overload.3

视频监控网络部分的协议ddns,的模块的实现代码,请大家大胆指正.

.Ve
.Sp
If one or two of these operations are not overloaded, the remaining ones can
be used instead.  \f(CW\*(C`bool\*(C'\fR is used in the flow control operators
(like \f(CW\*(C`while\*(C'\fR) and for the ternary \f(CW\*(C`?:\*(C'\fR 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.
.Sp
As a special case if the overload returns the object itself then it will
be used directly. An overloaded conversion returning the object is
probably a bug, because you're likely to get something that looks like
\&\f(CW\*(C`YourPackage=HASH(0x8172b34)\*(C'\fR.
.IP "\(bu" 5
\&\fIIteration\fR
.Sp
.Vb 1
\&    "<>"
.Ve
.Sp
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 \fIread-filehandle\fR syntax \f(CW\*(C`<$var>\*(C'\fR and
\&\fIglobbing\fR syntax \f(CW\*(C`<${var}>\*(C'\fR.
.Sp
\&\fB\s-1BUGS\s0\fR Even in list context, the iterator is currently called only
once and with scalar context.
.IP "\(bu" 5
\&\fIDereferencing\fR
.Sp
.Vb 1
\&    \*(Aq${}\*(Aq, \*(Aq@{}\*(Aq, \*(Aq%{}\*(Aq, \*(Aq&{}\*(Aq, \*(Aq*{}\*(Aq.
.Ve
.Sp
If not overloaded, the argument will be dereferenced \fIas is\fR, thus
should be of correct type.  These functions should return a reference
of correct type, or another object with overloaded dereferencing.
.Sp
As a special case if the overload returns the object itself then it
will be used directly (provided it is the correct type).
.Sp
The dereference operators must be specified explicitly they will not be passed to
\&\*(L"nomethod\*(R".
.IP "\(bu" 5
\&\fISpecial\fR
.Sp
.Vb 1
\&    "nomethod", "fallback", "=", "~~",
.Ve
.Sp
see "\s-1SPECIAL\s0 \s-1SYMBOLS\s0 \s-1FOR\s0 \f(CW\*(C`use overload\*(C'\fR".
.PP
See \*(L"Fallback\*(R" for an explanation of when a missing method can be
autogenerated.
.PP
A computer-readable form of the above table is available in the hash
\&\f(CW%overload::ops\fR, with values being space-separated lists of names:
.PP
.Vb 10
\& with_assign      => \*(Aq+ \- * / % ** << >> x .\*(Aq,
\& assign           => \*(Aq+= \-= *= /= %= **= <<= >>= x= .=\*(Aq,
\& num_comparison   => \*(Aq< <= > >= == !=\*(Aq,
\& \*(Aq3way_comparison\*(Aq=> \*(Aq<=> cmp\*(Aq,
\& str_comparison   => \*(Aqlt le gt ge eq ne\*(Aq,
\& binary           => \*(Aq& &= | |= ^ ^=\*(Aq,
\& unary            => \*(Aqneg ! ~\*(Aq,
\& mutators         => \*(Aq++ \-\-\*(Aq,
\& func             => \*(Aqatan2 cos sin exp abs log sqrt\*(Aq,
\& conversion       => \*(Aqbool "" 0+\*(Aq,
\& iterators        => \*(Aq<>\*(Aq,
\& dereferencing    => \*(Aq${} @{} %{} &{} *{}\*(Aq,
\& special          => \*(Aqnomethod fallback =\*(Aq
.Ve
.Sh "Inheritance and overloading"
.IX Subsection "Inheritance and overloading"
Inheritance interacts with overloading in two ways.
.ie n .IP "Strings as values of ""use overload"" directive" 4
.el .IP "Strings as values of \f(CWuse overload\fR directive" 4
.IX Item "Strings as values of use overload directive"
If \f(CW\*(C`value\*(C'\fR in
.Sp
.Vb 1
\&  use overload key => value;
.Ve
.Sp
is a string, it is interpreted as a method name.
.IP "Overloading of an operation is inherited by derived classes" 4
.IX Item "Overloading of an operation is inherited by derived classes"
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:
.Sp
If \f(CW\*(C`A\*(C'\fR inherits from \f(CW\*(C`B\*(C'\fR and \f(CW\*(C`C\*(C'\fR (in this order), \f(CW\*(C`B\*(C'\fR overloads
\&\f(CW\*(C`+\*(C'\fR with \f(CW\*(C`\e&D::plus_sub\*(C'\fR, and \f(CW\*(C`C\*(C'\fR overloads \f(CW\*(C`+\*(C'\fR by \f(CW"plus_meth"\fR,
then the subroutine \f(CW\*(C`D::plus_sub\*(C'\fR will be called to implement
operation \f(CW\*(C`+\*(C'\fR for an object in package \f(CW\*(C`A\*(C'\fR.
.PP
Note that since the value of the \f(CW\*(C`fallback\*(C'\fR key is not a subroutine,
its inheritance is not governed by the above rules.  In the current
implementation, the value of \f(CW\*(C`fallback\*(C'\fR in the first overloaded
ancestor is used, but this is accidental and subject to change.
.ie n .SH "SPECIAL SYMBOLS FOR ""use overload"""
.el .SH "SPECIAL SYMBOLS FOR \f(CWuse overload\fP"
.IX Header "SPECIAL SYMBOLS FOR use overload"
Three keys are recognized by Perl that are not covered by the above
description.
.Sh "Last Resort"
.IX Subsection "Last Resort"
\&\f(CW"nomethod"\fR 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, \f(CW\*(C`1\-$a\*(C'\fR can be equivalent to
.PP
.Vb 1
\&        &nomethodMethod($a,1,1,"\-")
.Ve
.PP
if the pair \f(CW"nomethod" => "nomethodMethod"\fR was specified in the
\&\f(CW\*(C`use overload\*(C'\fR directive.
.PP
The \f(CW"nomethod"\fR mechanism is \fInot\fR used for the dereference operators
( ${} @{} %{} &{} *{} ).
.PP
If some operation cannot be resolved, and there is no function
assigned to \f(CW"nomethod"\fR, then an exception will be raised via \fIdie()\fR\-\-
unless \f(CW"fallback"\fR was specified as a key in \f(CW\*(C`use overload\*(C'\fR directive.
.Sh "Fallback"
.IX Subsection "Fallback"
The key \f(CW"fallback"\fR governs what to do if a method for a particular
operation is not found.  Three different cases are possible depending on
the value of \f(CW"fallback"\fR:
.IP "\(bu" 16
\&\f(CW\*(C`undef\*(C'\fR
.Sp
Perl tries to use a
substituted method (see \*(L"\s-1MAGIC\s0 \s-1AUTOGENERATION\s0\*(R").  If this fails, it
then tries to calls \f(CW"nomethod"\fR value; if missing, an exception
will be raised.
.IP "\(bu" 16
\&\s-1TRUE\s0
.Sp
The same as for the \f(CW\*(C`undef\*(C'\fR value, but no exception is raised.  Instead,
it silently reverts to what it would have done were there no \f(CW\*(C`use overload\*(C'\fR
present.
.IP "\(bu" 16
defined, but \s-1FALSE\s0
.Sp
No autogeneration is tried.  Perl tries to call
\&\f(CW"nomethod"\fR value, and if this is missing, raises an exception.
.PP
\&\fBNote.\fR \f(CW"fallback"\fR inheritance via \f(CW@ISA\fR is not carved in stone
yet, see \*(L"Inheritance and overloading\*(R".
.Sh "Smart Match"
.IX Subsection "Smart Match"
The key \f(CW"~~"\fR allows you to override the smart matching used by
the switch construct. See feature.
.Sh "Copy Constructor"
.IX Subsection "Copy Constructor"
The value for \f(CW"="\fR is a reference to a function with three
arguments, i.e., it looks like the other values in \f(CW\*(C`use
overload\*(C'\fR. However, it does not overload the Perl assignment
operator. This would go against Camel hair.
.PP
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
.PP
.Vb 2
\&        $a=$b;
\&        ++$a;
.Ve
.PP
To make this change \f(CW$a\fR and not change \f(CW$b\fR, a copy of \f(CW$$a\fR is made,
and \f(CW$a\fR is assigned a reference to this new object.  This operation is
done during execution of the \f(CW\*(C`++$a\*(C'\fR, and not during the assignment,
(so before the increment \f(CW$$a\fR coincides with \f(CW$$b\fR).  This is only
done if \f(CW\*(C`++\*(C'\fR is expressed via a method for \f(CW\*(Aq++\*(Aq\fR or \f(CW\*(Aq+=\*(Aq\fR (or
\&\f(CW\*(C`nomethod\*(C'\fR).  Note that if this operation is expressed via \f(CW\*(Aq+\*(Aq\fR
a nonmutator, i.e., as in
.PP
.Vb 2
\&        $a=$b;
\&        $a=$a+1;
.Ve
.PP
then \f(CW$a\fR does not reference a new copy of \f(CW$$a\fR, since $$a does not
appear as lvalue when the above code is executed.
.PP
If the copy constructor is required during the execution of some mutator,
but a method for \f(CW\*(Aq=\*(Aq\fR was not specified, it can be autogenerated as a
string copy if the object is a plain scalar.
.IP "\fBExample\fR" 5
.IX Item "Example"
The actually executed code for
.Sp
.Vb 3
\&        $a=$b;
\&        Something else which does not modify $a or $b....
\&        ++$a;
.Ve
.Sp
may be
.Sp
.Vb 4
\&        $a=$b;
\&        Something else which does not modify $a or $b....
\&        $a = $a\->clone(undef,"");
\&        $a\->incr(undef,"");
.Ve
.Sp
if \f(CW$b\fR was mathemagical, and \f(CW\*(Aq++\*(Aq\fR was overloaded with \f(CW\*(C`\e&incr\*(C'\fR,
\&\f(CW\*(Aq=\*(Aq\fR was overloaded with \f(CW\*(C`\e&clone\*(C'\fR.
.PP
Same behaviour is triggered by \f(CW\*(C`$b = $a++\*(C'\fR, which is consider a synonym for
\&\f(CW\*(C`$b = $a; ++$a\*(C'\fR.
.SH "MAGIC AUTOGENERATION"
.IX Header "MAGIC AUTOGENERATION"
If a method for an operation is not found, and the value for  \f(CW"fallback"\fR is
\&\s-1TRUE\s0 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:
.IP "\fIAssignment forms of arithmetic operations\fR" 16
.IX Item "Assignment forms of arithmetic operations"
\&\f(CW\*(C`$a+=$b\*(C'\fR can use the method for \f(CW"+"\fR if the method for \f(CW"+="\fR
is not defined.
.IP "\fIConversion operations\fR" 16
.IX Item "Conversion operations"
String, numeric, and boolean conversion are calculated in terms of one
another if not all of them are defined.
.IP "\fIIncrement and decrement\fR" 16
.IX Item "Increment and decrement"
The \f(CW\*(C`++$a\*(C'\fR operation can be expressed in terms of \f(CW\*(C`$a+=1\*(C'\fR or \f(CW\*(C`$a+1\*(C'\fR,
and \f(CW\*(C`$a\-\-\*(C'\fR in terms of \f(CW\*(C`$a\-=1\*(C'\fR and \f(CW\*(C`$a\-1\*(C'\fR.
.ie n .IP """abs($a)""" 16
.el .IP "\f(CWabs($a)\fR" 16
.IX Item "abs($a)"
can be expressed in terms of \f(CW\*(C`$a<0\*(C'\fR and \f(CW\*(C`\-$a\*(C'\fR (or \f(CW\*(C`0\-$a\*(C'\fR).
.IP "\fIUnary minus\fR" 16
.IX Item "Unary minus"
can be expressed in terms of subtraction.
.IP "\fINegation\fR" 16
.IX Item "Negation"
\&\f(CW\*(C`!\*(C'\fR and \f(CW\*(C`not\*(C'\fR can be expressed in terms of boolean conversion, or
string or numerical conversion.
.IP "\fIConcatenation\fR" 16
.IX Item "Concatenation"
can be expressed in terms of string conversion.
.IP "\fIComparison operations\fR" 16
.IX Item "Comparison operations"
can be expressed in terms of its \*(L"spaceship\*(R" counterpart: either
\&\f(CW\*(C`<=>\*(C'\fR or \f(CW\*(C`cmp\*(C'\fR:
.Sp
.Vb 2
\&    <, >, <=, >=, ==, !=        in terms of <=>
\&    lt, gt, le, ge, eq, ne      in terms of cmp
.Ve
.IP "\fIIterator\fR" 16
.IX Item "Iterator"
.Vb 1
\&    <>                          in terms of builtin operations
.Ve
.IP "\fIDereferencing\fR" 16
.IX Item "Dereferencing"
.Vb 1
\&    ${} @{} %{} &{} *{}         in terms of builtin operations
.Ve
.IP "\fICopy operator\fR" 16
.IX Item "Copy operator"
can be expressed in terms of an assignment to the dereferenced value, if this
value is a scalar and not a reference.
.SH "Minimal set of overloaded operations"
.IX Header "Minimal set of overloaded operations"
Since some operations can be automatically generated from others, there is
a minimal set of operations that need to be overloaded in order to have
the complete set of overloaded operations at one's disposal.
Of course, the autogenerated operations may not do exactly what the user
expects. See \*(L"\s-1MAGIC\s0 \s-1AUTOGENERATION\s0\*(R" above. The minimal set is:
.PP
.Vb 4
\&    + \- * / % ** << >> x
\&    <=> cmp
\&    & | ^ ~
\&    atan2 cos sin exp log sqrt int
.Ve
.PP
Additionally, you need to define at least one of string, boolean or
numeric conversions because any one can be used to emulate the others.
The string conversion can also be used to emulate concatenation.
.SH "Losing overloading"
.IX Header "Losing overloading"
The restriction for the comparison operation is that even if, for example,
`\f(CW\*(C`cmp\*(C'\fR' should return a blessed reference, the autogenerated `\f(CW\*(C`lt\*(C'\fR'
function will produce only a standard logical value based on the
numerical value of the result of `\f(CW\*(C`cmp\*(C'\fR'.  In particular, a working
numeric conversion is needed in this case (possibly expressed in terms of
other conversions).
.PP
Similarly, \f(CW\*(C`.=\*(C'\fR  and \f(CW\*(C`x=\*(C'\fR operators lose their mathemagical properties
if the string conversion substitution is applied.
.PP
When you \fIchop()\fR a mathemagical object it is promoted to a string and its
mathemagical properties are lost.  The same can happen with other
operations as well.
.SH "Run-time Overloading"
.IX Header "Run-time Overloading"
Since all \f(CW\*(C`use\*(C'\fR directives are executed at compile-time, the only way to
change overloading during run-time is to
.PP
.Vb 1
\&    eval \*(Aquse overload "+" => \e&addmethod\*(Aq;
.Ve
.PP
You can also use
.PP
.Vb 1
\&    eval \*(Aqno overload "+", "\-\-", "<="\*(Aq;
.Ve
.PP
though the use of these constructs during run-time is questionable.
.SH "Public functions"
.IX Header "Public functions"
Package \f(CW\*(C`overload.pm\*(C'\fR provides the following public functions:
.IP "overload::StrVal(arg)" 5
.IX Item "overload::StrVal(arg)"
Gives string value of \f(CW\*(C`arg\*(C'\fR as in absence of stringify overloading. If you
are using this to get the address of a reference (useful for checking if two
references point to the same thing) then you may be better off using
\&\f(CW\*(C`Scalar::Util::refaddr()\*(C'\fR, which is faster.
.IP "overload::Overloaded(arg)" 5
.IX Item "overload::Overloaded(arg)"
Returns true if \f(CW\*(C`arg\*(C'\fR is subject to overloading of some operations.
.IP "overload::Method(obj,op)" 5
.IX Item "overload::Method(obj,op)"
Returns \f(CW\*(C`undef\*(C'\fR or a reference to the method that implements \f(CW\*(C`op\*(C'\fR.
.SH "Overloading constants"
.IX Header "Overloading constants"
For some applications, the Perl parser mangles constants too much.
It is possible to hook into this process via \f(CW\*(C`overload::constant()\*(C'\fR
and \f(CW\*(C`overload::remove_constant()\*(C'\fR functions.
.PP
These functions take a hash as an argument.  The recognized keys of this hash
are:
.IP "integer" 8
.IX Item "integer"
to overload integer constants,
.IP "float" 8
.IX Item "float"
to overload floating point constants,
.IP "binary" 8
.IX Item "binary"
to overload octal and hexadecimal constants,
.IP "q" 8
.IX Item "q"
to overload \f(CW\*(C`q\*(C'\fR\-quoted strings, constant pieces of \f(CW\*(C`qq\*(C'\fR\- and \f(CW\*(C`qx\*(C'\fR\-quoted
strings and here-documents,
.IP "qr" 8
.IX Item "qr"