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 $a++. See MAGIC AUTOGENERATION. (Mathemagical
methods refer to methods triggered by an overloaded mathematical
operator.)
Since overloading respects inheritance via the @ISA hierarchy, the
above declaration would also trigger overloading of + and *= in
all the packages which inherit from Number.
The functions specified in the use overload ... directive are called
with three (in one particular case with four, see Last Resort)
arguments. If the corresponding operation is binary, then the first
two arguments are the two arguments of the operation. However, due to
general object calling conventions, the first argument should always be
an object in the package, so in the situation of 7+$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:
the current operation is an assignment variant (as in
$a+=7), but the usual function is called instead. This additional
information can be used to generate some optimizations. Compare
Calling Conventions for Mutators.
Unary operation are considered binary operations with the second
argument being undef. Thus the functions that overloads {"++"}
is called with arguments ($a,undef,'') when $a++ is executed.
There is nothing special about these methods. They may change the
value of their arguments, and may leave it as is. The result is going
to be assigned to the value in the left-hand-side if different from
this value.
This allows for the same method to be used as overloaded += and
+. Note that this is allowed, but not recommended, since by the
semantic of Fallback Perl will call the method for + anyway,
if += is not overloaded.
Warning. Due to the presense of assignment versions of operations,
routines which may be called in assignment context may create
self-referential structures. Currently Perl will not free self-referential
structures until cycles are explicitly broken. You may get problems
when traversing your structures too.
Say,
use overload '+' => sub { bless [ \$_[0], \$_[1] ] };
is asking for trouble, since for code $obj += $foo the subroutine
is called as $obj = add($obj, $foo, undef), or $obj = [\$obj,
\$foo]. If using such a subroutine is an important optimization, one
can overload += explicitly by a non-``optimized'' version, or switch
to non-optimized version if not defined $_[2] (see
Calling Conventions for Binary Operations).
Even if no explicit assignment-variants of operators are present in
the script, they may be generated by the optimizer. Say, ",$obj," or
',' . $obj . ',' may be both optimized to
For these operations a substituted non-assignment variant can be called if
the assignment variant is not available. Methods for operations ``+'',
``-'', ``+='', and ``-='' can be called to automatically generate
increment and decrement methods. The operation ``-'' can be used to
autogenerate missing methods for unary minus or abs.
If the corresponding ``spaceship'' variant is available, it can be
used to substitute for the missing operation. During sorting
arrays, cmp is used to compare values subject to use overload.
``neg'' stands for unary minus. If the method for neg is not
specified, it can be autogenerated using the method for
subtraction. If the method for ``!'' is not specified, it can be
autogenerated using the methods for ``bool'', or ``\"\"'', or ``0+''.
If one or two of these operations are not overloaded, the remaining ones can
be used instead. bool is used in the flow control operators
(like while) and for the ternary ``?:'' operation. These functions can
return any arbitrary Perl value. If the corresponding operation for this value
is overloaded too, that operation will be called again with this value.
If not overloaded, the argument will be converted to a filehandle or
glob (which may require a stringification). The same overloading
happens both for the read-filehandle syntax <$var> and
globbing syntax <${var}>.
If not overloaded, the argument will be dereferenced as is, thus
should be of correct type. These functions should return a reference
of correct type, or another object with overloaded dereferencing.
Any class derived from an overloaded class is also overloaded. The
set of overloaded methods is the union of overloaded methods of all
the ancestors. If some method is overloaded in several ancestor, then
which description will be used is decided by the usual inheritance
rules:
If A inherits from B and C (in this order), B overloads
+ with \&D::plus_sub, and C overloads + by "plus_meth",
then the subroutine D::plus_sub will be called to implement
operation + for an object in package A.
Note that since the value of the fallback key is not a subroutine,
its inheritance is not governed by the above rules. In the current
implementation, the value of fallback in the first overloaded
ancestor is used, but this is accidental and subject to change.
"nomethod" should be followed by a reference to a function of four
parameters. If defined, it is called when the overloading mechanism
cannot find a method for some operation. The first three arguments of
this function coincide with the arguments for the corresponding method if
it were found, the fourth argument is the symbol
corresponding to the missing method. If several methods are tried,
the last one is used. Say, 1-$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.
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":
undef
Perl tries to use a
substituted method (see 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.
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 $$b). This is only
done if ++ is expressed via a method for '++' or '+=' (or
nomethod). Note that if this operation is expressed via '+'
a nonmutator, i.e., as in
$a=$b;
$a=$a+1;
then $a does not reference a new copy of $$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.
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:
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.
For some application Perl parser mangles constants too much. It is possible
to hook into this process via overload::constant() and overload::remove_constant()
functions.
These functions take a hash as an argument. The recognized keys of this hash
are
to overload constant pieces of regular expressions.
The corresponding values are references to functions which take three arguments:
the first one is the initial string form of the constant, the second one
is how Perl interprets this constant, the third one is how the constant is used.
Note that the initial string form does not
contain string delimiters, and has backslashes in backslash-delimiter
combinations stripped (thus the value of delimiter is not relevant for
processing of this string). The return value of this function is how this
constant is going to be interpreted by Perl. The third argument is undefined
unless for overloaded q- and qr- constants, it is q in single-quote
context (comes from strings, regular expressions, and single-quote HERE
documents), it is tr for arguments of tr/y operators,
it is s for right-hand side of s-operator, and it is qq otherwise.
Since an expression "ab$cd,," is just a shortcut for 'ab' . $cd . ',,',
it is expected that overloaded constant strings are equipped with reasonable
overloaded catenation operator, otherwise absurd results will result.
Similarly, negative numbers are considered as negations of positive constants.
Note that it is probably meaningless to call the functions overload::constant()
and overload::remove_constant() from anywhere but import() and unimport() methods.
From these methods they may be called as
sub import {
shift;
return unless @_;
die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant';
overload::constant integer => sub {Math::BigInt->new(shift)};
}
BUGS Currently overloaded-ness of constants does not propagate
into eval '...'.
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
