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{

Table of Contents

  • What is an object, really?
  • What happens when you assign a non-overloaded object?
  • Altering objects
  • Overloading addition
  • Perl's Promise
  • What the heck is a mutator?
  • Overloading a mutator
  • Overloading the copy constructor (`=')
  • Wrapping things up

Intro

The most confusing aspect of using operator overloading in Perl is undoubtedly the operator `='. It is natural to assume, at first glance, that overloading `=' means that you're overloading assignment for the class in question. When that doesn't happen, going to the documentation isn't much better, as overload.pm is quite cryptic on this matter.

The difficult thing to understand about `=' is what it really affects. Invariably, users think that overloading `=' causes your specified code to be called when an overloaded object is assigned to another via `my $newobj = $obj' or a similar construct. It is necessary to think about the process from a Perl point of view to understand why this is not the case.

What is an object, really?

In Perl, an object is simply a variable, be it scalar, array, hash, or something more exotic, that has been `bless'ed (note to self: try not to use the word "thingy" in this tutorial). We refer to this variable by holding a reference to it. If we do this:

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We've created an object of type MyPackage. However, $a itself is not the object, $a is not a member of MyPackage, and it hasn't been blessed. It simply points to the object we created.

/---\ /--------------------------------\ | a | ------> | instance of MyPackage: foo = 1 | \---/ \--------------------------------/

What happens when you assign a non-overloaded object?

And now, we create a new variable $b, and assign $a to it:

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What has happened here? Remember that $a and $b are references, and they're just pointing to other things. If $a was a reference to an unblessed array, and we executed the above assignment, would we create a brand new array for $b to point to? Of course not. $b would be pointing to the same array. So in our case, all that has changed is:

/---\ /--------------------------------\ | a | ------> | instance of MyPackage: foo = 1 | \---/ /--> \--------------------------------/ | /---\ | | b | ---/ \---/

We now have two variables pointing to the same object. Things are as they should be.

Altering objects

Ah, but what happens if we try to change one of them? Well, it depends on how you try to change it. If you treat it like a normal reference to an object, overloading has nothing to do with it. If you go and modify `$a->{foo}', it will of course be reflected when you ask for `$b->{foo}'. Just like always, it's your responsibility to create a new copy if you want one. So the only question is what happens if you try to modify them with an overloaded operation?

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Well, actually, nothing, because we forgot to overload addition, so Perl has no way of knowing how to add a MyPackage.

Overloading addition

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In the add subroutine, we're creating a new instance of MyPackage with its foo attribute initialized to the foo attribute of the first operand plus either the foo attribute of the second operand if it's a MyPackage and the actual value of the second operand if it's not. (In real code we'd probably do a little more error checking to make sure we're not passed, say, an IO::Socket as our second operand).

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This time, it works. If you take a look at $c, it points to a brand new instance of MyPackage.

/---\ /--------------------------------\ | a | ------> | instance of MyPackage: foo = 1 | \---/ /--> \--------------------------------/ | /---\ | | b | ---/ \---/ /---\ /--------------------------------\ | c | ------> | instance of MyPackage: foo = 2 | \---/ \--------------------------------/

So in the case of a binary operation, Perl relies on us to provide the right subroutine. If we'd had our add subroutine return an reference to a URI object, it would have still worked, and $c would now point to that URI object. This would be bizarre, of course, but unfortunately no one has come up with a module such that we could write `no bizarre qw/constructs/', and so Perl would merrily give us back the URI object.

Perl's Promise

So what happens when we use something like `++'? To understand this, we have to understand what happens inside the works when we assign one overloaded object to another, like we did above with `$b = $a'.

When we do a pure assignment, like we discussed above, Perl treats the operands ($b and $a) as normal references and has them point to the same thing. That's all. Well ... almost all. Perl also makes us a "promise".

Dear dlc, If at any point in this program you use a mutator on either $a or $b, I promise to create a new object for $b to point to so the mutator doesn't affect both variables. If you change either $a or $b by normal means before that, this promise is null and void. Your pal, -Perl

Why does it do this? Well, Perl's whole philosophy revolves around DWIM, which stands for Do What IMean. When I assign $a to $b, it doesn't yet know "what I mean" -- am I going to treat $a and $b like usual references, or like references to overloaded objects? It can't tell, so it hedges its bets and waits to see what I'll do with them. If I don't use a mutator, nothing changes. If I do use a mutator, it realizes that I'm using the overloaded properties, and clones me a new object, because if it didn't, and `$a++' also modified what $b was pointing to, that would almost certainly not be "what I mean".

What the heck is a mutator?

A mutator is an operator that modifies one of its operands. Mutators that we can overload include `++', `--', and all of the assignment operators, such as `+=', `&=', etc. Note the subtle difference between an operator that modifies an operand, which is a mutator, and one that sets an operand, which is the assignment operator `='.

So Perl is basically telling us that we can safely use mutators on references to objects, even when two references are pointing to the same object because we've used `=' to assign them.

This may still sound confusing, so let's keep plugging away at our example:

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Scroll up if you don't remember what our data structures looked like before -- $a and $b pointed to the same object, because we assigned $b to $a using `='. Perl remembers this, and so when we try to use the mutator `++' here, it says "Aha! I promised dlc that I'd make this work!"

And it does work, even though we haven't overloaded `++' or `='. Perl translates `$b++' into `$b = $b + 1', because it's smart enough to know how to construct non-overloaded operators from operators we have overloaded. And because we did overload addition, we get a reference to a new MyPackage object back from the call to add(), and it's assigned to $b:

/---\ /--------------------------------\ | a | ------> | instance of MyPackage: foo = 1 | \---/ \--------------------------------/ /---\ /--------------------------------\ | b | ------> | instance of MyPackage: foo = 2 | \---/ \--------------------------------/ /---\ /--------------------------------\ | c | ------> | instance of MyPackage: foo = 2 | \---/ \--------------------------------/

So why would we ever need to overload `=', then, if Perl is smart enough to do all of this for us? Well, when more complicated things happen, Perl won't necessarily be able to figure out a copy constructor (copy constructor is a fancy phrase for "the thing that gets called when Perl has to make good on its promise") and we'll have to write it ourselves.

Overloading a mutator

Let's say we decide that autoincrement for MyPackage has to do something special -- it's going to double the value in foo, rather than add 1 to it.

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Oops -- we get a fatal runtime error:

Operation `=': no method found, argument in overloaded package MyPackage at overload.pl line 26.

So now it's asking for the copy constructor, because we have a custom autoincrement.

Overloading the copy constructor (`=')

We'll add a very simple copy constructor:

[download]

All we're doing here is asking for a new object, which is pretty much exactly what was happening before we added our custom autoincrement. So, we try the above code again:

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Let's stop here and look at our structures after executing that last line of code:

/---\ /--------------------------------\ | a | ------> | instance of MyPackage: foo = 1 | \---/ \--------------------------------/ /---\ /--------------------------------\ | b | ------> | instance of MyPackage: foo = 2 | \---/ \--------------------------------/ /---\ /--------------------------------\ | c | ------> | instance of MyPackage: foo = 2 | \---/ /--> \--------------------------------/ | /---\ | | d | ---/ \---/

And now, we do the increment:

[download]

And voila, our new autoincrement has worked:

/---\ /--------------------------------\ | a | ------> | instance of MyPackage: foo = 1 | \---/ \--------------------------------/ /---\ /--------------------------------\ | b | ------> | instance of MyPackage: foo = 2 | \---/ \--------------------------------/ /---\ /--------------------------------\ | c | ------> | instance of MyPackage: foo = 2 | \---/ \--------------------------------/ /---\ /--------------------------------\ | d | ------> | instance of MyPackage: foo = 4 | \---/ \--------------------------------/

Wrapping things up

What does all of this mean?

  • You rarely need to worry about overloading the copy constructor unless you also overload `++' (or another mutator), which in turn you rarely need to worry about unless you want it to mean something other than += 1 (or the standard meaning for the mutator you've chosen).
  • When you do overload `++' (or some other mutator), you'll need to overload `=' as well, but in general it will be a very straightforward function, doing pretty much exactly what Perl does underneath when you don't overload your mutators but let it derive them for you.
  • The documentation for overload is confusing and oddly ordered. I hope to work on that soon.

-dlc

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