12.6. Determining the Caller's Package12.6.2. SolutionTo find the current package: $this_pack = _ _PACKAGE_ _; $that_pack = caller( ); 12.6.3. DiscussionThe _ _PACKAGE_ _ symbol returns the package that the code is currently being compiled into. This doesn't interpolate into double-quoted strings: print "I am in package _ _PACKAGE_ _\n"; # WRONG! I am in package _ _PACKAGE_ _ Needing to figure out the caller's package arose more often in older code that received as input a string of code to be eval uated, or a filehandle, format, or directory handle name. Consider a call to a hypothetical runit function: package Alpha; runit('$line = <TEMP>'); package Beta; sub runit { my $codestr = shift; eval $codestr; die if $@; } Because runit was compiled in a different package than was currently executing, when the eval runs, it acts as though it were passed $Beta::line and Beta::TEMP. The old workaround was to include your caller's package first: package Beta; sub runit { my $codestr = shift; my $hispack = caller; eval "package $hispack; $codestr"; die if $@; } That approach works only when $line is a global variable. If it's lexical, that won't help at all. Instead, arrange for runit to accept a reference to a subroutine: package Alpha; runit( sub { $line = <TEMP> } ); package Beta; sub runit { my $coderef = shift; &$coderef( ); } This not only works with lexicals, but has the added benefit of checking the code's syntax at compile time, which is a major win. If all that's being passed in is a filehandle, it's more portable to use the Symbol::qualify function. This function takes a name and package to qualify the name into. If the name needs qualification, it fixes it; otherwise, it's left alone. But that's considerably less efficient than a * prototype. Here's an example that reads and returns n lines from a filehandle. The function qualifies the handle before working with it. open (FH, "<", "/etc/termcap") or die "can't open /etc/termcap: $!"; ($a, $b, $c) = nreadline(3, "FH"); use Symbol ( ); use Carp; sub nreadline { my ($count, $handle) = @_; my(@retlist,$line); croak "count must be > 0" unless $count > 0; $handle = Symbol::qualify($handle, (caller( ))[0]); croak "need open filehandle" unless defined fileno($handle); push(@retlist, $line) while defined($line = <$handle>) && $count--; return @retlist; } If everyone who called your nreadline function passed the filehandle as a typeglob *FH, as a glob reference *FH, or using FileHandle or IO::Handle objects, you wouldn't need to do this. It's only the possibility of a bare "FH" string that requires qualification. 12.6.4. See AlsoThe documentation for the standard Symbol module, also found in Chapter 32 of Programming Perl; the descriptions of the special symbols _ _FILE_ _, _ _LINE_ _, and _ _PACKAGE_ _ in perldata(1); Recipe 12.14; Recipe 7.6 Copyright © 2003 O'Reilly & Associates. All rights reserved. |
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