overload(3perl) Perl Programmers Reference Guide overload(3perl)
NAME
overload - Package for overloading Perl operations
SYNOPSIS
package SomeThing;
use overload
'+' => \&myadd,
'-' => \&mysub;
# etc
...
package main;
$a = SomeThing->new( 57 );
$b = 5 + $a;
...
if (overload::Overloaded $b) {...}
...
$strval = overload::StrVal $b;
DESCRIPTION
This pragma allows overloading of Perl's operators for a class. To
overload built-in functions, see "Overriding Built-in Functions" in
perlsub instead.
Fundamentals
Declaration
Arguments of the "use overload" directive are (key, value) pairs. For
the full set of legal keys, see "Overloadable Operations" below.
Operator implementations (the values) can be subroutines, references to
subroutines, or anonymous subroutines - in other words, anything legal
inside a "&{ ... }" call. Values specified as strings are interpreted
as method names. Thus
package Number;
use overload
"-" => "minus",
"*=" => \&muas,
'""' => sub { ...; };
declares that subtraction is to be implemented by method minus() in the
class "Number" (or one of its base classes), and that the function
Number::muas() is to be used for the assignment form of multiplication,
"*=". It also defines an anonymous subroutine to implement
stringification: this is called whenever an object blessed into the
package "Number" is used in a string context (this subroutine might, for
example, return the number as a Roman numeral).
Calling Conventions and Magic Autogeneration
The following sample implementation of minus() (which assumes that
"Number" objects are simply blessed references to scalars) illustrates
the calling conventions:
package Number;
sub minus {
my ($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 exceptions - see below, particularly
"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
"Number"s:
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]}; }
If the "bitwise" feature is enabled (see feature), a fifth TRUE argument
is passed to subroutines handling "&", "|", "^" and "~". This indicates
that the caller is expecting numeric behaviour. The fourth argument
will be "undef", as that position ($_[3]) is reserved for use by
"nomethod".
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;
--$a;
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
%overload::ops:
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. This list is
subject to growth over time.
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
expression.
• 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 below.
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)".
qr
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 "<${var}>".
• 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 filetest.
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:
$obj->match([1,2,3],0);
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:
$obj->match(1,0);
$obj->match(2,0);
$obj->match(3,0);
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
operator.
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"
"nomethod"
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. If the "bitwise" feature is
enabled (see feature), a fifth TRUE argument is passed to subroutines
handling "&", "|", "^" and "~" to indicate that the caller is expecting
numeric behaviour.
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".
"fallback"
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) ...
++$b;
would be executed in a manner equivalent to
$a = $b;
# ...
$b = $b->clone(undef, "");
$b->incr(undef, "");
Note:
• 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
modified.
• 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
appropriate:
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
itself.)
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:
overload::StrVal(arg)
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 builtin::refaddr() or
Scalar::Util::refaddr(), which are faster.
overload::Overloaded(arg)
Returns true if "arg" is subject to overloading of some operations.
overload::Method(obj,op)
Returns "undef" or a reference to the method that implements "op".
Such a method always takes three arguments, which will be enforced
if it is an XS method.
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
as
sub import {
shift;
return unless @_;
die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant';
overload::constant integer => sub {Math::BigInt->new(shift)};
}
IMPLEMENTATION
What follows is subject to change RSN.
The table of methods for all operations is cached in magic for the
symbol table hash for the package. The cache is invalidated during
processing of "use overload", "no 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
Perl.)
If a package uses overload, it carries a special flag. This flag is
also set when new functions 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).
COOKBOOK
Please add examples to what follows!
Two-face Scalars
Put this in two_face.pm in your Perl library directory:
package two_face; # Scalars with separate string and
# numeric values.
sub new { my $p = shift; bless [@_], $p }
use overload '""' => \&str, '0+' => \&num, fallback => 1;
sub num {shift->[1]}
sub str {shift->[0]}
Use it as follows:
require two_face;
my $seven = 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;
\%h;
}
sub TIEHASH { my $p = shift; bless \ shift, $p }
my %fields;
my $i = 0;
$fields{$_} = $i++ foreach qw{zero one two three};
sub STORE {
my $self = ${shift()};
my $key = $fields{shift()};
defined $key or die "Out of band access";
$$self->[$key] = shift;
}
sub FETCH {
my $self = ${shift()};
my $key = $fields{shift()};
defined $key or die "Out of band access";
$$self->[$key];
}
Now one can access an object using both the array and hash syntax:
my $bar = two_refs->new(3,4,5,6);
$bar->[2] = 11;
$bar->{two} == 11 or die 'bad hash fetch';
Note several important features of this example. First of all, the
actual type of $bar is a scalar reference, and we do not overload the
scalar dereference. Thus we can get the actual non-overloaded contents
of $bar by just using $$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
$out;
}
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:
package two_refs1;
use overload
'%{}' => sub { ${shift()}->[1] },
'@{}' => sub { ${shift()}->[0] };
sub new {
my $p = shift;
my $a = [@_];
my %h;
tie %h, $p, $a;
bless \ [$a, \%h], $p;
}
sub gethash {
my %h;
my $self = shift;
tie %h, ref $self, $self;
\%h;
}
sub TIEHASH { my $p = shift; bless \ shift, $p }
my %fields;
my $i = 0;
$fields{$_} = $i++ foreach qw{zero one two three};
sub STORE {
my $a = ${shift()};
my $key = $fields{shift()};
defined $key or die "Out of band access";
$a->[$key] = shift;
}
sub FETCH {
my $a = ${shift()};
my $key = $fields{shift()};
defined $key or die "Out of band access";
$a->[$key];
}
Now if $baz is overloaded like this, then $baz is a 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->[$ind]" involves no overloaded
operations.
Symbolic Calculator
Put this in symbolic.pm in your Perl library directory:
package symbolic; # Primitive symbolic calculator
use overload nomethod => \&wrap;
sub new { shift; bless ['n', @_] }
sub wrap {
my ($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
methods.
Something is still amiss: consider the loop variable $cnt of the script.
It was a number, not an object. We cannot make this value of type
"symbolic", since then the loop will not terminate.
Indeed, to terminate the cycle, the $cnt should become false. However,
the operator "bool" for checking falsity is overloaded (this time via
overloaded ""), and returns a long string, thus any object of type
"symbolic" is true. To overcome this, we need a way to compare an
object to 0. In fact, it is easier to write a numeric conversion
routine.
Here is the text of symbolic.pm with such a routine added (and slightly
modified str()):
package symbolic; # Primitive symbolic calculator
use overload
nomethod => \&wrap, '""' => \&str, '0+' => \#
sub new { shift; bless ['n', @_] }
sub wrap {
my ($obj, $other, $inv, $meth) = @_;
($obj, $other) = ($other, $obj) if $inv;
bless [$meth, $obj, $other];
}
sub str {
my ($meth, $a, $b) = @{+shift};
$a = 'u' unless defined $a;
if (defined $b) {
"[$meth $a $b]";
} else {
"[$meth $a]";
}
}
my %subr = (
n => sub {$_[0]},
sqrt => sub {sqrt $_[0]},
'-' => sub {shift() - shift()},
'+' => sub {shift() + shift()},
'/' => sub {shift() / shift()},
'*' => sub {shift() * shift()},
'**' => sub {shift() ** shift()},
);
sub num {
my ($meth, $a, $b) = @{+shift};
my $subr = $subr{$meth}
or die "Do not know how to ($meth) in symbolic";
$a = $a->num if ref $a eq __PACKAGE__;
$b = $b->num if ref $b eq __PACKAGE__;
$subr->($a,$b);
}
All the work of numeric conversion is done in %subr and num(). Of
course, %subr is not complete, it contains only operators used in the
example below. Here is the extra-credit question: why do we need an
explicit recursion in num()? (Answer is at the end of this section.)
Use this module like this:
require symbolic;
my $iter = 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
pi=3.182598
The above module is very primitive. It does not implement mutator
methods ("++", "-=" and so on), does not do deep copying (not required
without mutators!), and implements only those arithmetic operations
which are used in the example.
To implement most arithmetic operations is easy; one should just use the
tables of operations, and change the code which fills %subr to
my %subr = ( 'n' => sub {$_[0]} );
foreach my $op (split " ", $overload::ops{with_assign}) {
$subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
}
my @bins = qw(binary 3way_comparison num_comparison str_comparison);
foreach my $op (split " ", "@overload::ops{ @bins }") {
$subr{$op} = eval "sub {shift() $op shift()}";
}
foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
print "defining '$op'\n";
$subr{$op} = eval "sub {$op shift()}";
}
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 { $_[0] = bless ['++', shift, 1]; }
instead.
As a final remark, note that one can fill %subr by
my %subr = ( 'n' => sub {$_[0]} );
foreach my $op (split " ", $overload::ops{with_assign}) {
$subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
}
my @bins = qw(binary 3way_comparison num_comparison str_comparison);
foreach my $op (split " ", "@overload::ops{ @bins }") {
$subr{$op} = eval "sub {shift() $op shift()}";
}
foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
$subr{$op} = eval "sub {$op shift()}";
}
$subr{'++'} = $subr{'+'};
$subr{'--'} = $subr{'-'};
This finishes implementation of a primitive symbolic 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
num().
If you wonder why defaults for conversion are different for str() and
num(), note how easy it was to write the symbolic calculator. This
simplicity is due to an 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 method
sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; }
Now
my ($a, $b);
symbolic->vars($a, $b);
my $c = sqrt($a**2 + $b**2);
$a = 3; $b = 4;
printf "c5 %s=%f\n", $c, $c;
$a = 12; $b = 5;
printf "c13 %s=%f\n", $c, $c;
shows that the numeric value of $c follows changes to the values of $a
and $b.
AUTHOR
Ilya Zakharevich <ilya@math.mps.ohio-state.edu>.
SEE ALSO
The "overloading" pragma can be used to enable or disable overloaded
operations within a lexical scope - see overloading.
DIAGNOSTICS
When Perl is run with the -Do switch or its equivalent, overloading
induces diagnostic messages.
Using the "m" command of Perl debugger (see perldebug) one can deduce
which operations are overloaded (and which ancestor triggers this
overloading). Say, if "eq" is 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?
BUGS AND PITFALLS
• 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 v5.40.1 2026-08-30 overload(3perl)
Generated by dwww version 1.16 on Sat Oct 3 07:40:03 CEST 2026.