overload(3)
NNAAMMEE
overload - Package for overloading perl operations
SSYYNNOOPPSSIISS
package SomeThing;
use overload
'+' => \&myadd,
'-' => \&mysub;
# etc
...
package main;
$a = new SomeThing 57;
$b=5+$a;
...
if (overload::Overloaded $b) {...}
...
$strval = overload::StrVal $b;
CCAAVVEEAATT SSCCRRIIPPTTOORR
Overloading of operators is a subject not to be taken
lightly. Neither its precise implementation, syntax, nor
semantics are 100% endorsed by Larry Wall. So any of
these may be changed at some point in the future.
DDEESSCCRRIIPPTTIIOONN
DDeeccllaarraattiioonn ooff oovveerrllooaaddeedd ffuunnccttiioonnss
The compilation directive
package Number;
use overload
"+" => \&add,
"*=" => "muas";
declares function Number::add() for addition, and method
muas() in the "class" Number (or one of its base classes)
for the assignment form *= of multiplication.
Arguments of this directive come in (key, value) pairs.
Legal values are values legal inside a &{ ... } call, so
the name of a subroutine, a reference to a subroutine, or
an anonymous subroutine will all work. Note that values
specified as strings are interpreted as methods, not
subroutines. Legal keys are listed below.
The subroutine add will be called to execute $a+$b if $a
is a reference to an object blessed into the package
Number, or if $a is not an object from a package with
defined mathemagic addition, but $b is a reference to a
Number. It can also be called in other situations, like
$a+=7, or $a++. See the section on MAGIC AUTOGENERATION.
(Mathemagical methods refer to methods triggered by an
overloaded mathematical operator.)
Since overloading respects inheritance via the @ISA
hierarchy, the above declaration would also trigger
overloading of + and *= in all the packages which inherit
from Number.
CCaalllliinngg CCoonnvveennttiioonnss ffoorr BBiinnaarryy OOppeerraattiioonnss
The functions specified in the use overload ... directive
are called with three (in one particular case with four,
see the section on Last Resort) arguments. If the
corresponding operation is binary, then the first two
arguments are the two arguments of the operation.
However, due to general object calling conventions, the
first argument should always be an object in the package,
so in the situation of 7+$a, the order of the arguments is
interchanged. It probably does not matter when
implementing the addition method, but whether the
arguments are reversed is vital to the subtraction method.
The method can query this information by examining the
third argument, which can take three different values:
FALSE the order of arguments is as in the current
operation.
TRUE the arguments are reversed.
undef the current operation is an assignment variant (as
in $a+=7), but the usual function is called
instead. This additional information can be used
to generate some optimizations.
CCaalllliinngg CCoonnvveennttiioonnss ffoorr UUnnaarryy OOppeerraattiioonnss
Unary operation are considered binary operations with the
second argument being undef. Thus the functions that
overloads {"++"} is called with arguments ($a,undef,'')
when $a++ is executed.
OOvveerrllooaaddaabbllee OOppeerraattiioonnss
The following symbols can be specified in use overload:
+o Arithmetic operations
"+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=",
"**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=",
For these operations a substituted non-assignment
variant can be called if the assignment variant is
not available. Methods for operations "+", "-",
"+=", and "-=" can be called to automatically
generate increment and decrement methods. The
operation "-" can be used to autogenerate missing
methods for unary minus or abs.
+o Comparison operations
"<", "<=", ">", ">=", "==", "!=", "<=>",
"lt", "le", "gt", "ge", "eq", "ne", "cmp",
If the corresponding "spaceship" variant is
available, it can be used to substitute for the
missing operation. During sorting arrays, cmp is
used to compare values subject to use overload.
+o Bit operations
"&", "^", "|", "neg", "!", "~",
"neg" stands for unary minus. If the method for neg
is not specified, it can be autogenerated using the
method for subtraction. If the method for "!" is not
specified, it can be autogenerated using the methods
for "bool", or "\"\"", or "0+".
+o Increment and decrement
"++", "--",
If undefined, addition and subtraction methods can be
used instead. These operations are called both in
prefix and postfix form.
+o Transcendental functions
"atan2", "cos", "sin", "exp", "abs", "log", "sqrt",
If abs is unavailable, it can be autogenerated using
methods for "<" or "<=>" combined with either unary
minus or subtraction.
+o Boolean, string and numeric conversion
"bool", "\"\"", "0+",
If one or two of these operations are unavailable,
the remaining ones can be used instead. bool is used
in the flow control operators (like while) and for
the ternary "?:" operation. These functions can
return any arbitrary Perl value. If the
corresponding operation for this value is overloaded
too, that operation will be called again with this
value.
+o Special
"nomethod", "fallback", "=",
see the section on SPECIAL SYMBOLS FOR use overload.
See the section on Fallback for an explanation of when a
missing method can be autogenerated.
IInnhheerriittaannccee aanndd oovveerrllooaaddiinngg
Inheritance interacts with overloading in two ways.
Strings as values of use overload directive
If value in
use overload key => value;
is a string, it is interpreted as a method name.
Overloading of an operation is inherited by derived
classes
Any class derived from an overloaded class is also
overloaded. The set of overloaded methods is the
union of overloaded methods of all the ancestors. If
some method is overloaded in several ancestor, then
which description will be used is decided by the
usual inheritance rules:
If A inherits from B and C (in this 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 since the value of the fallback key is not a
subroutine, its inheritance is not governed by the above
rules. In the current implementation, the value of
fallback in the first overloaded ancestor is used, but
this is accidental and subject to change.
SSPPEECCIIAALL SSYYMMBBOOLLSS FFOORR uussee oovveerrllooaadd
Three keys are recognized by Perl that are not covered by
the above description.
LLaasstt RReessoorrtt
"nomethod" should be followed by a reference to a function
of four parameters. If defined, it is called when the
overloading mechanism cannot find a method for some
operation. The first three arguments of this function
coincide with the arguments for the corresponding method
if it were found, the fourth argument is the symbol
corresponding to the missing method. If several methods
are tried, the last one is used. Say, 1-$a can be
equivalent to
&nomethodMethod($a,1,1,"-")
if the pair "nomethod" => "nomethodMethod" was specified
in the use overload directive.
If some operation cannot be resolved, and there is no
function assigned to "nomethod", then an exception will be
raised via die()-- unless "fallback" was specified as a
key in use overload directive.
FFaallllbbaacckk
The key "fallback" governs what to do if a method for a
particular operation is not found. Three different cases
are possible depending on the value of "fallback":
+o undef Perl tries to use a substituted method
(see the section on MAGIC AUTOGENERATION).
If this fails, it then tries to calls
"nomethod" value; if missing, an exception
will be raised.
+o TRUE The same as for the undef value, but no
exception is raised. Instead, it silently
reverts to what it would have done were
there no use overload present.
+o defined, but FALSE
No autogeneration is tried. Perl tries to
call "nomethod" value, and if this is
missing, raises an exception.
NNoottee.. "fallback" inheritance via @ISA is not carved in
stone yet, see the section on Inheritance and overloading.
CCooppyy CCoonnssttrruuccttoorr
The value for "=" is a reference to a function with three
arguments, i.e., it looks like the other values in use
overload. However, it does not overload the Perl
assignment operator. This would go against Camel hair.
This operation is called in the situations when a mutator
is applied to a reference that shares its object with some
other reference, such as
$a=$b;
$a++;
To make this change $a and not change $b, a copy of $$a is
made, and $a is assigned a reference to this new object.
This operation is done during execution of the $a++, and
not during the assignment, (so before the increment $$a
coincides with $$b). This is only done if ++ is expressed
via a method for '++' or '+='. Note that if this
operation is expressed via '+' a nonmutator, i.e., as in
$a=$b;
$a=$a+1;
then $a does not reference a new copy of $$a, since $$a
does not appear as lvalue when the above code is executed.
If the copy constructor is required during the execution
of some mutator, but a method for '=' was not specified,
it can be autogenerated as a string copy if the object is
a plain scalar.
EExxaammppllee
The actually executed code for
$a=$b;
Something else which does not modify $a or $b....
++$a;
may be
$a=$b;
Something else which does not modify $a or $b....
$a = $a->clone(undef,"");
$a->incr(undef,"");
if $b was mathemagical, and '++' was overloaded with
\&incr, '=' was overloaded with \&clone.
MMAAGGIICC AAUUTTOOGGEENNEERRAATTIIOONN
If a method for an operation is not found, and the value
for "fallback" is TRUE or undefined, Perl tries to
autogenerate a substitute method for the missing operation
based on the defined operations. Autogenerated method
substitutions are possible for the following operations:
Assignment forms of arithmetic operations
$a+=$b can use the method for "+" if the
method for "+=" is not defined.
Conversion operations
String, numeric, and boolean conversion
are calculated in terms of one another if
not all of them are defined.
Increment and decrement
The ++$a operation can be expressed in
terms of $a+=1 or $a+1, and $a-- in terms
of $a-=1 and $a-1.
abs($a) can be expressed in terms of $a<0 and -$a
(or 0-$a).
Unary minus can be expressed in terms of subtraction.
Negation ! and not can be expressed in terms of
boolean conversion, or string or numerical
conversion.
Concatenation can be expressed in terms of string
conversion.
Comparison operations
can be expressed in terms of its
"spaceship" counterpart: either <=> or
cmp:
<, >, <=, >=, ==, != in terms of <=>
lt, gt, le, ge, eq, ne in terms of cmp
Copy operator can be expressed in terms of an assignment
to the dereferenced value, if this value
is a scalar and not a reference.
WWAARRNNIINNGG
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.
RRuunn--ttiimmee OOvveerrllooaaddiinngg
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.
PPuubblliicc ffuunnccttiioonnss
Package overload.pm provides the following public
functions:
overload::StrVal(arg)
Gives string value of arg as in absence of stringify
overloading.
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.
IIMMPPLLEEMMEENNTTAATTIIOONN
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.
However, this invalidation remains unprocessed until the
next blessing into the package. Hence if you want to
change overloading structure dynamically, you'll need an
additional (fake) blessing to update the table.
(Every SVish thing has a magic queue, and magic is an
entry in that queue. This is how a single variable may
participate in multiple forms of magic simultaneously.
For instance, environment variables regularly have two
forms at once: their %ENV magic and their taint magic.
However, the magic which implements overloading is applied
to the stashes, which are rarely used directly, thus
should not slow down Perl.)
If an object belongs to a package using overload, it
carries a special flag. Thus the only speed penalty
during arithmetic operations without overloading is the
checking of this flag.
In fact, if use overload is not present, there is almost
no overhead for overloadable operations, so most programs
should not suffer measurable performance penalties. A
considerable effort was made to minimize the overhead when
overload is used in some package, but the arguments in
question do not belong to packages using overload. When
in doubt, test your speed with use overload and without
it. So far there have been no reports of substantial
speed degradation if Perl is compiled with optimization
turned on.
There is no size penalty for data if overload is not used.
The only size penalty if overload is used in some package
is that all the packages acquire a magic during the next
blessing into the package. This magic is three-words-long
for packages without overloading, and carries the cache
tabel if the package is overloaded.
Copying ($a=$b) is shallow; however, a one-level-deep
copying is carried out before any operation that can imply
an assignment to the object $a (or $b) refers to, like
$a++. You can override this behavior by defining your own
copy constructor (see the section on Copy Constructor).
It is expected that arguments to methods that are not
explicitly supposed to be changed are constant (but this
is not enforced).
AAUUTTHHOORR
Ilya Zakharevich lt;ilya@math.mps.ohio-state.edu.
DDIIAAGGNNOOSSTTIICCSS
When Perl is run with the --DDoo switch or its equivalent,
overloading induces diagnostic messages.
Using the m command of Perl debugger (see the perldebug
manpage) 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).
BBUUGGSS
Because it is used for overloading, the per-package hash
%OVERLOAD now has a special meaning in Perl. The symbol
table is filled with names looking like line-noise.
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.
This document is confusing.