Use Copy Constructor And Assignment Operator In Perl

overload

NAME

overload - Package for overloading Perl operations

SYNOPSIS

  1. package SomeThing;
  2. useoverload
  3. '+'=> \&myadd,
  4. '-'=> \&mysub;
  5. # etc
  6. ...
  7. package main;
  8. $a = newSomeThing57;
  9. $b=5+$a;
  10. ...
  11. if (overload::Overloaded$b){...}
  12. ...
  13. $strval = overload::StrVal$b;

DESCRIPTION

Declaration of overloaded functions

The compilation directive

  1. package Number;
  2. useoverload
  3. "+"=> \&add,
  4. "*="=>"muas";

declares function Number::add() for addition, and method muas() in the "class" (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 subroutines. Legal keys are listed below.

The subroutine will be called to execute if $a is a reference to an object blessed into the package , or if $a is not an object from a package with defined mathemagic addition, but $b is a reference to a . It can also be called in other situations, like , or . See MAGIC AUTOGENERATION. (Mathemagical methods refer to methods triggered by an overloaded mathematical operator.)

Since overloading respects inheritance via the @ISA hierarchy, the above declaration would also trigger overloading of and in all the packages which inherit from .

Calling Conventions for Binary Operations

The functions specified in the directive 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 , 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 information by examining the third argument, which can take three different values:

  • FALSE

    the order of arguments is as in the current operation.

  • TRUE

    the arguments are reversed.

  • the current operation is an assignment variant (as in ), 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 . Thus the functions that overloads is called with arguments 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,

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

    is an appropriate implementation of overloaded . Note that

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

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

  • 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 overloaded 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 broken. You may get problems when traversing your structures too.

Say,

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

is asking for trouble, since for code the subroutine is called as , or . If using such a subroutine is an important optimization, one can overload explicitly by a non-"optimized" version, or switch to non-optimized version if (see Calling Conventions for Binary Operations).

Even if no explicit assignment-variants of operators are present in the script, they may be generated by the optimizer. Say, or may be both optimized to

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

Overloadable Operations

The following symbols can be specified in directive:

  • Arithmetic operations
    1. "+","+=","-","-=","*","*=","/","/=","%","%=",
    2. "**","**=","<<","<<=",">>",">>=","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 generate increment and decrement methods. The operation can be used to autogenerate missing methods for unary minus or .

    See MAGIC AUTOGENERATION, Calling Conventions for Mutators and Calling Conventions for Binary Operations) for details of these substitutions.

  • Comparison operations
    1. "<","<=",">",">=","==","!=","<=>",
    2. "lt","le","gt","ge","eq","ne","cmp",

    If the corresponding "spaceship" variant is available, it can be used to substitute for the missing operation. During ing arrays, is used to compare values subject to .

  • Bit operations
    1. "&","^","|","neg","!","~",

    stands for unary minus. If the method for 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 , or , or .

  • Increment and decrement
    1. "++","--",

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

  • Transcendental functions
    1. "atan2","cos","sin","exp","abs","log","sqrt","int"

    If 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 is unavailable, it can be autogenerated using the overloading of .

  • Boolean, string and numeric conversion
    1. 'bool','""','0+',

    If one or two of these operations are not overloaded, the remaining ones can be used instead. is used in the flow control operators (like ) and for the ternary operation. These functions can return any arbitrary Perl value. If the corresponding operation for this value is overloaded too, that 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 .

  • Iteration
    1. "<>"

    If not overloaded, the argument will be converted to a filehandle or glob (which may require a stringification). The same overloading happens both for the read-filehandle syntax and globbing syntax .

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

  • Dereferencing
    1. '${}','@{}','%{}','&{}','*{}'.

    If not overloaded, the argument will be dereferenced as is, thus should be of correct type. These functions 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 explicitly they will not be passed to "nomethod".

  • Special
    1. "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-separated lists of names:

  1. with_assign=>'+ - * / % ** << >> x .',
  2. assign=>'+= -= *= /= %= **= <<= >>= x= .=',
  3. num_comparison=>'< <= > >= == !=',
  4. '3way_comparison'=>'<=> cmp',
  5. str_comparison=>'lt le gt ge eq ne',
  6. binary=>'& | ^',
  7. unary=>'neg ! ~',
  8. mutators=>'++ --',
  9. func=>'atan2 cos sin exp abs log sqrt',
  10. conversion=>'bool "" 0+',
  11. iterators=>'<>',
  12. dereferencing=>'${} @{} %{} &{} *{}',
  13. special=>'nomethod fallback ='

Inheritance and overloading

Inheritance interacts with overloading in two ways.

  • Strings as values of directive

    If in

    1. useoverloadkey=>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 inherits from and (in this order), overloads with , and overloads by , then the subroutine will be called to implement operation for an object in package .

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

SPECIAL SYMBOLS FOR

Three keys are recognized by Perl that are not covered by the above description.

Last Resort

should be followed by a reference to a function of four parameters. If defined, it is called when the overloading mechanism cannot find a method for some operation. 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, can be equivalent to

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

if the pair was specified in the directive.

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

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

Fallback

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

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

  • TRUE

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

  • defined, but FALSE

    No autogeneration is tried. Perl tries to call value, and if this is missing, raises an exception.

Note. 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 . However, it does not overload the Perl assignment 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 other reference, such as

  1. $a=$b;
  2. ++$a;

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

  1. $a=$b;
  2. $a=$a+1;

then does not reference a new copy of , 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

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

    may be

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

    if $b was mathemagical, and was overloaded with , was overloaded with .

Same behaviour is triggered by , which is consider a synonym for .

MAGIC AUTOGENERATION

If a method for an operation is not found, and the value for is TRUE or undefined, Perl tries to autogenerate 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

    can use the method for if the 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 operation can be expressed in terms of or , and in terms of and .

  • can be expressed in terms of and (or ).

  • Unary minus

    can be expressed in terms of subtraction.

  • Negation

    and can be expressed in terms of boolean conversion, or string or numerical conversion.

  • Concatenation

    can be expressed in terms of string conversion.

  • Comparison operations

    can be expressed in terms of its "spaceship" counterpart: either or :

    1. <, >, <=, >=, ==, != intermsof <=>
    2. lt, gt, le, ge, eq, ne intermsof cmp
  • Iterator
    1. <> in terms of builtin operations
  • Dereferencing
    1. ${} @{} %{} &{} *{} 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, ` ' should return a blessed reference, the autogenerated ` ' function will produce only a standard logical value based on the numerical value of the result of ` '. In particular, a working numeric conversion is needed in this case (possibly expressed in terms of other conversions).

Similarly, and 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.

Run-time Overloading

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

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

You can also use

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

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

Public functions

Package provides the following public functions:

  • overload::StrVal(arg)

    Gives string value of as in 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 , which is faster.

  • overload::Overloaded(arg)

    Returns true if is subject to overloading of some operations.

  • overload::Method(obj,op)

    Returns or a reference to the method that implements .

Overloading constants

For some applications, the Perl parser mangles constants too much. It is possible to hook into this process via and 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 -quoted strings, constant pieces of - and -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 - and - constants, it is in single-quote context (comes from strings, regular expressions, and single-quote HERE documents), it is for arguments of / operators, it is for right-hand side of -operator, and it is otherwise.

Since an expression is just a shortcut for , 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 as

  1. sub import{
  2. shift;
  3. return unless @_;
  4. die"unknown import: @_" unless @_ == 1 and $_[0] eq ':constant';
  5. overload::constantinteger=>sub{Math::BigInt->new(shift)};
  6. }

BUGS Currently overloaded-ness of constants does not propagate into .

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 , , new function definitions, and changes in @ISA. However, this invalidation remains unprocessed until the next ing into the package. Hence if you want to change overloading structure dynamically, you'll need an additional (fake) ing to update the table.

(Every SVish thing has a magic queue, and magic is an entry in that queue. This is how a single variable may participate in multiple forms of magic simultaneously. For instance, environment variables regularly have two forms at once: their %ENV magic and their taint magic. However, the magic which implements overloading is applied to the stashes, which are rarely used directly, thus should not slow down Perl.)

If an object belongs to a package using overload, it carries a special flag. Thus the only speed penalty during arithmetic operations without overloading is the checking of this flag.

In fact, if 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 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 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 ( ) 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 . 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 if $a and $b are objects. Perl-think implies that $a 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 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 preserving 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 Constructor.

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

  1. $a = newData23;
  2. ...
  3. $b = $a;# $b is "linked" to $a
  4. ...
  5. $a = $a->clone;# Unlink $b from $a
  6. $a->increment_by(4);

Note that overloaded access makes this transparent:

  1. $a = newData23;
  2. $b = $a;# $b is "linked" to $a
  3. $a += 4;# would unlink $b automagically

However, it would not make

  1. $a = newData23;
  2. $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). 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.)

Cookbook

Please add examples to what follows!

Two-face scalars

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

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

Use it as follows:

  1. requiretwo_face;
  2. my$seven = newtwo_face("vii",7);
  3. printf"seven=$seven, seven=%d, eight=%d\n",$seven,$seven+1;
  4. 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:

  1. seven=vii, seven=7, eight=8
  2. 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 better than a pseudo-hash by allowing index 0 to be treated as a normal element.

  1. package two_refs;
  2. useoverload'%{}'=> \&gethash,'@{}'=>sub{$ {shift()} };
  3. sub new{
  4. my$p = shift;
  5. bless \ [@_],$p;
  6. }
  7. sub gethash{
  8. my%h;
  9. my$self = shift;
  10. tie%h,ref$self,$self;
  11. \%h;
  12. }
  13. sub TIEHASH{my$p = shift;bless \ shift,$p}
  14. my%fields;
  15. my$i = 0;
  16. $fields{$_} = $i++ foreach qw{zero one two three};
  17. sub STORE{
  18. my$self = ${shift()};
  19. my$key = $fields{shift()};
  20. defined$key or die"Out of band access";
  21. $$self->[$key] = shift;
  22. }
  23. sub FETCH{
  24. my$self = ${shift()};
  25. my$key = $fields{shift()};
  26. defined$key or die"Out of band access";
  27. $$self->[$key];
  28. }

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

  1. my$bar = newtwo_refs3,4,5,6;
  2. $bar->[2] = 11;
  3. $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 (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:

  1. sub access_hash{
  2. my($self,$key) = (shift,shift);
  3. my$class = ref$self;
  4. bless$self,'overload::dummy';# Disable overloading of %{}
  5. my$out = $self->{$key};
  6. bless$self,$class;# Restore overloading
  7. $out;
  8. }

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

  1. package two_refs1;
  2. useoverload'%{}'=>sub{${shift()}->[1] },
  3. '@{}'=>sub{${shift()}->[0] };
  4. sub new{
  5. my$p = shift;
  6. my$a = [@_];
  7. my%h;
  8. tie%h,$p,$a;
  9. bless \ [$a, \%h],$p;
  10. }
  11. sub gethash{
  12. my%h;
  13. my$self = shift;
  14. tie%h,ref$self,$self;
  15. \%h;
  16. }
  17. sub TIEHASH{my$p = shift;bless \ shift,$p}
  18. my%fields;
  19. my$i = 0;
  20. $fields{$_} = $i++ foreach qw{zero one two three};
  21. sub STORE{
  22. my$a = ${shift()};
  23. my$key = $fields{shift()};
  24. defined$key or die"Out of band access";
  25. $a->[$key] = shift;
  26. }
  27. sub FETCH{
  28. my$a = ${shift()};
  29. my$key = $fields{shift()};
  30. defined$key or die"Out of band access";
  31. $a->[$key];
  32. }

Now if $baz is overloaded like this, then 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 are references to a reference to an array, thus references to a scalar. Thus the accessor expression involves no overloaded operations.

Symbolic calculator

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

  1. package symbolic;# Primitive symbolic calculator
  2. useoverloadnomethod=> \&wrap;
  3. sub new{shift;bless['n',@_]}
  4. sub wrap{
  5. my($obj,$other,$inv,$meth) = @_;
  6. ($obj,$other) = ($other,$obj) if $inv;
  7. bless[$meth,$obj,$other];
  8. }

This module is very unusual as overloaded modules go: it does not provide any usual overloaded operators, instead it provides the Last Resort operator . In this example the corresponding subroutine returns an object which encapsulates operations done over the objects: contains , contains .

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

  1. requiresymbolic;
  2. my$iter = 1;# 2**($iter+2) = 8
  3. my$side = newsymbolic1;
  4. my$cnt = $iter;
  5. while ($cnt--){
  6. $side = (sqrt(1 + $side**2) - 1)/$side;
  7. }
  8. print"OK\n";

The value of $side is

  1. ['/',['-',['sqrt',['+',1,['**',['n',1],2]],
  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 Option is set, and not via command.

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

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

  1. sub pretty{
  2. my($meth,$a,$b) = @{+shift};
  3. $a = 'u' unless defined$a;
  4. $b = 'u' unless defined$b;
  5. $a = $a->pretty if ref$a;
  6. $b = $b->pretty if ref$b;
  7. "[$meth $a $b]";
  8. }

Now one can finish the script by

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

The method 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 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

  1. useoverloadnomethod=> \&wrap,'""'=> \&str;
  2. sub str{
  3. my($meth,$a,$b) = @{+shift};
  4. $a = 'u' unless defined$a;
  5. $b = 'u' unless defined$b;
  6. "[$meth $a $b]";
  7. }

Now one can change the last line of the script to

  1. print"side = $side\n";

which outputs

  1. 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 , since then the loop will not terminate.

Indeed, to terminate the cycle, the $cnt should become false. However, the operator for checking falsity is overloaded (this time via overloaded ), and returns a long string, thus any object of type 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 routine.

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

  1. package symbolic;# Primitive symbolic calculator
  2. useoverload
  3. nomethod=> \&wrap,'""'=> \&str,'0+'=> \&num;
  4. sub new{shift;bless['n',@_]}
  5. sub wrap{
  6. my($obj,$other,$inv,$meth) = @_;
  7. ($obj,$other) = ($other,$obj) if $inv;
  8. bless[$meth,$obj,$other];
  9. }
  10. sub str{
  11. my($meth,$a,$b) = @{+shift};
  12. $a = 'u' unless defined$a;
  13. if (defined$b){
  14. "[$meth $a $b]";
  15. } else {
  16. "[$meth $a]";
  17. }
  18. }
  19. my%subr = (n=>sub{$_[0]},
  20. sqrt=>sub{sqrt$_[0]},
  21. '-'=>sub{shift() - shift()},
  22. '+'=>sub{shift() + shift()},
  23. '/'=>sub{shift() / shift()},
  24. '*'=>sub{shift() * shift()},
  25. '**'=>sub{shift() ** shift()},
  26. );
  27. sub num{
  28. my($meth,$a,$b) = @{+shift};
  29. my$subr = $subr{$meth}
  30. or die"Do not know how to ($meth) in symbolic";
  31. $a = $a->num if ref$a eq __PACKAGE__;
  32. $b = $b->num if ref$b eq __PACKAGE__;
  33. $subr->($a,$b);
  34. }

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:

  1. requiresymbolic;
  2. my$iter = newsymbolic2;# 16-gon
  3. my$side = newsymbolic1;
  4. my$cnt = $iter;
  5. while ($cnt){
  6. $cnt = $cnt - 1;# Mutator `--' not implemented
  7. $side = (sqrt(1 + $side**2) - 1)/$side;
  8. }
  9. printf"%s=%f\n",$side,$side;
  10. printf"pi=%f\n",$side*(2**($iter+2));

It prints (without so many line breaks)

  1. [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1]
  2. [n 1]] 2]]] 1]
  3. [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912
  4. 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

  1. my%subr = ('n'=>sub{

You can only overload certain operators, which are shown in Table 13-1. The operators are also listed in the hash made available when you , though the categorization is a little different there.

Note that , , , and are not actual Perl operators. The five dereferencers, , and probably don't seem like operators either. Nevertheless, they are all valid keys for the parameter list you provide to . This is not really a problem. We'll let you in on a little secret: it's a bit of a fib to say that the pragma overloads operators. It overloads the underlying operations, whether invoked explicitly via their "official" operators, or implicitly via some related operator. (The pseudo-operators we mentioned can only be invoked implicitly.) In other words, overloading happens not at the syntactic level, but at the semantic level. The point is not to look good. The point is to do the right thing. Feel free to generalize.

Note also that does not overload Perl's assignment operator, as you might expect. That would not do the right thing. More on that later.

We'll start by discussing the conversion operators, not because they're the most obvious (they aren't), but because they're the most useful. Many classes overload nothing but stringification, specified by the key. (Yes, that really is two double-quotes in a row.)

Conversion operators:,,

These three keys let you provide behaviors for Perl's automatic conversions to strings, numbers, and Boolean values, respectively.

We say that stringification occurs when any nonstring variable is used as a string. It's what happens when you convert a variable into a string via printing, interpolation, concatenation, or even by using it as a hash key. Stringification is also why you see something like when you try to an object.

We say that numification occurs when a nonnumeric variable is converted into a number in any numeric context, such as any mathematical expression, array index, or even as an operand of the range operator.

Finally, while nobody here quite has the nerve to call it boolification, you can define how an object should be interpreted in a Boolean context (such as , , , , , , , , , or the block of a expression) by creating a handler.

Any of the three conversion operators can be autogenerated if you have any one of them (we'll explain autogeneration later). Your handlers can return any value you like. Note that if the operation that triggered the conversion is also overloaded, that overloading will occur immediately afterward.

Here's a demonstration of that invokes an object's handler upon stringification. Don't forget to quote the quotes:

package Person; use overload q("") => \&as_string; sub new { my $class = shift; return bless { @_ } => $class; } sub as_string { my $self = shift; my ($key, $value, $result); while (($key, $value) = each %$self) { $result .= "$key => $value\n"; } return $result; } $obj = Person->new(height => 72, weight => 165, eyes => "brown"); print $obj;
Instead of something like , this prints (in hash order):
weight => 165 height => 72 eyes => brown
(We sincerely hope this person was not measured in kg and cm.)
Arithmetic operators:,,,,,,,,

These should all be familiar except for , which is a special overloading key for the unary minus: the in . The distinction between the and keys allows you to specify different behaviors for unary minus and binary minus, more commonly known as subtraction.

If you overload but not , and then try to use a unary minus, Perl will emulate a handler for you. This is known as autogeneration, where certain operators can be reasonably deduced from other operators (on the assumption that the overloaded operators will have the same relationships as the regular operators). Since unary minus can be expressed as a function of binary minus (that is, is equivalent to ), Perl doesn't force you to overload when will do. (Of course, if you've arbitrarily defined binary minus to divide the second argument by the first, unary minus will be a fine way to throw a divide-by-0 exception.)

Concatenation via the operator can be autogenerated via the stringification handler (see above).

Logical operator:

If a handler for is not specified, it can be autogenerated using the , , or handler. If you overload the operator, the operator will also trigger whatever behavior you requested. (Remember our little secret?)

You may be surprised at the absence of the other logical operators, but most logical operators can't be overloaded because they short-circuit. They're really control-flow operators that need to be able to delay evaluation of some of their arguments. That's also the reason the operator isn't overloaded.

Bitwise operators:,,,,,

The operator is a unary operator; all the others are binary. Here's how we could overload to do something like :

package ShiftString; use overload '>>' => \&right_shift, '""' => sub { ${ $_[0] } }; sub new { my $class = shift; my $value = shift; return bless \$value => $class; } sub right_shift { my ($x, $y) = @_; my $value = $$x; substr($value, -$y) = ""; return bless \$value => ref($x); } $camel = ShiftString->new("Camel"); $ram = $camel >> 2; print $ram; # Cam
Assignment operators:,,,,,,,,,,,

These assignment operators might change the value of their arguments or leave them as is. The result is assigned to the lefthand operand only if the new value differs from the old one. This allows the same handler to be used to overload both and . Although this is permitted, it is seldom recommended, since by the semantics described later under "When an Overload Handler Is Missing (nomethod and fallback)", Perl will invoke the handler for anyway, assuming hasn't been overloaded directly.

Concatenation () can be autogenerated using stringification followed by ordinary string concatenation. The and operators can be autogenerated from and (or and ).

Handlers implementing and are expected to mutate (alter) their arguments. If you wanted autodecrement to work on letters as well as numbers, you could do that with a handler as follows:

package MagicDec; use overload q(--) => \&decrement, q("") => sub { ${ $_[0] } }; sub new { my $class = shift; my $value = shift; bless \$value => $class; } sub decrement { my @string = reverse split(//, ${ $_[0] } ); my $i; for ($i = 0; $i < @string; $i++ ) { last unless $string[$i] =~ /a/i; $string[$i] = chr( ord($string[$i]) + 25 ); } $string[$i] = chr( ord($string[$i]) - 1 ); my $result = join('', reverse @string); $_[0] = bless \$result => ref($_[0]); } package main; for $normal (qw/perl NZ Pa/) { $magic = MagicDec->new($normal); $magic--; print "$normal goes to $magic\n"; }
That prints out:
perl goes to perk NZ goes to NY Pa goes to Oz
exactly reversing Perl's magical string autoincrement operator.

The operation can be autogenerated using or , and using or . However, this does not trigger the copying behavior that a real operator would. See "The Copy Constructor" later in this chapter.

Comparison operators:,,,,,,,,,,,,,

If is overloaded, it can be used to autogenerate behaviors for , , , , , and . Similarly, if is overloaded, it can be used to autogenerate behaviors for , , , , , and .

Note that overloading won't let you sort objects as easily as you'd like, because what will be compared are the stringified versions of the objects instead of the objects themselves. If that was your goal, you'd want to overload as well.

Mathematical functions:,,,,,,

If is unavailable, it can be autogenerated from or combined with either unary minus or subtraction.

An overloaded can be used to autogenerate missing handlers for unary minus or for the function, which may also be separately overloaded. (Yes, we know that looks like a function, whereas unary minus looks like an operator, but they aren't all that different as far as Perl's concerned.)

Iterative operator:

The handler can be triggered by using either (when it reads from a filehandle, as in ) or (when it is used for fileglobbing, as in ).

package LuckyDraw; use overload '<>' => sub { my $self = shift; return splice @$self, rand @$self, 1; }; sub new { my $class = shift; return bless [@_] => $class; } package main; $lotto = new LuckyDraw 1 .. 51; for (qw(1st 2nd 3rd 4th 5th 6th)) { $lucky_number = <$lotto>; print "The $_ lucky number is: $lucky_number.\n"; } $lucky_number = <$lotto>; print "\nAnd the bonus number is: $lucky_number.\n";
In California, this prints:
The 1st lucky number is: 18 The 2nd lucky number is: 11 The 3rd lucky number is: 40 The 4th lucky number is: 7 The 5th lucky number is: 51 The 6th lucky number is: 33 And the bonus number is: 5
Dereference operators:,,,,

Attempts to dereference scalar, array, hash, subroutine, and glob references can be intercepted by overloading these five symbols.

The online Perl documentation for demonstrates how you can use this operator to simulate your own pseudohashes. Here's a simpler example that implements an object as an anonymous array but permits hash referencing. Don't try to treat it as a real hash; you won't be able to key/value pairs from the object. If you want to combine array and hash notations, use a real pseudohash (as it were).

package PsychoHash; use overload '%{}' => \&as_hash; sub as_hash { my ($x) = shift; return { @$x }; } sub new { my $class = shift; return bless [ @_ ] => $class; } $critter = new PsychoHash( height => 72, weight => 365, type => "camel" ); print $critter->{weight}; # prints 365
Also see Chapter 14, "Tied Variables", for a mechanism to let you redefine basic operations on hashes, arrays, and scalars.

When overloading an operator, try not to create objects with references to themselves. For instance,

use overload '+' => sub { bless [ \$_[0], \$_[1] ] };
This is asking for trouble, since if you say , the result will make a reference to a blessed array reference whose first element is . This is a circular reference, which means that even if you destroy , its memory won't be freed until your process (or interpreter) terminates. See "Garbage Collection, Circular References, and Weak References" in Chapter 8, "References".

Copyright © 2002 O'Reilly & Associates. All rights reserved.

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