问题
In a Perl daemon reacting to various events I'm trying to use a Null object pattern in 2 cases by creating anonymous subroutines, which should just return a value of 1 aka "true" (please scroll to the right to see the check subroutines for LOGIN and ALIVE events):
package User;
our %EVENTS = (
LOGIN => {handler => \&handleLogin, check => sub {1}, },
CHAT => {handler => \&handleChat, check => \&mayChat, },
JOIN => {handler => \&handleJoin, check => \&mayJoin, },
LEAVE => {handler => \&handleLeave, check => \&mayLeave, },
ALIVE => {handler => sub {}, check => sub {1}, },
BID => {handler => \&handleBid, check => \&checkArgs, },
TAKE => {handler => \&handleTake, check => \&checkArgs, },
# .... more events ....
);
sub action($$$) {
my $user = shift;
my $event = shift;
my $arg = shift;
my $game = $user->{GAME};
unless (exists $EVENTS{$event}) {
print STDERR "wrong event: $event\n";
return;
}
my $handler = $EVENTS{$event}->{handler};
my $check = $EVENTS{$event}->{check};
return unless $user->$check->($arg); # XXX fails
$user->$handler->($arg);
}
sub mayChat($$) {
my $user = shift;
return if $user->{KIBITZER};
}
# ...... more methods here ...
1;
Unfortunately I get the runtime error for LOGIN event:
Can't use string ("1") as a subroutine ref while "strict refs" in use
Does anybody please know how to fix it here?
How to provide a "function pointer" to an anonymous Perl subroutine?
The handler => \&sub { 1 } doesn't do it either.
Using perl 5.8.8 and perl 5.10.1 on CentOS 5.x and 6.x
UPDATE:
I've also tried following:
my $check = $EVENTS{$event}->{check};
return unless $check->($user, $arg);
but it doesn't help. I think this rules out the "missing blessing" suggested in some answers.
UPDATE 2:
I have extended the source code snippet in my original question. The background is: I'm in the process of refactoring of my source code and thus I've created the %EVENTS hash as listed above, so that for each incoming event (a string sent over TCP-socket from a Flash client) there is a reference to a subroutine (check) which validates the event and a reference to another subroutine (handler) which performs some actions. I'm not sure if other subroutines work - I'm stuck already at the first LOGIN event.
I also don't understand why doesn't check => sub { 1 } above work - isn't sub supposed to return a reference to an anonymous subroutine (when the name is omitted - according to perldoc perlref section 4)?
UPDATE 3:
The output of print Dumper(\%EVENTS) -
$VAR1 = {
'PLAY' => {
'check' => sub { "DUMMY" },
'handler' => sub { "DUMMY" },
},
'JOIN' => {
'check' => sub { "DUMMY" },
'handler' => sub { "DUMMY" },
},
'OVER1' => {
'check' => sub { "DUMMY" },
'handler' => sub { "DUMMY" },
},
'ALIVE' => {
'check' => sub { "DUMMY" },
'handler' => sub { "DUMMY" },
},
'DISCARD' => {
'check' => $VAR1->{'PLAY'}{'check'},
'handler' => sub { "DUMMY" },
},
'MISS1' => {
'check' => sub { "DUMMY" },
'handler' => sub { "DUMMY" },
},
'LOGIN' => {
'check' => sub { "DUMMY" },
'handler' => sub { "DUMMY" },
},
'TAKE' => {
'check' => $VAR1->{'PLAY'}{'check'},
'handler' => sub { "DUMMY" },
},
'ONEMORE' => {
'check' => sub { "DUMMY" },
'handler' => sub { "DUMMY" },
},
'OVER2' => {
'check' => sub { "DUMMY" },
'handler' => sub { "DUMMY" },
},
'MISS2' => {
'check' => sub { "DUMMY" },
'handler' => sub { "DUMMY" },
},
'EXACT' => {
'check' => sub { "DUMMY" },
'handler' => sub { "DUMMY" },
},
'TRUST' => {
'check' => $VAR1->{'PLAY'}{'check'},
'handler' => sub { "DUMMY" },
},
'LEAVE' => {
'check' => sub { "DUMMY" },
'handler' => sub { "DUMMY" },
},
'DEFEND' => {
'check' => $VAR1->{'PLAY'}{'check'},
'handler' => sub { "DUMMY" },
},
'OPEN' => {
'check' => $VAR1->{'PLAY'}{'check'},
'handler' => sub { "DUMMY" },
},
'REVEAL' => {
'check' => sub { "DUMMY" },
'handler' => sub { "DUMMY" },
},
'CHAT' => {
'check' => sub { "DUMMY" },
'handler' => sub { "DUMMY" },
},
'DECLARE' => {
'check' => $VAR1->{'PLAY'}{'check'},
'handler' => sub { "DUMMY" },
},
'BACK' => {
'check' => sub { "DUMMY" },
'handler' => sub { "DUMMY" },
},
'MISERE' => {
'check' => sub { "DUMMY" },
'handler' => sub { "DUMMY" },
},
'BID' => {
'check' => $VAR1->{'PLAY'}{'check'},
'handler' => sub { "DUMMY" },
}
};
回答1:
The problem is not with the particular event that is surfacing the problem; the actual bug is in action. In particular, the line
return unless $user->$check->($arg); # XXX fails
doesn't do what you think it does. Between the presence of prototypes and Perl's willingness to try and call a sub specified by name, you wind up eventually calling User:: for the CHAT event. Which doesn't seem to be what you intended for it to do.
The more correct call looks like
return unless $check->($user, $arg);
This expects $check to contain a subref (which it does), dereferences it and calls it. This works even though sometimes $check will refer to a prototyped function.
That leaves the problem that this procedural code doesn't respect inheritance. To do that, you have to rephrase %EVENTS a bit. Thus:
our %EVENTS = (
LOGIN => {handler => \&handleLogin, check => sub {1}, },
CHAT => {handler => \&handleChat, check => sub { shift->mayChat(@_) },
...
);
Note that you are strongly discouraged to mix function prototypes and Perl OO programming precisely because it can lead to hard-to-diagnose problems like this one.
In reference to your other question: my $foo = sub { } is indeed how you construct anonymous subroutines. But you do need to call them appropriately.
回答2:
$check is already a code reference, so you could say
return unless $check->($arg);
Your existing code could also be salvaged if $check were a reference to code that returned a code reference:
our %EVENTS = ( LOGIN => { ..., check => sub { sub { 1 } }, } ... );
Think of sub { } as a "code reference" operator the way that \ is an operator to create a scalar reference, or [...] is an operator to create an array reference.
回答3:
Unless $check is a code reference,
$user->$check->($arg);
won't work.
As a self-contained example:
> perl -e 'use strict;use warnings;my $a=17;$a->("whatever");'
Can't use string ("17") as a subroutine ref while "strict refs" in use at -e line 1.
So you'll have to look more carefully at the structure of your data and avoid treating scalars as code references.
回答4:
looks like $event is either LOGIN or ALIVe, both have anonymous subs for the check key that return 1. $check is locally defined to that subroutine, and returns 1, the code then attempts to access the value '1' in the user hash/object as a hash
回答5:
My own answer - the following seems to work, I probably was dereferencing my sub refs in a wrong way...
sub action($$$) {
my $user = shift;
my $event = shift;
my $arg = shift;
my $handler = $EVENTS{$event}->{handler};
my $check = $EVENTS{$event}->{check};
return unless &$check($user, $arg);
&$handler($user, $arg);
}
回答6:
The key fact about your program that you left out of your question is this: mayChat and friends all return subroutine references. These are then evaluated at
return unless $user->$check->($arg); # XXX falis
. Entries in %EVENTS are references to routines that return subroutine references. Once we've stated it in those terms, the problem with your original code becomes obvious.
$EVENTS{LOGIN}->{check} is a reference to a sub that returns an integer, when what is expected is a reference to a sub that returns a reference to a sub. If we make it a reference to a sub that returns a reference to a sub:
LOGIN => {phase => undef, handler => \&handleLogin, check => sub { sub {1} }, },
, it works.
The fix for your problem is to (1) make those entries be subs that return subs and (2) document the interface of %EVENTS so you are the last person to have this problem.
来源：https://stackoverflow.com/questions/9929754/cant-use-string-1-as-a-subroutine-ref-while-strict-refs-in-use