package SomeThing;

%OVERLOAD = ( '+' => \&myadd, '-' => \&mysub, # etc ); ...

package main; $a = new SomeThing 57; $b=5+$a;

package Number; %OVERLOAD = ( "+" => \&add, "*=" => "muas" );declares function Number::add() for addition, and method muas() in the "class"

The subroutine **$OVERLOAD{"+"}** will be called to execute **$a+$b** if $a
is a reference to an object blessed into the package **Number**, or $a is
not an object from a package with defined mathemagic addition, but $b is a
reference to a **Number**. It can be called also in other situations, like
**$a+=7**, or **$a++**. See
MAGIC AUTOGENERATION
. (Mathemagical
methods refer to methods triggered by an overloaded mathematical
operator.)

**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.

******Arithmetic operations*-
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 operations "**-**" can be used to autogenerate missing methods for unary minus or abs . ******Comparison operations*-
If the corresponding "spaceship" variant is available, it can be
used to substitute for the missing operation. During
sort
ing
arrays,
**cmp**is used to compare values subject to %OVERLOAD. ******Bit operations*-
"
**neg**" stands for unary minus. If the method for**neg**is not specified, it can be autogenerated using on the method for subtraction. ******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*- If abs is unavailable, it can be autogenerated using methods for "<" or "<=>" combined with either unary minus or subtraction.
******Boolean, string and numeric conversion*-
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*- see SPECIAL KEYS OF %OVERLOAD .

"+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=", "**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=",

"<", "<=", ">", ">=", "==", "!=", "<=>", "lt", "le", "gt", "ge", "eq", "ne", "cmp",

"&", "^", "|", "neg", "!", "~",

"++", "--",

"atan2", "cos", "sin", "exp", "abs", "log", "sqrt",

"bool", "\"\"", "0+",

"nomethod", "fallback", "=",

&{ $Pack::OVERLOAD{"nomethod"} }($a,1,1,"-").If some operation cannot be resolved, and there is no

*** undef**-
Perl tries to use a
substituted method (see
MAGIC AUTOGENERATION
). If this fails, it
then tries to calls
**$OVERLOAD{"nomethod"}**; if missing, an exception will be raised. *** TRUE**- The same as for the undef value, but no exception is raised. Instead, it silently reverts to what it would have done were there no %OVERLOAD is present.
*** defined, but FALSE**-
No autogeneration is tried. Perl tries to call
**$OVERLOAD{"nomethod"}**, and if this is missing, raises an exception.

$a=$b; $a++;To make this change to $a and not to change $b, a freshly made copy of

$a=$b; $a=$a+1;then

If the copy constructor is required during execution of some mutator, but
**$OPERATOR{'='}** is missing, it can be autogenerated as a string
copy if an object of
the package is a plain scalar.

*Assignment forms of arithmetic operations*-
**$a=+$b**can use the**$OVERLOAD{"+"}**method if**$OVERLOAD{"+="}**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($a)**-
can be expressed in terms of C<$a<0> and
**-$a**(or**0-$a**). *Unary minus*- can be expressed in terms of subtraction.
**Concatenation**- can be expressed in terms of string conversion.
*Comparison operations*-
can be expressed in terms of its "spaceship" counterpart: either
C<<=>> or
**cmp**: <, >, <=, >=, ==, != in terms of <=> lt, gt, le, ge, eq, ne in terms of cmp *Copy operator*- can be expressed in terms of assignment to the dereferenced value, if this value is scalar but not a reference.

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

When you chop() a mathemagical object, it becomes promoted to a string first, and its mathemagical qualities is lost. The same can happen with other operations as well.

(Every SVish thing has a magic queue, and a 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.)

If an object belongs to a package with %OVERLOAD, it carries a special flag. Thus the only speed penalty during arithmetic operations without overload is the check of this flag.

In fact, if no %OVERLOAD is ever accessed, there is almost no overhead for overloadable operations, so most programs should not suffer measurable performance penalties. Considerable effort was made minimize overhead when %OVERLOAD is accessed and the current operation is overloadable but the arguments in question do not belong to packages with %OVERLOAD. When in doubt, test your speed with %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 there is no %OVERLOAD.

The copying like **$a=$b** is shallow; however, a one-level-deep
copying is
carried out before any operation that can imply an assignment to the
object $b (or $a) refers to, like **$b++**. You can override this
behavior by defining your copy constructor (see "Copy Constructor").

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

Although the copy constructor is specially designed to make overloading operations with references to an array simpler, as it now works it's useless for this because a subroutine cannot return an array in the same way as it returns a scalar (from the point of view of Perl internals). Expect a change of interface for the copy constructor.

As shipped, %OVERLOAD is not inherited via the @ISA tree. A patch for this is available from the author.

This document is confusing.