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overload

Section: Perl Programmers Reference Guide (3)
Updated: perl 5.004, patch 04
Index 

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 are100% 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() inthe ``class'' Number (or one of its base classes)for the assignment form *= of multiplication.

Arguments of this directive come in (key, value) pairs. Legal valuesare values legal inside a &{ ... } call, so the name of asubroutine, a reference to a subroutine, or an anonymous subroutinewill all work. Note that values specified as strings areinterpreted as methods, not subroutines. Legal keys are listed below.

The subroutine add will be called to execute $a+$b if $ais a reference to an object blessed into the package Number, or if $a isnot an object from a package with defined mathemagic addition, but $b is areference to a Number. It can also be called in other situations, like$a+=7, or $a++. See the section on MAGIC AUTOGENERATION. (Mathemagicalmethods refer to methods triggered by an overloaded mathematicaloperator.)

Since overloading respects inheritance via the @ISA hierarchy, theabove declaration would also trigger overloading of + and *= inall the packages which inherit from Number. 

Calling Conventions for Binary Operations

The functions specified in the use overload ... directive are calledwith three (in one particular case with four, see the section on Last Resort)arguments. If the corresponding operation is binary, then the firsttwo arguments are the two arguments of the operation. However, due togeneral object calling conventions, the first argument should always bean object in the package, so in the situation of 7+$a, theorder of the arguments is interchanged. It probably does not matterwhen implementing the addition method, but whether the argumentsare reversed is vital to the subtraction method. The method canquery this information by examining the third argument, which can takethree different values:
FALSE
the order of arguments is as in the current operation.
TRUE
the arguments are reversed.
undef
the current operation is an assignment variant (as in$a+=7), but the usual function is called instead. This additionalinformation can be used to generate some optimizations.
 

Calling Conventions for Unary Operations

Unary operation are considered binary operations with the secondargument 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 ifthe assignment variant is not available. Methods for operations ``+'',``-'', ``+='', and ``-='' can be called to automatically generateincrement and decrement methods. The operation ``-'' can be used toautogenerate 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 beused to substitute for the missing operation. During sortingarrays, cmp is used to compare values subject to use overload.
* Bit operations

    "&", "^", "|", "neg", "!", "~",
``neg'' stands for unary minus. If the method for neg is notspecified, it can be autogenerated using the method forsubtraction. If the method for ``!'' is not specified, it can beautogenerated using the methods for ``bool'', or ``\"\"'', or ``0+''.
* Increment and decrement

    "++", "--",
If undefined, addition and subtraction methods can beused instead. These operations are called both in prefix andpostfix form.
* Transcendental functions

    "atan2", "cos", "sin", "exp", "abs", "log", "sqrt",
If abs is unavailable, it can be autogenerated using methodsfor ``<'' 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 canbe used instead. bool is used in the flow control operators(like while) and for the ternary ``?:'' operation. These functions canreturn any arbitrary Perl value. If the corresponding operation for this valueis 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.
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. Theset of overloaded methods is the union of overloaded methods of allthe ancestors. If some method is overloaded in several ancestor, thenwhich description will be used is decided by the usual inheritancerules:

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 implementoperation + 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 currentimplementation, the value of fallback in the first overloadedancestor 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 abovedescription. 

Last Resort

"nomethod" should be followed by a reference to a function of fourparameters. If defined, it is called when the overloading mechanismcannot find a method for some operation. The first three arguments ofthis function coincide with the arguments for the corresponding method ifit were found, the fourth argument is the symbolcorresponding 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 theuse overload directive.

If some operation cannot be resolved, and there is no functionassigned 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 particularoperation is not found. Three different cases are possible depending onthe value of "fallback":
* undef
Perl tries to use asubstituted method (see the section on MAGIC AUTOGENERATION). If this fails, itthen tries to calls "nomethod" value; if missing, an exceptionwill 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 overloadpresent.
* 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 stoneyet, see the section on Inheritance and overloading. 

Copy Constructor

The value for "=" is a reference to a function with threearguments, i.e., it looks like the other values in useoverload. However, it does not overload the Perl assignmentoperator. This would go against Camel hair.

This operation is called in the situations when a mutator is appliedto a reference that shares its object with some other reference, suchas

        $a=$b;         $a++;
To make this change $a and not change $b, a copy of $$a is made,and $a is assigned a reference to this new object. This operation isdone during execution of the $a++, and not during the assignment,(so before the increment $$a coincides with $$b). This is onlydone if ++ is expressed via a method for '++' or '+='. Notethat if this operation is expressed via '+' a nonmutator, i.e., asin

        $a=$b;         $a=$a+1;
then $a does not reference a new copy of $$a, since $$a does notappear 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 astring copy if the object is a plain scalar.

Example
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.
 

MAGIC AUTOGENERATION

If a method for an operation is not found, and the value for "fallback" isTRUE or undefined, Perl tries to autogenerate a substitute method forthe missing operation based on the defined operations. Autogenerated methodsubstitutions 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 oneanother 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, orstring 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 thisvalue is a scalar and not a reference.
 

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 thenumerical value of the result of `cmp'. In particular, a workingnumeric conversion is needed in this case (possibly expressed in terms ofother conversions).

Similarly, .= and x= operators lose their mathemagical propertiesif the string conversion substitution is applied.

When you chop() a mathemagical object it is promoted to a string and itsmathemagical properties are lost. The same can happen with otheroperations as well. 

Run-time Overloading

Since all use directives are executed at compile-time, the only way tochange 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 thesymbol table hash for the package. The cache is invalidated duringprocessing of use overload, no overload, new functiondefinitions, and changes in @ISA. However, this invalidation remainsunprocessed until the next blessing into the package. Hence if youwant to change overloading structure dynamically, you'll need anadditional (fake) blessing to update the table.

(Every SVish thing has a magic queue, and magic is an entry in thatqueue. This is how a single variable may participate in multipleforms of magic simultaneously. For instance, environment variablesregularly have two forms at once: their %ENV magic and their taintmagic. However, the magic which implements overloading is applied tothe stashes, which are rarely used directly, thus should not slow downPerl.)

If an object belongs to a package using overload, it carries a specialflag. Thus the only speed penalty during arithmetic operations withoutoverloading is the checking of this flag.

In fact, if use overload is not present, there is almost no overheadfor overloadable operations, so most programs should not suffermeasurable performance penalties. A considerable effort was made tominimize the overhead when overload is used in some package, but thearguments in question do not belong to packages using overload. Whenin doubt, test your speed with use overload and without it. So farthere have been no reports of substantial speed degradation if Perl iscompiled with optimization turned on.

There is no size penalty for data if overload is not used. The onlysize penalty if overload is used in some package is that all thepackages acquire a magic during the next blessing into thepackage. This magic is three-words-long for packages withoutoverloading, 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 theobject $a (or $b) refers to, like $a++. You can override thisbehavior by defining your own copy constructor (see the section on Copy Constructor).

It is expected that arguments to methods that are not explicitly supposedto be changed are constant (but this is not enforced). 

AUTHOR

Ilya Zakharevich <ilyaAATTmath.mps.ohio-state.edu>. 

DIAGNOSTICS

When Perl is run with the -Do switch or its equivalent, overloadinginduces diagnostic messages.

Using the m command of Perl debugger (see the perldebug manpage) one candeduce which operations are overloaded (and which ancestor triggersthis overloading). Say, if eq is overloaded, then the method (eqis shown by debugger. The method () corresponds to the fallbackkey (in fact a presence of this method shows that this package hasoverloading enabled, and it is what is used by the Overloadedfunction). 

BUGS

Because it is used for overloading, the per-package hash %OVERLOAD nowhas a special meaning in Perl. The symbol table is filled with nameslooking like line-noise.

For the purpose of inheritance every overloaded package behaves as iffallback is present (possibly undefined). This may createinteresting effects if some package is not overloaded, but inheritsfrom two overloaded packages.

This document is confusing.

 

Index

NAME
SYNOPSIS
CAVEAT SCRIPTOR
DESCRIPTION
Declaration of overloaded functions
Calling Conventions for Binary Operations
Calling Conventions for Unary Operations
Overloadable Operations
Inheritance and overloading
SPECIAL SYMBOLS FOR use overload
Last Resort
Fallback
Copy Constructor
MAGIC AUTOGENERATION
WARNING
Run-time Overloading
Public functions
IMPLEMENTATION
AUTHOR
DIAGNOSTICS
BUGS
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