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<HTML><HEAD><TITLE>Recipe 12.5. Determining the Caller's Package (Perl Cookbook)</TITLE><METANAME="DC.title"CONTENT="Perl Cookbook"><METANAME="DC.creator"CONTENT="Tom Christiansen & Nathan Torkington"><METANAME="DC.publisher"CONTENT="O'Reilly & Associates, Inc."><METANAME="DC.date"CONTENT="1999-07-02T01:41:33Z"><METANAME="DC.type"CONTENT="Text.Monograph"><METANAME="DC.format"CONTENT="text/html"SCHEME="MIME"><METANAME="DC.source"CONTENT="1-56592-243-3"SCHEME="ISBN"><METANAME="DC.language"CONTENT="en-US"><METANAME="generator"CONTENT="Jade 1.1/O'Reilly DocBook 3.0 to HTML 4.0"><LINKREV="made"HREF="mailto:online-books@oreilly.com"TITLE="Online Books Comments"><LINKREL="up"HREF="ch12_01.htm"TITLE="12. Packages, Libraries, and Modules"><LINKREL="prev"HREF="ch12_05.htm"TITLE="12.4. Making Variables Private to a Module"><LINKREL="next"HREF="ch12_07.htm"TITLE="12.6. Automating Module Clean-Up"></HEAD><BODYBGCOLOR="#FFFFFF"><img alt="Book Home" border="0" src="gifs/smbanner.gif" usemap="#banner-map" /><map name="banner-map"><area shape="rect" coords="1,-2,616,66" href="index.htm" alt="Perl Cookbook"><area shape="rect" coords="629,-11,726,25" href="jobjects/fsearch.htm" alt="Search this book" /></map><div class="navbar"><p><TABLEWIDTH="684"BORDER="0"CELLSPACING="0"CELLPADDING="0"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch12_05.htm"TITLE="12.4. Making Variables Private to a Module"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 12.4. Making Variables Private to a Module"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1"><ACLASS="chapter"REL="up"HREF="ch12_01.htm"TITLE="12. Packages, Libraries, and Modules"></A></FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch12_07.htm"TITLE="12.6. Automating Module Clean-Up"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 12.6. Automating Module Clean-Up"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch12-chap12_determining_0">12.5. Determining the Caller's Package</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch12-pgfId-716">Problem<ACLASS="indexterm"NAME="ch12-idx-1000005210-0"></A><ACLASS="indexterm"NAME="ch12-idx-1000005210-1"></A></A></H3><PCLASS="para">You need to find out the current or calling package.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch12-pgfId-722">Solution</A></H3><PCLASS="para">To find the current package:</P><PRECLASS="programlisting">$this_pack = __PACKAGE__;</PRE><PCLASS="para">To find the <ACLASS="indexterm"NAME="ch12-idx-1000005227-0"></A>caller's package:</P><PRECLASS="programlisting">$that_pack = caller();</PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch12-pgfId-734">Discussion</A></H3><PCLASS="para">The <CODECLASS="literal">__PACKAGE__</CODE> symbol returns the package that the code is currently being compiled into. This doesn't interpolate into double-quoted strings:</P><PRECLASS="programlisting">print "I am in package __PACKAGE__\n"; # WRONG!<BCLASS="emphasis.bold">I am in package __PACKAGE__</B></PRE><PCLASS="para">Needing to figure out the caller's package arose more often in older code that received as input a string of code to be <CODECLASS="literal">eval</CODE>uated, or a filehandle, format, or directory handle name. Consider a call to a hypothetical <CODECLASS="literal">runit</CODE> function:</P><PRECLASS="programlisting">package Alpha;runit('$line = <TEMP>');package Beta;sub runit { my $codestr = shift; eval $codestr; die if $@;}</PRE><PCLASS="para">Because <CODECLASS="literal">runit</CODE> was compiled in a different package than was currently executing, when the <CODECLASS="literal">eval</CODE> runs, it will act as though it were passed <CODECLASS="literal">$Beta::line</CODE> and <CODECLASS="literal">Beta::TEMP</CODE>. The old workaround was to include your caller's package first:</P><PRECLASS="programlisting">package Beta;sub runit { my $codestr = shift; my $hispack = caller; eval "package $hispack; $codestr"; die if $@;}</PRE><PCLASS="para">That approach only works when <CODECLASS="literal">$line</CODE> is a global variable. If it's lexical, that won't help at all. Instead, arrange for <CODECLASS="literal">runit</CODE> to accept a reference to a subroutine:</P><PRECLASS="programlisting">package Alpha;runit( sub { $line = <TEMP> } );package Beta;sub runit { my $coderef = shift; &$coderef();}</PRE><PCLASS="para">This not only works with lexicals, it has the added benefit of checking the code's syntax at compile time, which is a major win.</P><PCLASS="para">If all that's being passed in is a filehandle, it's more portable to use the <CODECLASS="literal">Symbol::qualify</CODE><ACLASS="indexterm"NAME="ch12-idx-1000005220-0"></A> 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 <CODECLASS="literal">*</CODE> prototype.</P><PCLASS="para">Here's an example that reads and returns <EMCLASS="emphasis">n</EM> lines from a filehandle. The function qualifies the handle before working with it.</P><PRECLASS="programlisting">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, (<CODECLASS="literal">caller()</CODE>)[0]); croak "need open filehandle" unless defined fileno($handle); push(@retlist, $line) while defined($line = <$handle>) && $count--; return @retlist;}</PRE><PCLASS="para">If everyone who called your <CODECLASS="literal">nreadline</CODE> function <ACLASS="indexterm"NAME="ch12-idx-1000006218-0"></A>passed in the filehandle as a typeglob <CODECLASS="literal">*FH</CODE>, as a glob reference <CODECLASS="literal">\*FH</CODE>, or using FileHandle or IO::Handle objects, you wouldn't need to do this. It's only the possibility of a bare <CODECLASS="literal">"FH"</CODE> that requires qualification.<ACLASS="indexterm"NAME="ch12-idx-1000005212-0"></A></P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch12-pgfId-1000005916">See Also</A></H3><PCLASS="para">The documentation for the standard Symbol module, also found in <ACLASS="olink"HREF="../prog/ch07_01.htm">Chapter 7</A> of <ACLASS="citetitle"HREF="../prog/index.htm"TITLE="Programming Perl"><CITECLASS="citetitle">Programming Perl</CITE></A>; the descriptions of the special symbols <CODECLASS="literal">__FILE__ </CODE>, <CODECLASS="literal">__LINE__</CODE> , and <CODECLASS="literal">__PACKAGE__</CODE> in <ICLASS="filename">perldata</I> (1); <ACLASS="xref"HREF="ch12_13.htm"TITLE="Reporting Errors and Warnings Like Built-Ins">Recipe 12.12</A></P></DIV></DIV><DIVCLASS="htmlnav"><P></P><HRALIGN="LEFT"WIDTH="684"TITLE="footer"><TABLEWIDTH="684"BORDER="0"CELLSPACING="0"CELLPADDING="0"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch12_05.htm"TITLE="12.4. 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