PERLOVL

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 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 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: 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 .

See "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 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. What is special about this subroutine is that it should not return a blessed reference into a package (as most other methods are expected to), but rather a freshly made copy of its dereferenced argument (see "BUGS", though). 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 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.

WARNING

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 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 bless ing; so if it is changed dynamically, you'll need an additional fake bless ing 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 "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 <F

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.

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.