overload.3

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.\" ========================================================================
.\"
.IX Title "overload 3"
.TH overload 3 "2007-12-18" "perl v5.10.0" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
overload \- Package for overloading Perl operations
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\&    package SomeThing;
\&
\&    use overload
\&        \*(Aq+\*(Aq => \e&myadd,
\&        \*(Aq\-\*(Aq => \e&mysub;
\&        # etc
\&    ...
\&
\&    package main;
\&    $a = new SomeThing 57;
\&    $b=5+$a;
\&    ...
\&    if (overload::Overloaded $b) {...}
\&    ...
\&    $strval = overload::StrVal $b;
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
.Sh "Declaration of overloaded functions"
.IX Subsection "Declaration of overloaded functions"
The compilation directive
.PP
.Vb 4
\&    package Number;
\&    use overload
\&        "+" => \e&add,
\&        "*=" => "muas";
.Ve
.PP
declares function \fINumber::add()\fR for addition, and method \fImuas()\fR in
the \*(L"class\*(R" \f(CW\*(C`Number\*(C'\fR (or one of its base classes)
for the assignment form \f(CW\*(C`*=\*(C'\fR of multiplication.
.PP
Arguments of this directive come in (key, value) pairs.  Legal values
are values legal inside a \f(CW\*(C`&{ ... }\*(C'\fR call, so the name of a
subroutine, a reference to a subroutine, or an anonymous subroutine
will all work.  Note that values specified as strings are
interpreted as methods, not subroutines.  Legal keys are listed below.
.PP
The subroutine \f(CW\*(C`add\*(C'\fR will be called to execute \f(CW\*(C`$a+$b\*(C'\fR if \f(CW$a\fR
is a reference to an object blessed into the package \f(CW\*(C`Number\*(C'\fR, or if \f(CW$a\fR is
not an object from a package with defined mathemagic addition, but \f(CW$b\fR is a
reference to a \f(CW\*(C`Number\*(C'\fR.  It can also be called in other situations, like
\&\f(CW\*(C`$a+=7\*(C'\fR, or \f(CW\*(C`$a++\*(C'\fR.  See \*(L"\s-1MAGIC\s0 \s-1AUTOGENERATION\s0\*(R".  (Mathemagical
methods refer to methods triggered by an overloaded mathematical
operator.)
.PP
Since overloading respects inheritance via the \f(CW@ISA\fR hierarchy, the
above declaration would also trigger overloading of \f(CW\*(C`+\*(C'\fR and \f(CW\*(C`*=\*(C'\fR in
all the packages which inherit from \f(CW\*(C`Number\*(C'\fR.
.Sh "Calling Conventions for Binary Operations"
.IX Subsection "Calling Conventions for Binary Operations"
The functions specified in the \f(CW\*(C`use overload ...\*(C'\fR directive are called
with three (in one particular case with four, see \*(L"Last Resort\*(R")
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 \f(CW\*(C`7+$a\*(C'\fR, the
order of the arguments is interchanged.  It probably does not matter
when implementing the addition method, but whether the arguments
are reversed is vital to the subtraction method.  The method can
query this information by examining the third argument, which can take
three different values:
.IP "\s-1FALSE\s0" 7
.IX Item "FALSE"
the order of arguments is as in the current operation.
.IP "\s-1TRUE\s0" 7
.IX Item "TRUE"
the arguments are reversed.
.ie n .IP """undef""" 7
.el .IP "\f(CWundef\fR" 7
.IX Item "undef"
the current operation is an assignment variant (as in
\&\f(CW\*(C`$a+=7\*(C'\fR), but the usual function is called instead.  This additional
information can be used to generate some optimizations.  Compare
\&\*(L"Calling Conventions for Mutators\*(R".
.Sh "Calling Conventions for Unary Operations"
.IX Subsection "Calling Conventions for Unary Operations"
Unary operation are considered binary operations with the second
argument being \f(CW\*(C`undef\*(C'\fR.  Thus the functions that overloads \f(CW\*(C`{"++"}\*(C'\fR
is called with arguments \f(CW\*(C`($a,undef,\*(Aq\*(Aq)\*(C'\fR when \f(CW$a\fR++ is executed.
.Sh "Calling Conventions for Mutators"
.IX Subsection "Calling Conventions for Mutators"
Two types of mutators have different calling conventions:
.ie n .IP """++""\fR and \f(CW""\-\-""" 4
.el .IP "\f(CW++\fR and \f(CW\-\-\fR" 4
.IX Item "++ and --"
The routines which implement these operators are expected to actually
\&\fImutate\fR their arguments.  So, assuming that \f(CW$obj\fR is a reference to a
number,
.Sp
.Vb 1
\&  sub incr { my $n = $ {$_[0]}; ++$n; $_[0] = bless \e$n}
.Ve
.Sp
is an appropriate implementation of overloaded \f(CW\*(C`++\*(C'\fR.  Note that
.Sp
.Vb 1
\&  sub incr { ++$ {$_[0]} ; shift }
.Ve
.Sp
is \s-1OK\s0 if used with preincrement and with postincrement. (In the case
of postincrement a copying will be performed, see \*(L"Copy Constructor\*(R".)
.ie n .IP """x="" and other assignment versions" 4
.el .IP "\f(CWx=\fR and other assignment versions" 4
.IX Item "x= and other assignment versions"
There is nothing special about these methods.  They may change the
value of their arguments, and may leave it as is.  The result is going
to be assigned to the value in the left-hand-side if different from
this value.
.Sp
This allows for the same method to be used as overloaded \f(CW\*(C`+=\*(C'\fR and
\&\f(CW\*(C`+\*(C'\fR.  Note that this is \fIallowed\fR, but not recommended, since by the
semantic of \*(L"Fallback\*(R" Perl will call the method for \f(CW\*(C`+\*(C'\fR anyway,
if \f(CW\*(C`+=\*(C'\fR is not overloaded.
.PP
\&\fBWarning.\fR  Due to the presence of assignment versions of operations,
routines which may be called in assignment context may create
self-referential structures.  Currently Perl will not free self-referential
structures until cycles are \f(CW\*(C`explicitly\*(C'\fR broken.  You may get problems
when traversing your structures too.
.PP
Say,
.PP
.Vb 1
\&  use overload \*(Aq+\*(Aq => sub { bless [ \e$_[0], \e$_[1] ] };
.Ve
.PP
is asking for trouble, since for code \f(CW\*(C`$obj += $foo\*(C'\fR the subroutine
is called as \f(CW\*(C`$obj = add($obj, $foo, undef)\*(C'\fR, or \f(CW\*(C`$obj = [\e$obj,
\&\e$foo]\*(C'\fR.  If using such a subroutine is an important optimization, one
can overload \f(CW\*(C`+=\*(C'\fR explicitly by a non\-\*(L"optimized\*(R" version, or switch
to non-optimized version if \f(CW\*(C`not defined $_[2]\*(C'\fR (see
\&\*(L"Calling Conventions for Binary Operations\*(R").
.PP
Even if no \fIexplicit\fR assignment-variants of operators are present in
the script, they may be generated by the optimizer.  Say, \f(CW",$obj,"\fR or
\&\f(CW\*(Aq,\*(Aq . $obj . \*(Aq,\*(Aq\fR may be both optimized to
.PP
.Vb 1
\&  my $tmp = \*(Aq,\*(Aq . $obj;    $tmp .= \*(Aq,\*(Aq;
.Ve
.Sh "Overloadable Operations"
.IX Subsection "Overloadable Operations"
The following symbols can be specified in \f(CW\*(C`use overload\*(C'\fR directive:
.IP "\(bu" 5
\&\fIArithmetic operations\fR
.Sp
.Vb 2
\&    "+", "+=", "\-", "\-=", "*", "*=", "/", "/=", "%", "%=",
\&    "**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=",
.Ve
.Sp
For these operations a substituted non-assignment variant can be called if
the assignment variant is not available.  Methods for operations \f(CW\*(C`+\*(C'\fR,
\&\f(CW\*(C`\-\*(C'\fR, \f(CW\*(C`+=\*(C'\fR, and \f(CW\*(C`\-=\*(C'\fR can be called to automatically generate
increment and decrement methods.  The operation \f(CW\*(C`\-\*(C'\fR can be used to
autogenerate missing methods for unary minus or \f(CW\*(C`abs\*(C'\fR.
.Sp
See \*(L"\s-1MAGIC\s0 \s-1AUTOGENERATION\s0\*(R", \*(L"Calling Conventions for Mutators\*(R" and
\&\*(L"Calling Conventions for Binary Operations\*(R") for details of these
substitutions.
.IP "\(bu" 5
\&\fIComparison operations\fR
.Sp
.Vb 2
\&    "<",  "<=", ">",  ">=", "==", "!=", "<=>",
\&    "lt", "le", "gt", "ge", "eq", "ne", "cmp",
.Ve
.Sp
If the corresponding \*(L"spaceship\*(R" variant is available, it can be
used to substitute for the missing operation.  During \f(CW\*(C`sort\*(C'\fRing
arrays, \f(CW\*(C`cmp\*(C'\fR is used to compare values subject to \f(CW\*(C`use overload\*(C'\fR.
.IP "\(bu" 5
\&\fIBit operations\fR
.Sp
.Vb 1
\&    "&", "&=", "^", "^=", "|", "|=", "neg", "!", "~",
.Ve
.Sp
\&\f(CW\*(C`neg\*(C'\fR stands for unary minus.  If the method for \f(CW\*(C`neg\*(C'\fR is not
specified, it can be autogenerated using the method for
subtraction. If the method for \f(CW\*(C`!\*(C'\fR is not specified, it can be
autogenerated using the methods for \f(CW\*(C`bool\*(C'\fR, or \f(CW""\fR, or \f(CW\*(C`0+\*(C'\fR.
.Sp
The same remarks in \*(L"Arithmetic operations\*(R" about
assignment-variants and autogeneration apply for
bit operations \f(CW"&"\fR, \f(CW"^"\fR, and \f(CW"|"\fR as well.
.IP "\(bu" 5
\&\fIIncrement and decrement\fR
.Sp
.Vb 1
\&    "++", "\-\-",
.Ve
.Sp
If undefined, addition and subtraction methods can be
used instead.  These operations are called both in prefix and
postfix form.
.IP "\(bu" 5
\&\fITranscendental functions\fR
.Sp
.Vb 1
\&    "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", "int"
.Ve
.Sp
If \f(CW\*(C`abs\*(C'\fR is unavailable, it can be autogenerated using methods
for \*(L"<\*(R" or \*(L"<=>\*(R" combined with either unary minus or subtraction.
.Sp
Note that traditionally the Perl function int rounds to 0, thus for
floating-point-like types one should follow the same semantic.  If
\&\f(CW\*(C`int\*(C'\fR is unavailable, it can be autogenerated using the overloading of
\&\f(CW\*(C`0+\*(C'\fR.
.IP "\(bu" 5
\&\fIBoolean, string and numeric conversion\fR
.Sp
.Vb 1
\&    \*(Aqbool\*(Aq, \*(Aq""\*(Aq, \*(Aq0+\*(Aq,