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 $a++. See
MAGIC AUTOGENERATION
. (Mathemagical
methods refer to methods triggered by an overloaded mathematical
operator.)
7+$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:
$a+=7), but the usual function is called instead. This additional
information can be used to generate some optimizations.
$OVERLOAD{"++"} is called with
arguments ($a,undef,'') when $a++ is executed.
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
.
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.
"neg" stands for unary minus. If the method for neg is not
specified, it can be autogenerated using on the method for subtraction.
If undefined, addition and subtraction methods can be used instead. These operations are called both in prefix and postfix form.
If abs is unavailable, it can be autogenerated using methods for <" or <=>" combined with either unary minus or subtraction.
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.
$OVERLOAD{"nomethod"} is a reference to a function of four parameters.
If defined, it is called when the overloading mechanism cannot find a
method for some operation. The first three arguments of this function
coincide with arguments for the corresponding method if it were found, the
fourth argument is the key of %OVERLOAD corresponding to the missing
method. If several methods are tried, the last one is used. Say, 1-$a
can be equivalent to
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.
$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"}:
$OVERLOAD{"nomethod"}; if missing, an exception
will be raised.
$OVERLOAD{"nomethod"}, and if this is missing, raises an exception.
$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
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
$$
b), and only if ++ is expressed via $OPERATOR{'++'} or
$OPERATOR{'+='}. Note that if this operation is expressed via '+',
i.e., as
then
$$
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.
$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:
$a=+$b can use the $OVERLOAD{"+"} method if $OVERLOAD{"+="}
is not defined.
++$a operation can be expressed in terms of $a+=1 or $a+1,
and $a-- in terms of $a-=1 and $a-1.
$a<0 and -$a (or 0-$a).
<=> or cmp:
<, >,<=, >=, ==, != in terms of<=>
lt, gt, le, ge, eq, ne in terms of cmp
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.
(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).
As shipped, %OVERLOAD is not inherited via the @ISA tree. A patch for this is available from the author.
This document is confusing.