in reply to Rethrowing with die $@ considered harmful
Similarly, I've started noticing issues with the common practice of doing local $_. In some situations, that's not enough to prevent damage to the caller. Using local *_ fixes those problems. For example, consider the following function:
sub dequote { local $_ = @_ ? $_[0] : $_; s/^"//; s/"$//; s/\\(.)/$1/g; return $_; }
It assigns its parameter (or $_ if there are no parameters) to $_. It attempts to prevent damage to the caller by localizing $_ first, but I will show it doesn't always work.
Case 1
$_ is tied, or $_ is an alias to something which is tied.
Running
use strict; use warnings; sub dequote { local $_ = @_ ? $_[0] : $_; s/^"//; s/"$//; s/\\(.)/$1/g; return $_; } { package MyTie; sub TIESCALAR { bless(\my $var, shift) } sub FETCH { my $self = shift; print("FETCH\n"); return $$self; } sub STORE { my $self = shift; $$self = shift; print("STORE $$self\n"); return $$self;} } tie my $var, 'MyTie'; $var = '"John \"Foo\" Bar"'; print dequote, "\n" foreach $var;
gives
STORE "John \"Foo\" Bar" FETCH STORE FETCH Use of uninitialized value in concatenation (.) or string at 561931.pl + line 18. STORE FETCH Use of uninitialized value in substitution (s///) at 561931.pl line 6. FETCH Use of uninitialized value in substitution (s///) at 561931.pl line 7. FETCH Use of uninitialized value in substitution (s///) at 561931.pl line 8. FETCH STORE Use of uninitialized value in print at 561931.pl line 25.
Case 2
pos($_) is used by the caller.
Running
use strict; use warnings; sub dequote { local $_ = @_ ? $_[0] : $_; s/^"//; s/"$//; s/\\(.)/$1/g; return $_; } $_ = 'abcd'; /\G .. /gcx; print(pos(), "\n"); dequote('"John \"Foo\" Bar"'); print(pos(), "\n");
gives
2 Use of uninitialized value in print at 561931.pl line 18.
Changing
local $_ = @_ ? $_[0] : $_;
to
my $s = @_ ? $_[0] : $_;
local *_ = \$s;
gives
2 2
The Solution
sub dequote { #local $_ = @_ ? $_[0] : $_; # XXX my $s = @_ ? $_[0] : $_; # Fix local *_ = \$s; # Fix s/^"//; s/"$//; s/\\(.)/$1/g; return $_; }
After making that change, the programs output
STORE "John \"Foo\" Bar" FETCH John "Foo" Bar
and
2 2
respectively.
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Re^2: Rethrowing with die $@ considered harmful
by Ieronim (Friar) on Jul 18, 2006 at 07:48 UTC | |
Re^2: Rethrowing with die $@ considered harmful
by xmath (Hermit) on Jul 22, 2006 at 10:44 UTC |
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