What is the global hash %_ ?

[ozarfreo]

I've seen that %_ is treated as a special variable in Perl, as $_ and
@_ are: no warning is issued when using it without a package name, no
'my' is allowed on that var; the problem is that I could not find it
among Perl's documentation.

I'd like to use it in a program but am not sure whether I might be
interfering with some other intended use for %_ . Can someone point out
what is this variable exactly, and whether it is safe to use it?

 

[John W. Krahn]

I've seen that %_ is treated as a special variable in Perl, as $_ and
@_ are: no warning is issued when using it without a package name, no
'my' is allowed on that var; the problem is that I could not find it
among Perl's documentation.

I'd like to use it in a program but am not sure whether I might be
interfering with some other intended use for %_ . Can someone point out
what is this variable exactly, and whether it is safe to use it?

If it is not listed in perlvar then perl doesn't care what you do with it.


John

 

[ozarfreo]

If it is not listed in perlvar then perl doesn't care what you do with it.

Then is there any reason for it to be treated specially?

 

[John W. Krahn]

Then is there any reason for it to be treated specially?

All Sigil-Punctuation variables are special, whether perl uses them or not.


John

 

[ozarfreo]

All Sigil-Punctuation variables are special, whether perl uses them or not.

Is this behavior documented somewhere?

 

[Gunnar Hjalmarsson]

I've seen that %_ is treated as a special variable in Perl, as $_ and
@_ are: no warning is issued when using it without a package name, no
'my' is allowed on that var; the problem is that I could not find it
among Perl's documentation.

I'd like to use it in a program but am not sure whether I might be
interfering with some other intended use for %_ .

Considering similar variables that are used by Perl and documented in
perlvar, it does not seem unlikely that it will be predefined in future
Perl versions. Hence, I for one would not use it. To be safe.

 

[Dr.Ruud]

(e-mail address removed) schreef:

I've seen that %_ is treated as a special variable in Perl, as $_ and
@_ are: no warning is issued when using it without a package name, no
'my' is allowed on that var; the problem is that I could not find it
among Perl's documentation.

I'd like to use it in a program but am not sure whether I might be
interfering with some other intended use for %_ . Can someone point
out what is this variable exactly, and whether it is safe to use it?

$ perl -Mstrict -wle '
$, = "<\n";
@_ = (one=>1, two=>2, three=>3);
print @_, "$_[1] <--"
'
one<
1<
two<
2<
three<
3<
1 <--


$ perl -Mstrict -wle '
$, = "<\n";
%_ = (one=>1, two=>2, three=>3);
print %_, "$_{one} <--"
'
three<
3<
one<
1<
two<
2<
1 <--

 

[ozarfreo]

Those described in perlvar have additional functionality.
Those not described there may have one in the future, so you shouldn't
use them nevertheless.

From perlmod:
<< Only identifiers starting with letters (or underscore) are stored
in a package's symbol table. All other symbols are kept in package
"main", including all punctuation variables, like $_. >>

Thanks to all! That completely answers my question.

 

[Brian McCauley]

Considering similar variables that are used by Perl and documented in
perlvar, it does not seem unlikely that it will be predefined in future
Perl versions. Hence, I for one would not use it. To be safe.

It seems moderately unlikely to be and I do sometimes %_ it to
represent the "current context" in callback APIs.

{
local *_ = \%context;
$foo_callback->($foo_callback_arg1,$foo_callback_arg2);
}

 

[Dr.Ruud]

Brian McCauley schreef:

It seems moderately unlikely to be and I do sometimes %_ it to
represent the "current context" in callback APIs.

{
local *_ = \%context;
$foo_callback->($foo_callback_arg1,$foo_callback_arg2);
}

I once tried to make &_ do as much as possible what say() will, a bit
like this:

sub _{print @_, "\n"}

sub _{print +(@_ ? @_ : $_), $/}

sub _(*@)
{
local $\ = "\n" ;
0 == @_ and return print ;
1 == @_ and ref $_[0] ? return print @{ $_[0] }
: return print $_[0] ;
my $fh = $_[0] ;
ref $_[1] ? print $fh @{ $_[1] }
: print $fh $_[1] ;
}

See also:
perl -MPerl6::Say -e 'say for (1,2,3)'