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<HTML><HEAD><TITLE>Recipe 11.12. Copying Data Structures (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="1999-07-02T01:40:52Z"><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="ch11_01.htm"TITLE="11. References and Records"><LINKREL="prev"HREF="ch11_12.htm"TITLE="11.11. Printing Data Structures"><LINKREL="next"HREF="ch11_14.htm"TITLE="11.13. Storing Data Structures to Disk"></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="ch11_12.htm"TITLE="11.11. Printing Data Structures"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 11.11. Printing Data Structures"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1"><ACLASS="chapter"REL="up"HREF="ch11_01.htm"TITLE="11. References and Records"></A></FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch11_14.htm"TITLE="11.13. Storing Data Structures to Disk"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 11.13. Storing Data Structures to Disk"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch11-chap11_copying_0">11.12. Copying Data Structures</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch11-pgfId-1286">Problem<ACLASS="indexterm"NAME="ch11-idx-1000004864-0"></A><ACLASS="indexterm"NAME="ch11-idx-1000004864-1"></A><ACLASS="indexterm"NAME="ch11-idx-1000004864-2"></A><ACLASS="indexterm"NAME="ch11-idx-1000004864-3"></A></A></H3><PCLASS="para">You need to copy a complex data structure.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch11-pgfId-1292">Solution</A></H3><PCLASS="para">Use the <CODECLASS="literal">dclone</CODE> function from the Storable module from CPAN:</P><PRECLASS="programlisting">use Storable;$r2 = dclone($r1);</PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch11-pgfId-1304">Discussion</A></H3><PCLASS="para">Two types of "copy" are sometimes confused. A <ICLASS="firstterm">surface copy</I><ACLASS="indexterm"NAME="ch11-idx-1000004865-0"></A><ACLASS="indexterm"NAME="ch11-idx-1000004865-1"></A><ACLASS="indexterm"NAME="ch11-idx-1000004865-2"></A> (also known as <ICLASS="firstterm">shallow copy</I>) simply copies references without creating copies of the data behind them:</P><PRECLASS="programlisting">@original = ( \@a, \@b, \@c );@surface = @original;</PRE><PCLASS="para">A <ICLASS="firstterm">deep copy</I><ACLASS="indexterm"NAME="ch11-idx-1000004866-0"></A> creates an entirely new structure with no overlapping references. This copies references to 1 layer deep:</P><PRECLASS="programlisting">@deep = map { [ @$_ ] } @original;</PRE><PCLASS="para">If <CODECLASS="literal">@a</CODE>, <CODECLASS="literal">@b</CODE>, and <CODECLASS="literal">@c</CODE> themselves contain references, the preceding <CODECLASS="literal">map</CODE> is no longer adequate. Writing your own code to deep-copy structures is laborious and rapidly becomes tiresome.</P><PCLASS="para">The Storable module, found on CPAN, provides a function called <CODECLASS="literal">dclone</CODE> that recursively copies its argument:</P><PRECLASS="programlisting">use Storable qw(dclone); $r2 = dclone($r1);</PRE><PCLASS="para">This only works on references or blessed objects of type SCALAR, ARRAY, or HASH; references of type CODE, GLOB, and IO and more esoteric types are not supported. The <CODECLASS="literal">safeFreeze</CODE> function from the <CODECLASS="literal">FreezeThaw</CODE> module supports these when used in the same address space by using a reference cache that could interfere with garbage collection and object destructors under some circumstances.</P><PCLASS="para">Because <CODECLASS="literal">dclone</CODE> takes and returns references, you must add extra punctuation if you have a hash of arrays that you want to copy:</P><PRECLASS="programlisting">%newhash = %{ dclone(\%oldhash) };<ACLASS="indexterm"NAME="ch11-idx-1000004868-0"></A><ACLASS="indexterm"NAME="ch11-idx-1000004868-1"></A><ACLASS="indexterm"NAME="ch11-idx-1000004868-2"></A></PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch11-pgfId-1332">See Also</A></H3><PCLASS="para">The documentation for the CPAN modules Storable, Data::Dumper, and FreezeThaw</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="ch11_12.htm"TITLE="11.11. 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