http://www.perlmonks.org?node_id=436457


in reply to Perl oddities

I'm not sure if this one counts as an oddity, or a bug. I think it's defined behaviour, but I'm not sure.

substr() is implemented as an Lvalue subroutine, and it's a fatal error when assigned to incorrectly. I think means that when substr() is passed as a parameter, it gets treated like an Lvalue, since it can be assigned back to by modifying @_.

This code crashes on the subroutine call: 

use strict;
use warnings;
my $x="";
foo( substr($x,2,1) ); # crashes here
print "Alive!\n"; # not reached
sub foo {}

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This code doesn't: 

use strict;
use warnings;
my $x="";
my $y=substr($x,2,1); # warns, doesn't crash
foo($y); # no-op
print "Alive\n"; # we get here just fine
sub foo {}

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I maintain that this is an odd behaviour, one way or the other. My hunch is that it counts as an "oddity" as opposed to a "bug": can the experts confirm or deny this?
--
Ytrew

Replies are listed 'Best First'.
Re: substr oddity
by BrowserUk (Patriarch) on Mar 04, 2005 at 06:05 UTC

    This doesn't seem to happen on Win32 5.8.4 (AS 810). Which version are you using? 

    P:\test>perl -wle"sub foo{}; my $s =''; foo( substr $s, 2, 1 );"
substr outside of string at -e line 1.

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    Examine what is said, not who speaks.
    Silence betokens consent.
    Love the truth but pardon error.
[reply]
[d/l]
      This doesn't seem to happen on Win32 5.8.4 (AS 810). Which version are you using?

      Well, I'm running version "5.8.6 built for PA-RISC2.0-thread-multi-LP64", running on HP/UX. I also get the same behaviour for 5.6.1.

      But your snippet doesn't actually demonstrate the problem: you get the warning message, but can't actually detect whether the code crashed. Here's what I get when I add a trace statement after the call to substr(). 

      $ perl -wle 'sub foo{}; my $s=""; foo( substr($s,2,1) );print "Not rea
+ched\n"'
substr outside of string at -e line 1.
$

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      Note that "not reached" is never reached, and is not printed, because the code crashes on the substr() call.
      --
      Ytrew
[reply]
[d/l]

        Fair enough, but your snippet also doesn't crash on my machine for exactly the same reasons: 

        >perl # 5.8.4
use strict;
use warnings;
my $x="";
foo( substr($x,2,1) ); # crashes here
print "Alive!\n"; # not reached
sub foo {}
^Z
substr outside of string at - line 4.

>c:\perl561\bin\perl5.6.1.exe
use strict;
use warnings;
my $x="";
foo( substr($x,2,1) ); # crashes here
print "Alive!\n"; # not reached
sub foo {}
^Z
substr outside of string at - line 4.

        [download]
        Examine what is said, not who speaks.
        Silence betokens consent.
        Love the truth but pardon error.
[reply]
[d/l]

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