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8.169. overload
Lets you
substitute class methods or your own subroutines for standard Perl
operators. For example, the code:
package Number;
use overload
"+" => \add,
"*=" => "muas";
declares function add for addition and method
muas in the Number class (or one of its base
classes) for the assignment form *= of
multiplication.
Arguments to use overload are key/value pairs, in
which the key is the operation being overloaded, and the value is the
function or method that is to be substituted. Legal values are values
permitted inside a &{ ... } call, so a
subroutine name, a subroutine reference, or an anonymous subroutine
are all legal. Legal keys (overloadable operations) are:
Type | Operations |
|---|---|
Arithmetic | + - * / % ** << >> x . += -= *= /= %= **= <<= >>= x= .= |
Comparison | < <= > >= == != <=> lt le gt ge eq ne cmp |
Bit and unary | % ^ | neg ! ~ |
Increment, decrement | ++ - |
Transcendental | atan2 cos sin exp abs log sqrt |
Boolean, string, numeric conversion | bool " 0+ |
Special | nomethod fallback = |
directive are typically called with three arguments. If the
corresponding operation is binary, then the first two arguments are
the two arguments of the operation. However, the first argument
should always be an object in the package, so in some cases, the
order of the arguments will be interchanged before the method is
called. The third argument provides information on the order and can
have these values:
False (0)
The order of arguments is as in the current operation.True (1)
The arguments are reversed.Undefined (undef)
The current operation is an assignment variant, but the usual
function is called instead.
Unary operations are considered binary operations with the second
argument undefined.
The special nomethod key should be followed by a
reference to a function of four parameters and called when the
overloading mechanism cannot find a method for an operation. The
first three arguments are the arguments for the corresponding method
if it was found; the fourth argument is the symbol corresponding to
the missing method. If several methods are tried, the last one is
used.
For example, 1-$a can be equivalent to: &nomethodMethod($a, 1, 1, "-")
if the pair "nomethod" => "nomethodMethod" was
specified in the use overload directive.
The special fallback key governs what to do if a
method for a particular operation is not found. There are three
possible cases, depending on the value associated with the
fallback key:
Undefined
Tries to use a substituted method. If that fails, it tries to call
the method specified for nomethod; if that also
fails, an exception is raised.
True
The same as undefined, but no exception is raised. Instead, Perl
silently reverts to non-overloaded operation.
Defined, but false
Tries to call the method specified for nomethod;
if that fails, an exception is raised.
The overload module provides the following public functions.
| Method |
overload::Method(obj, op)
Returns the undefined value or a reference to the method that
implements op.
| Overloaded |
overload::Overloaded(arg)
Returns true if arg is subject to
overloading of some operations.
| StrVal |
overload::StrVal(arg)
Gives string value of arg if stringify
overloading is absent.
|  | |
| 8.168. ops |  | 8.170. PerlIO |
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