overload(3perl)

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version 5.44.0 · 14 versions
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overload(3perl) — Perl Programmers Reference Guide 🫶

✨ 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 💖

okay so bestie, this pragma lets you overload Perl's operators for a class. If you wanna overload built-in functions instead, go peek at "Overriding Built-in Functions" in perlsub. That's a different vibe.

🦋 Fundamentals 🦋

🎀 Declaration 🎀

The arguments you pass to use overload are (key, value) pairs. For the full list of legal keys, scroll down to "Overloadable Operations".

Operator implementations — the values — can be subroutines, references to subroutines, or anonymous subroutines. Basically anything legal inside a &{ ... } call. Values given as strings are treated as method names. So this:

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

declares that subtraction is handled by method minus() in class "Number" (or one of its base classes), and that Number::muas() is used for the assignment form of multiplication, "*=". It also defines an anonymous subroutine for stringification: this gets called whenever an object blessed into package "Number" is used in a string context (this subroutine might, for example, return the number as a Roman numeral). No cap.

✨ Calling Conventions and Magic Autogeneration ✨

Here's a sample implementation of minus() — it assumes "Number" objects are just blessed references to scalars — that shows off 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, especially "nomethod").

The first one 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 make sure the first argument ($self) is an object implementing the overloaded operation, matching 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 that were not 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 "-=". So:

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

Notice the "undef"s: when autogeneration makes the method for a standard operator that doesn't change either of its operands — like "-" — get used to implement an operator that does change 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 it gives the subroutine a chance to change its behaviour in these cases.

In all the examples above, minus() only has to return the result of the subtraction: Perl takes care of assigning 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 for implementations of "++" and "--": those are expected to modify their operand. A fitting 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 means the caller is expecting numeric behaviour. The fourth argument will be "undef", because 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. Check this code:

$a = $b;
--$a;

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

$a == $b - 1

But an object 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 $b referring to the same object data. So you might expect the operation "--$a" to decrement $b as well as $a. However, that 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 example above, 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. So the object metaphor is preserved as far as possible, while mathemagical operations still work according to the arithmetic metaphor. Iconic.

Note: the paragraph right before this 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 you can put in the use overload directive is 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 that aren't obvious from this hash, are in the notes below. This list is subject to growth over time. Slay.

A warning is issued if you try to register an operator that isn't found above. Noted.

  • "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 so it doesn't get confused with binary "-". Smart.
  • "++", "--" — Assuming they 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 that involves using tie(), not overload — see "tie" in perlfunc and the "COOKBOOK" examples below.

The subroutine for the assignment variant of an operator only has 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 that's 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 "&" while 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 you 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.

Okay so when you're in a context like "$obj + 1", Perl is gonna call $obj's own implementation of "+" — it does not convert $obj to a number using the numify method '0+'. (The one little exception? 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. And if the corresponding operation for that value is overloaded too? Bestie, the operation just gets called again with this value. 🔁

And as a special case — if the overload returns the object itself, then it will be used directly. But an overloaded conversion returning the object is probably a bug, because you're likely to get something that looks like "YourPackage=HASH(0x8172b34)". 💀 Not the vibe.

  • 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 — these iconic ones:
${}  @{}  %{}  &{}  *{}

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 only gets called right before a call to a function that's declared to implement a mutator. Like, if ++$b; in the code above goes through a method declared for the key ++ (or nomethod, which gets ++ handed to it as the fourth argument) or, via autogeneration, +=. It does not get called if the increment happens through a call to the method for +, because in the equivalent code
    $a = $b;
    $b = $b + 1;

the data that $a points at is literally unchanged by assigning $b a reference to new object data. 💅

  • The copy constructor is skipped if Perl figures out it's unnecessary because literally nothing else holds a reference to the data being modified. Lowkey efficient of it. ✨
  • If fallback is undefined or TRUE then a copy constructor can be autogenerated — but only for objects based on scalars, bestie. In every other case you gotta define it explicitly yourself. Where an object's data is stored as, say, an array of scalars, something like this might be the move:
    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 plain assignment — aka assigning the object reference. Which in effect disables the whole 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. King of vibes, honestly.)

🧠 How Perl Chooses an Operator Implementation 🧠

Okay so, big questions: which gets checked first, nomethod or fallback? And if the two operands of an operator are different types and both overload the operator, whose implementation wins? Here's the tea, the precedence rules, in order:

  1. If the first operand has declared a subroutine to overload the operator, use that implementation. Periodt.
  2. Otherwise, if fallback is TRUE or undefined for the first operand, see if the rules for autogeneration let another of its operators be used instead.
  3. Unless the operator is an assignment (+=, -=, etc. — we skip those), repeat step (1) for the second operand.
  4. Repeat step (2) for the second operand.
  5. If the first operand has a nomethod method, use that.
  6. If the second operand has a nomethod method, use that.
  7. If fallback is TRUE for both operands, do the usual operation for the operator, treating the operands as numbers, strings, or booleans as appropriate for the operator (see the note).
  8. Nothing worked — die. 💀

If there's only one operand (or only one operand with overloading), the checks for the other operand above get skipped. Bye.

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

Note on Step 7: some operators have a different semantic depending on the type of their operands. And since there's no way to instruct Perl to treat the operands as, like, 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: even if, for example, cmp should return a blessed reference, the autogenerated lt function will only produce a standard logical value based on the numerical value of the result of cmp. So in particular, a working numeric conversion is needed here (possibly expressed in terms of other conversions). No cap.

Similarly, the .= and x= operators lose their mathemagical properties if the string conversion substitution gets applied. Sad.

And when you chop() a mathemagical object, it gets promoted to a string and its mathemagical properties are gone. 🪦 The same can happen with other operations too, so watch out.

👑 Inheritance and Overloading 👑

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

Method names in the use overload directive

If value in

    use overload key => value;

is a string, it's 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, 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.

Heads up: in Perl versions prior to 5.18, inheritance of the fallback key was not governed by the above rules. Back then, the value of fallback from the first overloaded ancestor was used. This got fixed in 5.18 to follow the usual rules of inheritance. Glow-up. ✨

🔥 Run-time Overloading 🔥

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

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

You can also do

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

though, ngl, using 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're using this to get the address of a reference (handy for checking whether two references point to the same thing), then you're probably 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, and that's enforced if it's an XS method.

🌈 Overloading Constants 🌈

For some applications, the Perl parser mangles constants way too much. Luckily you can hook into this process via the 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, and 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 (so the value of the delimiter isn't 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's q in single-quote context (comes from strings, regular expressions, and single-quote HERE documents), it's tr for arguments of tr/y operators, it's s for right-hand side of s-operator, and it's qq otherwise.

Since an expression ab$cd,, is just a shortcut for 'ab' . $cd . ',,', it's 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: it's 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 🛠️

Everything that follows is subject to change RSN, bestie.

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 can 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, so it should not slow down Perl. Slay.)

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'll be a slight speed penalty on the very first operation afterwards that supports overloading, while the overload tables get updated. If there's no overloading present, the flag is turned off. So the only speed penalty after that is the checking of this flag.

It's expected that arguments to methods that aren't 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 — dual threat, iconic.) 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];
}

okay so now the moment we've ALL been waiting for: you can hit an object with both the array syntax and the hash syntax at the same time. iconic. 💅

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

bestie there are several important things happening in this little example, and you need to clock every one of them. ✨

first of all — the actual type of $bar is a scalar reference</>, and we do not overload the scalar dereference. that's the whole trick. so we can get the actual, non-overloaded contents of $bar by just using $$bar (which is literally what we do inside the functions that overload dereference). and same energy for the object returned by the TIEHASH() method — it's a scalar reference too. 🎀

second — we make a brand new tied hash</> every single time the hash syntax gets used. and the reason that's a win is that it means we never have to worry about a possible reference loop, which would lead to a memory leak. no cap. 💀

Both these problems can be cured, though. say you wanna overload hash dereference on a reference to an object that's implemented as a hash itself — then the only problem you have to dodge is: how do you get at that real hash (as opposed to the virtual hash the overloaded dereference operator is serving you)? here's one possible fetching routine that does exactly that:

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;
}

And if you wanna stop creating a tied hash on every single access (the audacity of that overhead, honestly), you can throw in an extra level of indirection, which gives you 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];
}

So now if $baz is overloaded like this, then $baz is a reference to a reference to the intermediate array — and that intermediate array keeps a reference to an actual array, plus 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 ✨

Drop this in symbolic.pm in your Perl library directory, bestie:

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], and 2 + symbolic->new(3) contains ['+', 2, ['n', 3]]. it's giving abstract syntax tree but make it a whole personality. 🌳

Here's 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]]

okay so note that even though we got this value from a cute 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. love that for us.

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. 💀 not the vibe check we wanted.

bestie. that's the classic cursed recursion. do not pass go. 🚨

so we 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. slay. 🔥

Something is still amiss though 😭 — 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 would 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+' => \&num;

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

All the work of numeric conversion is done in %subr and num(). Of course, %subr is not complete — it contains only operators used in the example below. And here's the extra-credit question for you, bestie: why do we need an explicit recursion in num()? (Answer is at the end of this section. no peeking. 🙈)

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. lowkey minimalism, highkey unfinished. 🫶

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()}";
}

So here's the tea on assignment operators 💅 — the subroutines that implement them are not required to modify their operands (see "Overloadable Operations" up above, bestie), so we do not need anything special to make += and its little entourage work, besides adding these operators to %subr and defining a copy constructor. And why do we need the copy constructor? Because Perl has no way to know that the implementation of += does not mutate the argument — see "Copy Constructor" for the receipts 🔍.

To implement a copy constructor, add '=' => \&cpy to the use overload line, and then this code (it 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're continuing to do things inside nomethod, so add

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

after the first line of wrap(). Ngl this is not a most effective implementation, so 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{'-'};

And that, bestie, finishes the 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 — f in the chat 😭.

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. Delayed gratification, but make it math 💅.

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. Iconic behaviour, honestly 🌈.

🫶 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.

And that's the whole vibe check, bestie — this page came to you from perl v5.44.0, dated 2026-10-17, living its best life as overload(3perl) 🌸. Go overload something cute 💖✨

📦 where this page comes from