects" which are blessed into the class
           "two_refs1" are references to a reference to an array,
           thus references to a scalar.  Thus the accessor
           expression "$$foo->[$ind]" involves no overloaded
           operations.






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overload(3p)     Perl Programmers Reference Guide    overload(3p)


       Symbolic calculator

       Put this in symbolic.pm in your Perl library directory:

         package symbolic;             # Primitive symbolic calculator
         use overload nomethod => \&wrap;

         sub new { shift; bless ['n', @_] }
         sub wrap {
           my ($obj, $other, $inv, $meth) = @_;
           ($obj, $other) = ($other, $obj) if $inv;
           bless [$meth, $obj, $other];
         }

       This module is very unusual as overloaded modules go: it
       does not provide any usual overloaded operators, instead
       it provides the "Last Resort" operator "nomethod".  In
       this example the corresponding subroutine returns an
       object which encapsulates operations done over the
       objects: "new symbolic 3" contains "['n', 3]", "2 + new
       symbolic 3" contains "['+', 2, ['n', 3]]".

       Here is an example of the script which "calculates" the
       side of circumscribed octagon using the above package:

         require symbolic;
         my $iter = 1;                 # 2**($iter+2) = 8
         my $side = new symbolic 1;
         my $cnt = $iter;

         while ($cnt--) {
           $side = (sqrt(1 + $side**2) - 1)/$side;
         }
         print "OK\n";

       The value of $side is

         ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]],
                              undef], 1], ['n', 1]]

       Note that while we obtained this value using a nice little
       script, there is no simple way to use this value.  In fact
       this value may be inspected in debugger (see perldebug),
       but ony if "bareStringify" Option is set, and not via "p"
       command.

       If one attempts to print this value, then the overloaded
       operator "" will be called, which will call "nomethod"
       operator.  The result of this operator will be stringified
       again, but this result is again of type "symbolic", which
       will lead to an infinite loop.

       Add a pretty-printer method to the module symbolic.pm:




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overload(3p)     Perl Programmers Reference Guide    overload(3p)


         sub pretty {
           my ($meth, $a, $b) = @{+shift};
           $a = 'u' unless defined $a;
           $b = 'u' unless defined $b;
           $a = $a->pretty if ref $a;
           $b = $b->pretty if ref $b;
           "[$meth $a $b]";
         }

       Now one can finish the script by

         print "side = ", $side->pretty, "\n";

       The method "pretty" is doing object-to-string conversion,
       so it is natural to overload the operator "" using this
       method.  However, inside such a method it is not necessary
       to pretty-print the components $a and $b of an object.  In
       the above subroutine "[$meth $a $b]" is a catenation of
       some strings and components $a and $b.  If these compo-
       nents use overloading, the catenation operator will look
       for an overloaded operator "."; if not present, it will
       look for an overloaded operator "".  Thus it is enough to
       use

         use overload nomethod => \&wrap, '""' => \&str;
         sub str {
           my ($meth, $a, $b) = @{+shift};
           $a = 'u' unless defined $a;
           $b = 'u' unless defined $b;
           "[$meth $a $b]";
         }

       Now one can change the last line of the script to

         print "side = $side\n";

       which outputs

         side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]]

       and one can inspect the value in debugger using all the
       possible methods.

       Something is still amiss: consider the loop variable $cnt
       of the script.  It was a number, not an object.  We cannot
       make this value of type "symbolic", since then the loop
       will not terminate.

       Indeed, to terminate the cycle, the $cnt should become
       false.  However, the operator "bool" for checking falsity
       is overloaded (this time via overloaded ""), and returns a
       long string, thus any object of type "symbolic" is true.
       To overcome this, we need a way to compare an object to 0.
       In fact, it is easier to write a numeric conversion



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overload(3p)     Perl Programmers Reference Guide    overload(3p)


       routine.

       Here is the text of symbolic.pm with such a routine added
       (and slightly modified str()):

         package symbolic;             # Primitive symbolic calculator
         use overload
           nomethod => \&wrap, '""' => \&str, '0+' => \&num;

         sub new { shift; bless ['n', @_] }
         sub wrap {
           my ($obj, $other, $inv, $meth) = @_;
           ($obj, $other) = ($other, $obj) if $inv;
           bless [$meth, $obj, $other];
         }
         sub str {
           my ($meth, $a, $b) = @{+shift};
           $a = 'u' unless defined $a;
           if (defined $b) {
             "[$meth $a $b]";
           } else {
             "[$meth $a]";
           }
         }
         my %subr = ( n => sub {$_[0]},
                      sqrt => sub {sqrt $_[0]},
                      '-' => sub {shift() - shift()},
                      '+' => sub {shift() + shift()},
                      '/' => sub {shift() / shift()},
                      '*' => sub {shift() * shift()},
                      '**' => sub {shift() ** shift()},
                    );
         sub num {
           my ($meth, $a, $b) = @{+shift};
           my $subr = $subr{$meth}
             or die "Do not know how to ($meth) in symbolic";
           $a = $a->num if ref $a eq __PACKAGE__;
           $b = $b->num if ref $b eq __PACKAGE__;
           $subr->($a,$b);
         }

       All the work of numeric conversion is done in %subr and
       num().  Of course, %subr is not complete, it contains only
       operators used in the example below.  Here is the extra-
       credit question: why do we need an explicit recursion in
       num()?  (Answer is at the end of this section.)

       Use this module like this:

         require symbolic;
         my $iter = new symbolic 2;    # 16-gon
         my $side = new symbolic 1;
         my $cnt = $iter;




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overload(3p)     Perl Programmers Reference Guide    overload(3p)


         while ($cnt) {
           $cnt = $cnt - 1;            # Mutator `--' not implemented
           $side = (sqrt(1 + $side**2) - 1)/$side;
         }
         printf "%s=%f\n", $side, $side;
         printf "pi=%f\n", $side*(2**($iter+2));

       It prints (without so many line breaks)

         [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1]
                                 [n 1]] 2]]] 1]
            [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912
         pi=3.182598

       The above module is very primitive.  It does not implement
       mutator methods ("++", "-=" and so on), does not do deep
       copying (not required without mutators!), and implements
       only those arithmetic operations which are used in the
       example.

       To implement most arithmetic operations is easy; one
       should just use the tables of operations, and change the
       code which fills %subr to

         my %subr = ( 'n' => sub {$_[0]} );
         foreach my $op (split " ", $overload::ops{with_assign}) {
           $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
         }
         my @bins = qw(binary 3way_comparison num_comparison str_comparison);
         foreach my $op (split " ", "@overload::ops{ @bins }") {
           $subr{$op} = eval "sub {shift() $op shift()}";
         }
         foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
           print "defining `$op'\n";
           $subr{$op} = eval "sub {$op shift()}";
         }

       Due to "Calling Conventions for Mutators", we do not need
       anything special to make "+=" and friends work, except
       filling "+=" entry of %subr, and defining a copy construc-
       tor (needed since Perl has no way to know that the imple-
       mentation of '+=' does not mutate the argument, compare
       "Copy Constructor").

       To implement a copy constructor, add "'=' => \&cpy" to
       "use overload" line, and code (this code assumes that
       mutators change things one level deep only, so recursive
       copying is not needed):

         sub cpy {
           my $self = shift;
           bless [@$self], ref $self;
         }




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overload(3p)     Perl Programmers Reference Guide    overload(3p)


       To make "++" and "--" work, we need to implement actual
       mutators, either directly, or in "nomethod".  We continue
       to do things inside "nomethod", thus add

           if ($meth eq '++' or $meth eq '--') {
             @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference
             return $obj;
           }

       after the first line of wrap().  This is not a most effec-
       tive implementation, one may consider

         sub inc { $_[0] = bless ['++', shift, 1]; }

       instead.

       As a final remark, note that one can fill %subr by

         my %subr = ( 'n' => sub {$_[0]} );
         foreach my $op (split " ", $overload::ops{with_assign}) {
           $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
         }
         my @bins = qw(binary 3way_comparison num_comparison str_comparison);
         foreach my $op (split " ", "@overload::ops{ @bins }") {
           $subr{$op} = eval "sub {shift() $op shift()}";
         }
         foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
           $subr{$op} = eval "sub {$op shift()}";
         }
         $subr{'++'} = $subr{'+'};
         $subr{'--'} = $subr{'-'};

       This finishes implementation of a primitive symbolic cal-
       culator in 50 lines of Perl code.  Since the numeric val-
       ues of subexpressions are not cached, the calculator is
       very slow.

       Here is the answer for the exercise: In the case of str(),
       we need no explicit recursion since the overloaded
       "."-operator will fall back to an existing overloaded
       operator "".  Overloaded arithmetic operators do not fall
       back to numeric conversion if "fallback" is not explicitly
       requested.  Thus without an explicit recursion num() would
       convert "['+', $a, $b]" to "$a + $b", which would just
       rebuild the argument of num().

       If you wonder why defaults for conversion are different
       for str() and num(), note how easy it was to write the
       symbolic calculator.  This simplicity is due to an appro-
       priate choice of defaults.  One extra note: due to the
       explicit recursion num() is more fragile than sym(): we
       need to explicitly check for the type of $a and $b.  If
       components $a and $b happen to be of some related type,
       this may lead to problems.



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overload(3p)     Perl Programmers Reference Guide    overload(3p)


       Really symbolic calculator

       One may wonder why we call the above calculator symbolic.
       The reason is that the actual calculation of the value of
       expression is postponed until the value is used.

       To see it in action, add a method

         sub STORE {
           my $obj = shift;
           $#$obj = 1;
           @$obj->[0,1] = ('=', shift);
         }

       to the package "symbolic".  After this change one can do

         my $a = new symbolic 3;
         my $b = new symbolic 4;
         my $c = sqrt($a**2 + $b**2);

       and the numeric value of $c becomes 5.  However, after
       calling

         $a->STORE(12);  $b->STORE(5);

       the numeric value of $c becomes 13.  There is no doubt now
       that the module symbolic provides a symbolic calculator
       indeed.

       To hide the rough edges under the hood, provide a tie()d
       interface to the package "symbolic" (compare with
       "Metaphor clash").  Add methods

         sub TIESCALAR { my $pack = shift; $pack->new(@_) }
         sub FETCH { shift }
         sub nop {  }          # Around a bug

       (the bug is described in "BUGS").  One can use this new
       interface as

         tie $a, 'symbolic', 3;
         tie $b, 'symbolic', 4;
         $a->nop;  $b->nop;    # Around a bug

         my $c = sqrt($a**2 + $b**2);

       Now numeric value of $c is 5.  After "$a = 12; $b = 5" the
       numeric value of $c becomes 13.  To insulate the user of
       the module add a method

         sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; }

       Now




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overload(3p)     Perl Programmers Reference Guide    overload(3p)


         my ($a, $b);
         symbolic->vars($a, $b);
         my $c = sqrt($a**2 + $b**2);

         $a = 3; $b = 4;
         printf "c5  %s=%f\n", $c, $c;

         $a = 12; $b = 5;
         printf "c13  %s=%f\n", $c, $c;

       shows that the numeric value of $c follows changes to the
       values of $a and $b.

AUTHOR
       Ilya Zakharevich <ilya@math.mps.ohio-state.edu>.

DIAGNOSTICS
       When Perl is run with the -Do switch or its equivalent,
       overloading induces diagnostic messages.

       Using the "m" command of Perl debugger (see perldebug) one
       can deduce which operations are overloaded (and which
       ancestor triggers this overloading). Say, if "eq" is over-
       loaded, then the method "(eq" is shown by debugger. The
       method "()" corresponds to the "fallback" key (in fact a
       presence of this method shows that this package has over-
       loading enabled, and it is what is used by the "Over-
       loaded" function of module "overload").

       The module might issue the following warnings:

       Odd number of arguments for overload::constant
           (W) The call to overload::constant contained an odd
           number of arguments.  The arguments should come in
           pairs.

       `%s' is not an overloadable type
           (W) You tried to overload a constant type the overload
           package is unaware of.

       `%s' is not a code reference
           (W) The second (fourth, sixth, ...) argument of over-
           load::constant needs to be a code reference. Either an
           anonymous subroutine, or a reference to a subroutine.

BUGS
       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.

       For the purpose of inheritance every overloaded package
       behaves as if "fallback" is present (possibly undefined).
       This may create interesting effects if some package is not
       overloaded, but inherits from two overloaded packages.



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overload(3p)     Perl Programmers Reference Guide    overload(3p)


       Relation between overloading and tie()ing is broken.
       Overloading is triggered or not basing on the previous
       class of tie()d value.

       This happens because the presence of overloading is
       checked too early, before any tie()d access is attempted.
       If the FETCH()ed class of the tie()d value does not
       change, a simple workaround is to access the value immedi-
       ately after tie()ing, so that after this call the previous
       class coincides with the current one.

       Needed: a way to fix this without a speed penalty.

       Barewords are not covered by overloaded string constants.

       This document is confusing.  There are grammos and mis-
       leading language used in places.  It would seem a total
       rewrite is needed.







































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