overload(3) overload(3)
NAME
overload - Package for overloading perl operations
SYNOPSIS
package SomeThing;
use overload
'+' => \&myadd,
'-' => \&mysub;
# etc
...
package main;
$a = new SomeThing 57;
$b=5+$a;
...
if (overload::Overloaded $b) {...}
...
$strval = overload::StrVal $b;
CAVEAT SCRIPTOR
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.
DESCRIPTION
Declaration of overloaded functions
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
$a++. See the section on MAGIC AUTOGENERATION. (Mathemagical methods
refer to methods triggered by an overloaded mathematical operator.)
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Since overloading respects inheritance via the @ISA hierarchy, the above
declaration would also trigger overloading of + and *= in all the
packages which inherit from Number.
Calling Conventions for Binary Operations
The functions specified in the use overload ... directive are called with
three (in one particular case with four, see the section on Last Resort)
arguments. If the corresponding operation is binary, then the first two
arguments are the two arguments of the operation. However, due to
general object calling conventions, the first argument should always be
an object in the package, so in the situation of 7+$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 $a+=7), but
the usual function is called instead. This additional information
can be used to generate some optimizations.
Calling Conventions for Unary Operations
Unary operation are considered binary operations with the second argument
being undef. Thus the functions that overloads {"++"} is called with
arguments ($a,undef,'') when $a++ is executed.
Overloadable Operations
The following symbols can be specified in use overload:
⊕ 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.
⊕ Comparison operations
"<", "<=", ">", ">=", "==", "!=", "<=>",
"lt", "le", "gt", "ge", "eq", "ne", "cmp",
If the corresponding "spaceship" variant is available, it can be
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used to substitute for the missing operation. During sorting
arrays, cmp is used to compare values subject to use overload.
⊕ 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+".
⊕ Increment and decrement
"++", "--",
If undefined, addition and subtraction methods can be used instead.
These operations are called both in prefix and postfix form.
⊕ 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.
⊕ 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.
⊕ 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.
Inheritance and overloading
Inheritance interacts with overloading in two ways.
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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.
SPECIAL SYMBOLS FOR use overload
Three keys are recognized by Perl that are not covered by the above
description.
Last Resort
"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.
Fallback
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":
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⊕ 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.
⊕ 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.
⊕ defined, but FALSE
No autogeneration is tried. Perl tries to call
"nomethod" value, and if this is missing, raises an
exception.
Note. "fallback" inheritance via @ISA is not carved in stone yet, see the
section on Inheritance and overloading.
Copy Constructor
The value for "=" is a reference to a function with three arguments,
i.e., it looks like the other values in 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.
Example
The actually executed code for
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$a=$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.
MAGIC AUTOGENERATION
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.
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WARNING
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.
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 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.
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. 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.
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(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).
AUTHOR
Ilya Zakharevich <ilya@math.mps.ohio-state.edu>.
DIAGNOSTICS
When Perl is run with the -Do 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).
BUGS
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.
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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.
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