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.

\&  sub pretty {
\&    my ($meth, $a, $b) = @{+shift};
\&    $a = \*(Aqu\*(Aq unless defined $a;
\&    $b = \*(Aqu\*(Aq unless defined $b;
\&    $a = $a\->pretty if ref $a;
\&    $b = $b\->pretty if ref $b;
\&    "[$meth $a $b]";
\&  }
.Ve
.PP
Now one can finish the script by
.PP
.Vb 1
\&  print "side = ", $side\->pretty, "\en";
.Ve
.PP
The method \f(CW\*(C`pretty\*(C'\fR is doing object-to-string conversion, so it
is natural to overload the operator \f(CW""\fR using this method.  However,
inside such a method it is not necessary to pretty-print the
\&\fIcomponents\fR \f(CW$a\fR and \f(CW$b\fR of an object.  In the above subroutine
\&\f(CW"[$meth $a $b]"\fR is a catenation of some strings and components \f(CW$a\fR
and \f(CW$b\fR.  If these components use overloading, the catenation operator
will look for an overloaded operator \f(CW\*(C`.\*(C'\fR; if not present, it will
look for an overloaded operator \f(CW""\fR.  Thus it is enough to use
.PP
.Vb 7
\&  use overload nomethod => \e&wrap, \*(Aq""\*(Aq => \e&str;
\&  sub str {
\&    my ($meth, $a, $b) = @{+shift};
\&    $a = \*(Aqu\*(Aq unless defined $a;
\&    $b = \*(Aqu\*(Aq unless defined $b;
\&    "[$meth $a $b]";
\&  }
.Ve
.PP
Now one can change the last line of the script to
.PP
.Vb 1
\&  print "side = $side\en";
.Ve
.PP
which outputs
.PP
.Vb 1
\&  side = [/ [\- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]]
.Ve
.PP
and one can inspect the value in debugger using all the possible
methods.
.PP
Something is still amiss: consider the loop variable \f(CW$cnt\fR of the
script.  It was a number, not an object.  We cannot make this value of
type \f(CW\*(C`symbolic\*(C'\fR, since then the loop will not terminate.
.PP
Indeed, to terminate the cycle, the \f(CW$cnt\fR should become false.
However, the operator \f(CW\*(C`bool\*(C'\fR for checking falsity is overloaded (this
time via overloaded \f(CW""\fR), and returns a long string, thus any object
of type \f(CW\*(C`symbolic\*(C'\fR is true.  To overcome this, we need a way to
compare an object to 0.  In fact, it is easier to write a numeric
conversion routine.
.PP
Here is the text of \fIsymbolic.pm\fR with such a routine added (and
slightly modified \fIstr()\fR):
.PP
.Vb 3
\&  package symbolic;             # Primitive symbolic calculator
\&  use overload
\&    nomethod => \e&wrap, \*(Aq""\*(Aq => \e&str, \*(Aq0+\*(Aq => \e#
\&
\&  sub new { shift; bless [\*(Aqn\*(Aq, @_] }
\&  sub wrap {
\&    my ($obj, $other, $inv, $meth) = @_;
\&    ($obj, $other) = ($other, $obj) if $inv;
\&    bless [$meth, $obj, $other];
\&  }
\&  sub str {
\&    my ($meth, $a, $b) = @{+shift};
\&    $a = \*(Aqu\*(Aq unless defined $a;
\&    if (defined $b) {
\&      "[$meth $a $b]";
\&    } else {
\&      "[$meth $a]";
\&    }
\&  }
\&  my %subr = ( n => sub {$_[0]},
\&               sqrt => sub {sqrt $_[0]},
\&               \*(Aq\-\*(Aq => sub {shift() \- shift()},
\&               \*(Aq+\*(Aq => sub {shift() + shift()},
\&               \*(Aq/\*(Aq => sub {shift() / shift()},
\&               \*(Aq*\*(Aq => sub {shift() * shift()},
\&               \*(Aq**\*(Aq => sub {shift() ** shift()},
\&             );
\&  sub num {
\&    my ($meth, $a, $b) = @{+shift};
\&    my $subr = $subr{$meth}
\&      or die "Do not know how to ($meth) in symbolic";
\&    $a = $a\->num if ref $a eq _\|_PACKAGE_\|_;
\&    $b = $b\->num if ref $b eq _\|_PACKAGE_\|_;
\&    $subr\->($a,$b);
\&  }
.Ve
.PP
All the work of numeric conversion is done in \f(CW%subr\fR and \fInum()\fR.  Of
course, \f(CW%subr\fR is not complete, it contains only operators used in the
example below.  Here is the extra-credit question: why do we need an
explicit recursion in \fInum()\fR?  (Answer is at the end of this section.)
.PP
Use this module like this:
.PP
.Vb 4
\&  require symbolic;
\&  my $iter = new symbolic 2;    # 16\-gon
\&  my $side = new symbolic 1;
\&  my $cnt = $iter;
\&
\&  while ($cnt) {
\&    $cnt = $cnt \- 1;            # Mutator \`\-\-\*(Aq not implemented
\&    $side = (sqrt(1 + $side**2) \- 1)/$side;
\&  }
\&  printf "%s=%f\en", $side, $side;
\&  printf "pi=%f\en", $side*(2**($iter+2));
.Ve
.PP
It prints (without so many line breaks)
.PP
.Vb 4
\&  [/ [\- [sqrt [+ 1 [** [/ [\- [sqrt [+ 1 [** [n 1] 2]]] 1]
\&                          [n 1]] 2]]] 1]
\&     [/ [\- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912
\&  pi=3.182598
.Ve
.PP
The above module is very primitive.  It does not implement
mutator methods (\f(CW\*(C`++\*(C'\fR, \f(CW\*(C`\-=\*(C'\fR and so on), does not do deep copying
(not required without mutators!), and implements only those arithmetic
operations which are used in the example.
.PP
To implement most arithmetic operations is easy; one should just use
the tables of operations, and change the code which fills \f(CW%subr\fR to
.PP
.Vb 12
\&  my %subr = ( \*(Aqn\*(Aq => sub {$_[0]} );
\&  foreach my $op (split " ", $overload::ops{with_assign}) {
\&    $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
\&  }
\&  my @bins = qw(binary 3way_comparison num_comparison str_comparison);
\&  foreach my $op (split " ", "@overload::ops{ @bins }") {
\&    $subr{$op} = eval "sub {shift() $op shift()}";
\&  }
\&  foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
\&    print "defining \`$op\*(Aq\en";
\&    $subr{$op} = eval "sub {$op shift()}";
\&  }
.Ve
.PP
Due to \*(L"Calling Conventions for Mutators\*(R", we do not need anything
special to make \f(CW\*(C`+=\*(C'\fR and friends work, except filling \f(CW\*(C`+=\*(C'\fR entry of
\&\f(CW%subr\fR, and defining a copy constructor (needed since Perl has no
way to know that the implementation of \f(CW\*(Aq+=\*(Aq\fR does not mutate
the argument, compare \*(L"Copy Constructor\*(R").
.PP
To implement a copy constructor, add \f(CW\*(C`\*(Aq=\*(Aq => \e&cpy\*(C'\fR to \f(CW\*(C`use overload\*(C'\fR
line, and code (this code assumes that mutators change things one level
deep only, so recursive copying is not needed):
.PP
.Vb 4
\&  sub cpy {
\&    my $self = shift;
\&    bless [@$self], ref $self;
\&  }
.Ve
.PP
To make \f(CW\*(C`++\*(C'\fR and \f(CW\*(C`\-\-\*(C'\fR work, we need to implement actual mutators,
either directly, or in \f(CW\*(C`nomethod\*(C'\fR.  We continue to do things inside
\&\f(CW\*(C`nomethod\*(C'\fR, thus add
.PP
.Vb 4
\&    if ($meth eq \*(Aq++\*(Aq or $meth eq \*(Aq\-\-\*(Aq) {
\&      @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference
\&      return $obj;
\&    }
.Ve
.PP
after the first line of \fIwrap()\fR.  This is not a most effective
implementation, one may consider
.PP
.Vb 1
\&  sub inc { $_[0] = bless [\*(Aq++\*(Aq, shift, 1]; }
.Ve
.PP
instead.
.PP
As a final remark, note that one can fill \f(CW%subr\fR by
.PP
.Vb 10
\&  my %subr = ( \*(Aqn\*(Aq => sub {$_[0]} );
\&  foreach my $op (split " ", $overload::ops{with_assign}) {
\&    $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
\&  }
\&  my @bins = qw(binary 3way_comparison num_comparison str_comparison);
\&  foreach my $op (split " ", "@overload::ops{ @bins }") {
\&    $subr{$op} = eval "sub {shift() $op shift()}";
\&  }
\&  foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
\&    $subr{$op} = eval "sub {$op shift()}";
\&  }
\&  $subr{\*(Aq++\*(Aq} = $subr{\*(Aq+\*(Aq};
\&  $subr{\*(Aq\-\-\*(Aq} = $subr{\*(Aq\-\*(Aq};
.Ve
.PP
This finishes implementation of a primitive symbolic calculator in
50 lines of Perl code.  Since the numeric values of subexpressions
are not cached, the calculator is very slow.
.PP
Here is the answer for the exercise: In the case of \fIstr()\fR, we need no
explicit recursion since the overloaded \f(CW\*(C`.\*(C'\fR\-operator will fall back
to an existing overloaded operator \f(CW""\fR.  Overloaded arithmetic
operators \fIdo not\fR fall back to numeric conversion if \f(CW\*(C`fallback\*(C'\fR is
not explicitly requested.  Thus without an explicit recursion \fInum()\fR
would convert \f(CW\*(C`[\*(Aq+\*(Aq, $a, $b]\*(C'\fR to \f(CW\*(C`$a + $b\*(C'\fR, which would just rebuild
the argument of \fInum()\fR.
.PP
If you wonder why defaults for conversion are different for \fIstr()\fR and
\&\fInum()\fR, note how easy it was to write the symbolic calculator.  This
simplicity is due to an appropriate choice of defaults.  One extra
note: due to the explicit recursion \fInum()\fR is more fragile than \fIsym()\fR:
we need to explicitly check for the type of \f(CW$a\fR and \f(CW$b\fR.  If components
\&\f(CW$a\fR and \f(CW$b\fR happen to be of some related type, this may lead to problems.
.Sh "\fIReally\fP symbolic calculator"
.IX Subsection "Really symbolic calculator"
One may wonder why we call the above calculator symbolic.  The reason
is that the actual calculation of the value of expression is postponed
until the value is \fIused\fR.
.PP
To see it in action, add a method
.PP
.Vb 5
\&  sub STORE {
\&    my $obj = shift;
\&    $#$obj = 1;
\&    @$obj\->[0,1] = (\*(Aq=\*(Aq, shift);
\&  }
.Ve
.PP
to the package \f(CW\*(C`symbolic\*(C'\fR.  After this change one can do
.PP
.Vb 3
\&  my $a = new symbolic 3;
\&  my $b = new symbolic 4;
\&  my $c = sqrt($a**2 + $b**2);
.Ve
.PP
and the numeric value of \f(CW$c\fR becomes 5.  However, after calling
.PP
.Vb 1
\&  $a\->STORE(12);  $b\->STORE(5);
.Ve
.PP
the numeric value of \f(CW$c\fR becomes 13.  There is no doubt now that the module
symbolic provides a \fIsymbolic\fR calculator indeed.
.PP
To hide the rough edges under the hood, provide a \fItie()\fRd interface to the
package \f(CW\*(C`symbolic\*(C'\fR (compare with \*(L"Metaphor clash\*(R").  Add methods
.PP
.Vb 3
\&  sub TIESCALAR { my $pack = shift; $pack\->new(@_) }
\&  sub FETCH { shift }
\&  sub nop {  }          # Around a bug
.Ve
.PP
(the bug is described in \*(L"\s-1BUGS\s0\*(R").  One can use this new interface as
.PP
.Vb 3
\&  tie $a, \*(Aqsymbolic\*(Aq, 3;
\&  tie $b, \*(Aqsymbolic\*(Aq, 4;
\&  $a\->nop;  $b\->nop;    # Around a bug
\&
\&  my $c = sqrt($a**2 + $b**2);
.Ve
.PP
Now numeric value of \f(CW$c\fR is 5.  After \f(CW\*(C`$a = 12; $b = 5\*(C'\fR the numeric value
of \f(CW$c\fR becomes 13.  To insulate the user of the module add a method
.PP
.Vb 1
\&  sub vars { my $p = shift; tie($_, $p), $_\->nop foreach @_; }
.Ve
.PP
Now
.PP
.Vb 3
\&  my ($a, $b);
\&  symbolic\->vars($a, $b);
\&  my $c = sqrt($a**2 + $b**2);
\&
\&  $a = 3; $b = 4;
\&  printf "c5  %s=%f\en", $c, $c;
\&
\&  $a = 12; $b = 5;
\&  printf "c13  %s=%f\en", $c, $c;
.Ve
.PP
shows that the numeric value of \f(CW$c\fR follows changes to the values of \f(CW$a\fR
and \f(CW$b\fR.
.SH "AUTHOR"
.IX Header "AUTHOR"
Ilya Zakharevich <\[e-mail]\-state.edu\fR>.
.SH "DIAGNOSTICS"
.IX Header "DIAGNOSTICS"
When Perl is run with the \fB\-Do\fR switch or its equivalent, overloading
induces diagnostic messages.
.PP
Using the \f(CW\*(C`m\*(C'\fR command of Perl debugger (see perldebug) one can
deduce which operations are overloaded (and which ancestor triggers
this overloading). Say, if \f(CW\*(C`eq\*(C'\fR is overloaded, then the method \f(CW\*(C`(eq\*(C'\fR
is shown by debugger. The method \f(CW\*(C`()\*(C'\fR corresponds to the \f(CW\*(C`fallback\*(C'\fR
key (in fact a presence of this method shows that this package has
overloading enabled, and it is what is used by the \f(CW\*(C`Overloaded\*(C'\fR
function of module \f(CW\*(C`overload\*(C'\fR).
.PP
The module might issue the following warnings:
.IP "Odd number of arguments for overload::constant" 4
.IX Item "Odd number of arguments for overload::constant"
(W) The call to overload::constant contained an odd number of arguments.
The arguments should come in pairs.
.IP "`%s' is not an overloadable type" 4
.IX Item "`%s' is not an overloadable type"
(W) You tried to overload a constant type the overload package is unaware of.
.IP "`%s' is not a code reference" 4
.IX Item "`%s' is not a code reference"
(W) The second (fourth, sixth, ...) argument of overload::constant needs
to be a code reference. Either an anonymous subroutine, or a reference
to a subroutine.
.SH "BUGS"
.IX Header "BUGS"
Because it is used for overloading, the per-package hash \f(CW%OVERLOAD\fR now
has a special meaning in Perl. The symbol table is filled with names
looking like line-noise.
.PP
For the purpose of inheritance every overloaded package behaves as if
\&\f(CW\*(C`fallback\*(C'\fR is present (possibly undefined). This may create
interesting effects if some package is not overloaded, but inherits
from two overloaded packages.
.PP
Relation between overloading and \fItie()\fRing is broken.  Overloading is
triggered or not basing on the \fIprevious\fR class of \fItie()\fRd value.
.PP
This happens because the presence of overloading is checked too early,
before any \fItie()\fRd access is attempted.  If the \s-1\fIFETCH\s0()\fRed class of the
\&\fItie()\fRd value does not change, a simple workaround is to access the value
immediately after \fItie()\fRing, so that after this call the \fIprevious\fR class
coincides with the current one.
.PP
\&\fBNeeded:\fR a way to fix this without a speed penalty.
.PP
Barewords are not covered by overloaded string constants.
.PP
This document is confusing.  There are grammos and misleading language
used in places.  It would seem a total rewrite is needed.