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

       overload - Package for overloading Perl operations

	   package SomeThing;

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

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

       This pragma allows overloading of Perl's operators for a class.	To
       overload built-in functions, see "Overriding Built-in Functions" in
       perlsub instead.


       Arguments of the "use overload" directive are (key, value) pairs.  For
       the full set of legal keys, see "Overloadable Operations" below.

       Operator implementations (the values) can be subroutines, references to
       subroutines, or anonymous subroutines - in other words, anything legal
       inside a "&{ ... }" call.  Values specified as strings are interpreted
       as method names.	 Thus

	   package Number;
	   use overload
	       "-" => "minus",
	       "*=" => \&muas,
	       '""' => sub { ...; };

       declares that subtraction is to be implemented by method "minus()" in
       the class "Number" (or one of its base classes), and that the function
       "Number::muas()" is to be used for the assignment form of
       multiplication, "*=".  It also defines an anonymous subroutine to
       implement stringification: this is called whenever an object blessed
       into the package "Number" is used in a string context (this subroutine
       might, for example, return the number as a Roman numeral).

       Calling Conventions and Magic Autogeneration

       The following sample implementation of "minus()" (which assumes that
       "Number" objects are simply blessed references to scalars) illustrates
       the calling conventions:

	   package Number;
	   sub minus {
	       my ($self, $other, $swap) = @_;
	       my $result = $$self - $other;	     # *
	       $result = -$result if $swap;
	       ref $result ? $result : bless \$result;
	   # * may recurse once - see table below

       Three arguments are passed to all subroutines specified in the "use
       overload" directive (with one exception - see "nomethod").  The first
       of these is the operand providing the overloaded operator
       implementation - in this case, the object whose "minus()" method is
       being called.

       The second argument is the other operand, or "undef" in the case of a
       unary operator.

       The third argument is set to TRUE if (and only if) the two operands
       have been swapped.  Perl may do this to ensure that the first argument
       ($self) is an object implementing the overloaded operation, in line
       with general object calling conventions.	 For example, if $x and $y are

	   operation   |   generates a call to
	   $x - $y     |   minus($x, $y, '')
	   $x - 7      |   minus($x, 7, '')
	   7 - $x      |   minus($x, 7, 1)

       Perl may also use "minus()" to implement other operators which have not
       been specified in the "use overload" directive, according to the rules
       for "Magic Autogeneration" described later.  For example, the "use
       overload" above declared no subroutine for any of the operators "--",
       "neg" (the overload key for unary minus), or "-=".  Thus

	   operation   |   generates a call to
	   -$x	       |   minus($x, 0, 1)
	   $x--	       |   minus($x, 1, undef)
	   $x -= 3     |   minus($x, 3, undef)

       Note the "undef"s: where autogeneration results in the method for a
       standard operator which does not change either of its operands, such as
       "-", being used to implement an operator which changes the operand
       ("mutators": here, "--" and "-="), Perl passes undef as the third
       argument.  This still evaluates as FALSE, consistent with the fact that
       the operands have not been swapped, but gives the subroutine a chance
       to alter its behaviour in these cases.

       In all the above examples, "minus()" is required only to return the
       result of the subtraction: Perl takes care of the assignment to $x.  In
       fact, such methods should not modify their operands, even if "undef" is
       passed as the third argument (see "Overloadable Operations").

       The same is not true of implementations of "++" and "--": these are
       expected to modify their operand.  An appropriate implementation of
       "--" might look like

	   use overload '--' => "decr",
	       # ...
	   sub decr { --${$_[0]}; }

       Mathemagic, Mutators, and Copy Constructors

       The term 'mathemagic' describes the overloaded implementation of
       mathematical operators.	Mathemagical operations raise an issue.
       Consider the code:

	   $a = $b;

       If $a and $b are scalars then after these statements

	   $a == $b - 1

       An object, however, is a reference to blessed data, so if $a and $b are
       objects then the assignment "$a = $b" copies only the reference,
       leaving $a and $b referring to the same object data.  One might
       therefore expect the operation "--$a" to decrement $b as well as $a.
       However, this would not be consistent with how we expect the
       mathematical operators to work.

       Perl resolves this dilemma by transparently calling a copy constructor
       before calling a method defined to implement a mutator ("--", "+=", and
       so on.).	 In the above example, when Perl reaches the decrement
       statement, it makes a copy of the object data in $a and assigns to $a a
       reference to the copied data.  Only then does it call "decr()", which
       alters the copied data, leaving $b unchanged.  Thus the object metaphor
       is preserved as far as possible, while mathemagical operations still
       work according to the arithmetic metaphor.

       Note: the preceding paragraph describes what happens when Perl
       autogenerates the copy constructor for an object based on a scalar.
       For other cases, see "Copy Constructor".

   Overloadable Operations
       The complete list of keys that can be specified in the "use overload"
       directive are given, separated by spaces, in the values of the hash

	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 int',
	conversion	 => 'bool "" 0+ qr',
	iterators	 => '<>',
	filetest	 => '-X',
	dereferencing	 => '${} @{} %{} &{} *{}',
	matching	 => '~~',
	special		 => 'nomethod fallback ='

       Most of the overloadable operators map one-to-one to these keys.
       Exceptions, including additional overloadable operations not apparent
       from this hash, are included in the notes which follow.

       A warning is issued if an attempt is made to register an operator not
       found above.

       ·    "not"

	    The operator "not" is not a valid key for "use overload".
	    However, if the operator "!" is overloaded then the same
	    implementation will be used for "not" (since the two operators
	    differ only in precedence).

       ·    "neg"

	    The key "neg" is used for unary minus to disambiguate it from
	    binary "-".

       ·    "++", "--"

	    Assuming they are to behave analogously to Perl's "++" and "--",
	    overloaded implementations of these operators are required to
	    mutate their operands.

	    No distinction is made between prefix and postfix forms of the
	    increment and decrement operators: these differ only in the point
	    at which Perl calls the associated subroutine when evaluating an

       ·    Assignments

		+=  -=	*=  /=	%=  **=	 <<=  >>=  x=  .=
		&=  |=	^=

	    Simple assignment is not overloadable (the '=' key is used for the
	    "Copy Constructor").  Perl does have a way to make assignments to
	    an object do whatever you want, but this involves using tie(), not
	    overload - see "tie" in perlfunc and the "COOKBOOK" examples

	    The subroutine for the assignment variant of an operator is
	    required only to return the result of the operation.  It is
	    permitted to change the value of its operand (this is safe because
	    Perl calls the copy constructor first), but this is optional since
	    Perl assigns the returned value to the left-hand operand anyway.

	    An object that overloads an assignment operator does so only in
	    respect of assignments to that object.  In other words, Perl never
	    calls the corresponding methods with the third argument (the
	    "swap" argument) set to TRUE.  For example, the operation

		$a *= $b

	    cannot lead to $b's implementation of "*=" being called, even if
	    $a is a scalar.  (It can, however, generate a call to $b's method
	    for "*").

       ·    Non-mutators with a mutator variant

		 +  -  *  /  %	**  <<	>>  x  .
		 &  |  ^

	    As described above, Perl may call methods for operators like "+"
	    and "&" in the course of implementing missing operations like
	    "++", "+=", and "&=".  While these methods may detect this usage
	    by testing the definedness of the third argument, they should in
	    all cases avoid changing their operands.  This is because Perl
	    does not call the copy constructor before invoking these methods.

       ·    "int"

	    Traditionally, the Perl function "int" rounds to 0 (see "int" in
	    perlfunc), and so for floating-point-like types one should follow
	    the same semantic.

       ·    String, numeric, boolean, and regexp conversions

		""  0+	bool

	    These conversions are invoked according to context as necessary.
	    For example, the subroutine for '""' (stringify) may be used where
	    the overloaded object is passed as an argument to "print", and
	    that for 'bool' where it is tested in the condition of a flow
	    control statement (like "while") or the ternary "?:" operation.

	    Of course, in contexts like, for example, "$obj + 1", Perl will
	    invoke $obj's implementation of "+" rather than (in this example)
	    converting $obj to a number using the numify method '0+' (an
	    exception to this is when no method has been provided for '+' and
	    "fallback" is set to TRUE).

	    The subroutines for '""', '0+', and 'bool' can return any
	    arbitrary Perl value.  If the corresponding operation for this
	    value is overloaded too, the operation will be called again with
	    this value.

	    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)".


	    The subroutine for 'qr' is used wherever the object is
	    interpolated into or used as a regexp, including when it appears
	    on the RHS of a "=~" or "!~" operator.

	    "qr" must return a compiled regexp, or a ref to a compiled regexp
	    (such as "qr//" returns), and any further overloading on the
	    return value will be ignored.

       ·    Iteration

	    If "<>" is overloaded then the same implementation is used for
	    both the read-filehandle syntax "<$var>" and globbing syntax

       ·    File tests

	    The key '-X' is used to specify a subroutine to handle all the
	    filetest operators ("-f", "-x", and so on: see "-X" in perlfunc
	    for the full list); it is not possible to overload any filetest
	    operator individually.  To distinguish them, the letter following
	    the '-' is passed as the second argument (that is, in the slot
	    that for binary operators is used to pass the second operand).

	    Calling an overloaded filetest operator does not affect the stat
	    value associated with the special filehandle "_".  It still refers
	    to the result of the last "stat", "lstat" or unoverloaded

	    This overload was introduced in Perl 5.12.

       ·    Matching

	    The key "~~" allows you to override the smart matching logic used
	    by the "~~" operator and the switch construct ("given"/"when").
	    See "Switch Statements" in perlsyn and feature.

	    Unusually, the overloaded implementation of the smart match
	    operator does not get full control of the smart match behaviour.
	    In particular, in the following code:

		package Foo;
		use overload '~~' => 'match';

		my $obj =  Foo->new();
		$obj ~~ [ 1,2,3 ];

	    the smart match does not invoke the method call like this:


	    rather, the smart match distributive rule takes precedence, so
	    $obj is smart matched against each array element in turn until a
	    match is found, so you may see between one and three of these
	    calls instead:


	    Consult the match table in	"Smartmatch Operator" in perlop for
	    details of when overloading is invoked.

       ·    Dereferencing

		${}  @{}  %{}  &{}  *{}

	    If these operators are not explicitly overloaded then they work in
	    the normal way, yielding the underlying scalar, array, or whatever
	    stores the object data (or the appropriate error message if the
	    dereference operator doesn't match it).  Defining a catch-all
	    'nomethod' (see below) makes no difference to this as the catch-
	    all function will not be called to implement a missing dereference

	    If a dereference operator is overloaded then it must return a
	    reference of the appropriate type (for example, the subroutine for
	    key '${}' should return a reference to a scalar, not a scalar), or
	    another object which overloads the operator: that is, the
	    subroutine only determines what is dereferenced and the actual
	    dereferencing is left to Perl.  As a special case, if the
	    subroutine returns the object itself then it will not be called
	    again - avoiding infinite recursion.

       ·    Special

		nomethod  fallback  =

	    See "Special Keys for "use overload"".

   Magic Autogeneration
       If a method for an operation is not found then Perl tries to
       autogenerate a substitute implementation from the operations that have
       been defined.

       Note: the behaviour described in this section can be disabled by
       setting "fallback" to FALSE (see "fallback").

       In the following tables, numbers indicate priority.  For example, the
       table below states that, if no implementation for '!' has been defined
       then Perl will implement it using 'bool' (that is, by inverting the
       value returned by the method for 'bool'); if boolean conversion is also
       unimplemented then Perl will use '0+' or, failing that, '""'.

	   operator | can be autogenerated from
		    | 0+   ""	bool   .   x
	      0+    |	    1	  2
	      ""    |  1	  2
	      bool  |  1    2
	      int   |  1    2	  3
	      !	    |  2    3	  1
	      qr    |  2    1	  3
	      .	    |  2    1	  3
	      x	    |  2    1	  3
	      .=    |  3    2	  4    1
	      x=    |  3    2	  4	   1
	      <>    |  2    1	  3
	      -X    |  2    1	  3

       Note: The iterator ('<>') and file test ('-X') operators work as
       normal: if the operand is not a blessed glob or IO reference then it is
       converted to a string (using the method for '""', '0+', or 'bool') to
       be interpreted as a glob or filename.

	   operator | can be autogenerated from
		    |  <   <=>	 neg   -=    -
	      neg   |			     1
	      -=    |			     1
	      --    |			1    2
	      abs   | a1    a2	  b1	    b2	  [*]
	      <	    |	     1
	      <=    |	     1
	      >	    |	     1
	      >=    |	     1
	      ==    |	     1
	      !=    |	     1

	   * one from [a1, a2] and one from [b1, b2]

       Just as numeric comparisons can be autogenerated from the method for
       '<=>', string comparisons can be autogenerated from that for 'cmp':

	    operators	       |  can be autogenerated from
	    lt gt le ge eq ne  |  cmp

       Similarly, autogeneration for keys '+=' and '++' is analogous to '-='
       and '--' above:

	   operator | can be autogenerated from
		    |  +=    +
	       +=   |	     1
	       ++   |	1    2

       And other assignment variations are analogous to '+=' and '-=' (and
       similar to '.=' and 'x=' above):

		     operator ||  *= /= %= **= <<= >>= &= ^= |=
	   autogenerated from ||  *  /	%  **  <<  >>  &  ^  |

       Note also that the copy constructor (key '=') may be autogenerated, but
       only for objects based on scalars.  See "Copy Constructor".

       Minimal Set of Overloaded Operations

       Since some operations can be automatically generated from others, there
       is a minimal set of operations that need to be overloaded in order to
       have the complete set of overloaded operations at one's disposal.  Of
       course, the autogenerated operations may not do exactly what the user
       expects.	 The minimal set is:

	   + - * / % ** << >> x
	   <=> cmp
	   & | ^ ~
	   atan2 cos sin exp log sqrt int
	   "" 0+ bool

       Of the conversions, only one of string, boolean or numeric is needed
       because each can be generated from either of the other two.

   Special Keys for "use overload"

       The 'nomethod' key is used to specify a catch-all function to be called
       for any operator that is not individually overloaded.  The specified
       function will be passed four parameters.	 The first three arguments
       coincide with those that would have been passed to the corresponding
       method if it had been defined.  The fourth argument is the "use
       overload" key for that missing method.

       For example, if $a is an object blessed into a package declaring

	   use overload 'nomethod' => 'catch_all', # ...

       then the operation

	   3 + $a

       could (unless a method is specifically declared for the key '+') result
       in a call

	   catch_all($a, 3, 1, '+')

       See "How Perl Chooses an Operator Implementation".


       The value assigned to the key 'fallback' tells Perl how hard it should
       try to find an alternative way to implement a missing operator.

       ·   defined, but FALSE

	       use overload "fallback" => 0, # ... ;

	   This disables "Magic Autogeneration".

       ·   "undef"

	   In the default case where no value is explicitly assigned to
	   "fallback", magic autogeneration is enabled.

       ·   TRUE

	   The same as for "undef", but if a missing operator cannot be
	   autogenerated then, instead of issuing an error message, Perl is
	   allowed to revert to what it would have done for that operator if
	   there had been no "use overload" directive.

	   Note: in most cases, particularly the "Copy Constructor", this is
	   unlikely to be appropriate behaviour.

       See "How Perl Chooses an Operator Implementation".

       Copy Constructor

       As mentioned above, this operation is called when a mutator is applied
       to a reference that shares its object with some other reference.	 For
       example, if $b is mathemagical, and '++' is overloaded with 'incr', and
       '=' is overloaded with 'clone', then the code

	   $a = $b;
	   # ... (other code which does not modify $a or $b) ...

       would be executed in a manner equivalent to

	   $a = $b;
	   # ...
	   $b = $b->clone(undef, "");
	   $b->incr(undef, "");


       ·   The subroutine for '=' does not overload the Perl assignment
	   operator: it is used only to allow mutators to work as described
	   here.  (See "Assignments" above.)

       ·   As for other operations, the subroutine implementing '=' is passed
	   three arguments, though the last two are always "undef" and ''.

       ·   The copy constructor is called only before a call to a function
	   declared to implement a mutator, for example, if "++$b;" in the
	   code above is effected via a method declared for key '++' (or
	   'nomethod', passed '++' as the fourth argument) or, by
	   autogeneration, '+='.  It is not called if the increment operation
	   is effected by a call to the method for '+' since, in the
	   equivalent code,

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

	   the data referred to by $a is unchanged by the assignment to $b of
	   a reference to new object data.

       ·   The copy constructor is not called if Perl determines that it is
	   unnecessary because there is no other reference to the data being

       ·   If 'fallback' is undefined or TRUE then a copy constructor can be
	   autogenerated, but only for objects based on scalars.  In other
	   cases it needs to be defined explicitly.  Where an object's data is
	   stored as, for example, an array of scalars, the following might be

	       use overload '=' => sub { bless [ @{$_[0]} ] },	# ...

       ·   If 'fallback' is TRUE and no copy constructor is defined then, for
	   objects not based on scalars, Perl may silently fall back on simple
	   assignment - that is, assignment of the object reference.  In
	   effect, this disables the copy constructor mechanism since no new
	   copy of the object data is created.	This is almost certainly not
	   what you want.  (It is, however, consistent: for example, Perl's
	   fallback for the "++" operator is to increment the reference

   How Perl Chooses an Operator Implementation
       Which is checked first, "nomethod" or "fallback"?  If the two operands
       of an operator are of different types and both overload the operator,
       which implementation is used?  The following are the precedence rules:

       1.  If the first operand has declared a subroutine to overload the
	   operator then use that implementation.

       2.  Otherwise, if fallback is TRUE or undefined for the first operand
	   then see if the rules for autogeneration allows another of its
	   operators to be used instead.

       3.  Unless the operator is an assignment ("+=", "-=", etc.), repeat
	   step (1) in respect of the second operand.

       4.  Repeat Step (2) in respect of the second operand.

       5.  If the first operand has a "nomethod" method then use that.

       6.  If the second operand has a "nomethod" method then use that.

       7.  If "fallback" is TRUE for both operands then perform the usual
	   operation for the operator, treating the operands as numbers,
	   strings, or booleans as appropriate for the operator (see note).

       8.  Nothing worked - die.

       Where there is only one operand (or only one operand with overloading)
       the checks in respect of the other operand above are skipped.

       There are exceptions to the above rules for dereference operations
       (which, if Step 1 fails, always fall back to the normal, built-in
       implementations - see Dereferencing), and for "~~" (which has its own
       set of rules - see "Matching" under "Overloadable Operations" above).

       Note on Step 7: some operators have a different semantic depending on
       the type of their operands.  As there is no way to instruct Perl to
       treat the operands as, e.g., numbers instead of strings, the result
       here may not be what you expect.	 See "BUGS AND PITFALLS".

   Losing Overloading
       The restriction for the comparison operation is that even if, for
       example, "cmp" should return a blessed reference, 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 mathemagical properties
       if the string conversion substitution is applied.

       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.

   Inheritance and Overloading
       Overloading respects inheritance via the @ISA hierarchy.	 Inheritance
       interacts with overloading in two ways.

       Method names in the "use overload" directive
	   If "value" in

	     use overload key => value;

	   is a string, it is interpreted as a method name - which may (in the
	   usual way) be inherited from another class.

       Overloading of an operation is inherited by derived classes
	   Any class derived from an overloaded class is also overloaded and
	   inherits its operator implementations.  If the same operator is
	   overloaded in more than one ancestor then the implementation is
	   determined by the usual inheritance rules.

	   For example, if "A" inherits from "B" and "C" (in that 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 in Perl version prior to 5.18 inheritance of the "fallback"
       key was not governed by the above rules.	 The value of "fallback" in
       the first overloaded ancestor was used.	This was fixed in 5.18 to
       follow the usual rules of inheritance.

   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 functions:

	    Gives the string value of "arg" as in the absence of stringify
	    overloading.  If you are using this to get the address of a
	    reference (useful for checking if two references point to the same
	    thing) then you may be better off using "Scalar::Util::refaddr()",
	    which is faster.

	    Returns true if "arg" is subject to overloading of some

	    Returns "undef" or a reference to the method that implements "op".

   Overloading Constants
       For some applications, the Perl parser mangles constants too much.  It
       is possible to hook into this process via "overload::constant()" and
       "overload::remove_constant()" functions.

       These functions take a hash as an argument.  The recognized 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 expressions.

       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 interprets 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-delimiter
       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.  Similarly, 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

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

       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 overload", new function definitions,
       and changes in @ISA.

       (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

       If a package uses overload, it carries a special flag.  This flag is
       also set when new function are defined or @ISA is modified.  There will
       be a slight speed penalty on the very first operation thereafter that
       supports overloading, while the overload tables are updated.  If there
       is no overloading present, the flag is turned off.  Thus the only speed
       penalty thereafter is the checking of this flag.

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

       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 $seven = two_face->new("vii", 7);
	 printf "seven=$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.

	 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;

	 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";

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

	 my $bar = two_refs->new(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
       dereference).  Similarly, the object returned by the TIEHASH() method
       is a scalar reference.

       Second, we create a new tied hash each time the hash syntax is used.
       This allows us not to worry about a possibility 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 overloaded
       dereference operator).  Here is one possible fetching 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

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

	 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;

	 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";

       Now if $baz is overloaded like this, then $baz is a reference 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

       ·   There are no loops of references.

       ·   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

   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 an
       implementation for ""nomethod"".	 In this example the "nomethod"
       subroutine returns an object which encapsulates operations done over
       the objects: "symbolic->new(3)" contains "['n', 3]", "2 +
       symbolic->new(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 = symbolic->new(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 only 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:

	 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
       components 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

       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

       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+' => \#

	 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__;

       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 = symbolic->new(2);  # 16-gon
	 my $side = symbolic->new(1);
	 my $cnt = $iter;

	 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

       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()}";

       Since subroutines implementing assignment operators are not required to
       modify their operands (see "Overloadable Operations" above), we do not
       need anything special to make "+=" and friends work, besides adding
       these operators to %subr and defining a copy constructor (needed since
       Perl has no way to know that the implementation of '+=' does not mutate
       the argument - see "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;

       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 effective
       implementation, one may consider

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


       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 calculator in 50
       lines of Perl code.  Since the numeric values 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

       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 appropriate 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.

   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 = symbolic->new(3);
	 my $b = symbolic->new(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".	Add methods

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

       (the bug, fixed in Perl 5.14, 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

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


	 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.

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

       The "overloading" pragma can be used to enable or disable overloaded
       operations within a lexical scope - see overloading.

       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 overloaded, 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
       overloading enabled, and it is what is used by the "Overloaded"
       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 overload::constant
	   needs to be a code reference.  Either an anonymous subroutine, or a
	   reference to a subroutine.

       overload arg '%s' is invalid
	   (W) "use overload" was passed an argument it did not recognize.
	   Did you mistype an operator?

       ·   A pitfall when fallback is TRUE and Perl resorts to a built-in
	   implementation of an operator is that some operators have more than
	   one semantic, for example "|":

		   use overload '0+' => sub { $_[0]->{n}; },
		       fallback => 1;
		   my $x = bless { n => 4 }, "main";
		   my $y = bless { n => 8 }, "main";
		   print $x | $y, "\n";

	   You might expect this to output "12".  In fact, it prints "<": the
	   ASCII result of treating "|" as a bitwise string operator - that
	   is, the result of treating the operands as the strings "4" and "8"
	   rather than numbers.	 The fact that numify ("0+") is implemented
	   but stringify ("") isn't makes no difference since the latter is
	   simply autogenerated from the former.

	   The only way to change this is to provide your own subroutine for

       ·   Magic autogeneration increases the potential for inadvertently
	   creating self-referential structures.  Currently Perl will not free
	   self-referential structures until cycles are explicitly broken.
	   For example,

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

	   is asking for trouble, since

	       $obj += $y;

	   will effectively become

	       $obj = add($obj, $y, undef);

	   with the same result as

	       $obj = [\$obj, \$foo];

	   Even if no explicit assignment-variants of operators are present in
	   the script, they may be generated by the optimizer.	For example,

	       "obj = $obj\n"

	   may be optimized to

	       my $tmp = 'obj = ' . $obj;  $tmp .= "\n";

       ·   The symbol table is filled with names looking like line-noise.

       ·   This bug was fixed in Perl 5.18, but may still trip you up if you
	   are using older versions:

	   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.

       ·   Before Perl 5.14, the relation between overloading and tie()ing was
	   broken.  Overloading was triggered or not based on the previous
	   class of the tie()d variable.

	   This happened because the presence of overloading was checked too
	   early, before any tie()d access was attempted.  If the class of the
	   value FETCH()ed from the tied variable does not change, a simple
	   workaround for code that is to run on older Perl versions is to
	   access the value (via "() = $foo" or some such) immediately after
	   tie()ing, so that after this call the previous class coincides with
	   the current one.

       ·   Barewords are not covered by overloaded string constants.

       ·   The range operator ".." cannot be overloaded.

perl v5.18.2			  2013-11-04		       overload(3perl)

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