PERLOVL(1)PERLOVL(1)NAME
perlovl - perl overloading semantics
SYNOPSIS
package SomeThing;
%OVERLOAD = (
'+' => \&myadd,
'-' => \&mysub,
# etc
);
...
package main;
$a = new SomeThing 57;
$b=5+$a;
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
package Number;
%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. Legal values of this hash array are values legal
inside &{ ... } call, so the name of a subroutine, a reference to a
subroutine, or an anonymous subroutine will all work.
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 $a++. See the section on MAGIC AUTOGENERATION.
(Mathemagical methods refer to methods triggered by an overloaded
mathematical operator.)
Calling Conventions for Binary Operations
The functions in values %OVERLOAD are called with three (in one
particular case with four, see the section on Last Resort) arguments.
If the corresponding operation is binary, then the first two arguments
are the two arguments of the operation. However, due to general object
calling conventions, the first argument should be always an object in
the package, so in the situation of 7+$a, the order of arguments is
interchanged. Most probably it does not matter for implementation of
the addition method, but whether the arguments are reversed is vital
for the subtraction method. The subroutine 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 $OVERLOAD{"++"} is called with arguments
($a,undef,'') when $a++ is executed.
Overloadable Operations
The following keys of %OVERLOAD are recognized:
· 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 operations "-" 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
used to substitute for the missing operation. During sorting
arrays, cmp is used to compare values subject to %OVERLOAD.
· Bit operations
"&", "^", "⎪", "&=", "^=", "⎪=", "neg", "!", "~",
"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
"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 KEYS OF %OVERLOAD.
See the section on Fallback for an explanation of when a missing method
can be autogenerated.
SPECIAL KEYS OF %OVERLOAD
Three keys are recognized by Perl that are not covered by the above
description.
Last Resort
$OVERLOAD{"nomethod"} is a reference to a function of four parameters.
If defined, it is called when the overloading mechanism cannot find a
method for some operation. The first three arguments of this function
coincide with arguments for the corresponding method if it were found,
the fourth argument is the key of %OVERLOAD corresponding to the
missing method. If several methods are tried, the last one is used.
Say, 1-$a can be equivalent to
&{ $Pack::OVERLOAD{"nomethod"} }($a,1,1,"-").
If some operation cannot be resolved, and there is no
$OVERLOAD{"nomethod"}, then an exception will be raised via die()--
unless $OVERLOAD{"fallback"} is true.
Fallback
$OVERLOAD{"fallback"} governs what to do if a method for a particular
operation is not found. Three different cases are possible depending
on value of $OVERLOAD{"fallback"}:
· undef Perl tries to use a substituted method (see the section
on 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.
Copy Constructor
$OVERLOAD{"="} is a reference to a function with three arguments, i.e.,
it looks like a usual value of %OVERLOAD. 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 to $a and not to change $b, a freshly made copy of
$$a is made, and $a is assigned a reference to this object. This
operation is executed during $a++, (so before this $$a coincides with
$$b), and only if ++ is expressed via $OPERATOR{'++'} or
$OPERATOR{'+='}. Note that if this operation is expressed via '+',
i.e., as
$a=$b;
$a=$a+1;
then $$a and $$b do not appear as lvalues.
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.
MAGIC AUTOGENERATION
If a method for an operation is not found, and $OVERLOAD{"fallback"} is
TRUE or undefined, Perl tries to to autogenerate a substitute method
for the missing operation based on defined operations. Autogenerated
method substitutions are possible for the following operations:
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 $a<0 and -$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 <=> 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.
WARNING
The restriction for the comparison operation is that even if, for
example, `cmp' should return a reference to a blessed object, 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 becomes promoted to a string
first, and its mathemagical qualities is lost. The same can happen
with other operations as well.
IMPLEMENTATION
The table of methods for all operations is cached as a magic for the
symbol table hash of the package. It is rechecked for changes of
%OVERLOAD and @ISA only during blessing; so if it is changed
dynamically, you'll need an additional fake blessing to update the
table.
(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 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.
BUGS
Because it's used for overloading, the per-package associative array
%OVERLOAD now has a special meaning in Perl.
As shipped, %OVERLOAD is not inherited via the @ISA tree. A patch for
this is available from the author.
This document is confusing.
3rd Berkeley DistributionPERLOVL(1)