📄 ch10_11.htm
字号:
<HTML><HEAD><TITLE>Recipe 10.10. Returning Failure (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="1999-07-02T01:39:58Z"><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="ch10_01.htm"TITLE="10. Subroutines"><LINKREL="prev"HREF="ch10_10.htm"TITLE="10.9. Returning More Than One Array or Hash"><LINKREL="next"HREF="ch10_12.htm"TITLE="10.11. Prototyping Functions"></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="ch10_10.htm"TITLE="10.9. Returning More Than One Array or Hash"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 10.9. Returning More Than One Array or Hash"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1"><ACLASS="chapter"REL="up"HREF="ch10_01.htm"TITLE="10. Subroutines"></A></FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch10_12.htm"TITLE="10.11. Prototyping Functions"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 10.11. Prototyping Functions"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch10-32225">10.10. Returning Failure</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch10-pgfId-760">Problem<ACLASS="indexterm"NAME="ch10-idx-1000004702-0"></A><ACLASS="indexterm"NAME="ch10-idx-1000004702-1"></A><ACLASS="indexterm"NAME="ch10-idx-1000004702-2"></A><ACLASS="indexterm"NAME="ch10-idx-1000004702-3"></A></A></H3><PCLASS="para">You want to return a value indicating that your function failed.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch10-pgfId-766">Solution</A></H3><PCLASS="para">Use a bare <CODECLASS="literal">return</CODE><ACLASS="indexterm"NAME="ch10-idx-1000004706-0"></A> statement without any argument, which returns <CODECLASS="literal">undef</CODE> in scalar context and the empty list <CODECLASS="literal">()</CODE> in list context.</P><PRECLASS="programlisting">return;</PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch10-pgfId-774">Discussion</A></H3><PCLASS="para">A <CODECLASS="literal">return</CODE> without an argument means:</P><PRECLASS="programlisting">sub empty_retval { return ( wantarray ? () : undef );}</PRE><PCLASS="para">You can't use just <CODECLASS="literal">return</CODE> <CODECLASS="literal">undef</CODE> because in list context you will get a list of one value: <CODECLASS="literal">undef</CODE>. If your caller says:</P><PRECLASS="programlisting">if (@a = yourfunc()) { ... }</PRE><PCLASS="para">Then the "error" condition will be perceived as true, because <CODECLASS="literal">@a</CODE> will be assigned (<CODECLASS="literal">undef</CODE>) and then evaluated in scalar context. This yields <CODECLASS="literal">1</CODE>, the number of elements in <CODECLASS="literal">@a</CODE>, which is true. You could use the <CODECLASS="literal">wantarray</CODE> function to see what context you were called in, but a bare <CODECLASS="literal">return</CODE> is a clear and tidy solution that always works:</P><PRECLASS="programlisting">unless ($a = sfunc()) { die "sfunc failed" }unless (@a = afunc()) { die "afunc failed" }unless (%a = hfunc()) { die "hfunc failed" }</PRE><PCLASS="para">Some of Perl's built-in functions have a peculiar return value. Both <CODECLASS="literal">fcntl</CODE><ACLASS="indexterm"NAME="ch10-idx-1000004707-0"></A><ACLASS="indexterm"NAME="ch10-idx-1000004707-1"></A> and <CODECLASS="literal">ioctl</CODE> have the curious habit of returning the string <ACLASS="indexterm"NAME="ch10-idx-1000006361-0"></A><CODECLASS="literal">"0</CODE> <CODECLASS="literal">but</CODE> <CODECLASS="literal">true"</CODE> in some circumstances. (This magic string is conveniently exempt from the <BCLASS="emphasis.bold">-w</B> flag's incessant numerical conversion warnings.) This has the advantage of letting you write code like this:</P><PRECLASS="programlisting">ioctl(....) or die "can't ioctl: $!";</PRE><PCLASS="para">That way, code doesn't have to check for a defined zero as distinct from the undefined value, as it would for the <CODECLASS="literal">read</CODE> or <CODECLASS="literal">glob</CODE> functions. <CODECLASS="literal">"0</CODE> <CODECLASS="literal">but</CODE> <CODECLASS="literal">true"</CODE> is zero when used numerically. It's rare that this kind of return value is needed. A more common (and spectacular) way to indicate failure in a function is to raise an exception, as described in <ACLASS="xref"HREF="ch10_13.htm"TITLE="Handling Exceptions">Recipe 10.12</A>.<ACLASS="indexterm"NAME="ch10-idx-1000004709-0"></A><ACLASS="indexterm"NAME="ch10-idx-1000004709-1"></A><ACLASS="indexterm"NAME="ch10-idx-1000004709-2"></A><ACLASS="indexterm"NAME="ch10-idx-1000004709-3"></A></P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch10-pgfId-802">See Also</A></H3><PCLASS="para">The <ACLASS="olink"HREF="../prog/ch03_180.htm"><CODECLASS="literal">undef</CODE></A>, <ACLASS="olink"HREF="../prog/ch03_191.htm"><CODECLASS="literal">wantarray</CODE></A>, and <ACLASS="olink"HREF="../prog/ch03_125.htm"><CODECLASS="literal">return</CODE></A> functions in <ACLASS="olink"HREF="../prog/ch03_01.htm">Chapter 3</A> of <ACLASS="citetitle"HREF="../prog/index.htm"TITLE="Programming Perl"><CITECLASS="citetitle">Programming Perl</CITE></A> and <ICLASS="filename">perlfunc </I>(1); <ACLASS="xref"HREF="ch10_13.htm"TITLE="Handling Exceptions">Recipe 10.12</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="ch10_10.htm"TITLE="10.9. Returning More Than One Array or Hash"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 10.9. Returning More Than One Array or Hash"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><ACLASS="book"HREF="index.htm"TITLE="Perl Cookbook"><IMGSRC="../gifs/txthome.gif"ALT="Perl Cookbook"BORDER="0"></A></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch10_12.htm"TITLE="10.11. Prototyping Functions"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 10.11. Prototyping Functions"BORDER="0"></A></TD></TR><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="228">10.9. Returning More Than One Array or Hash</TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><ACLASS="index"HREF="index/index.htm"TITLE="Book Index"><IMGSRC="../gifs/index.gif"ALT="Book Index"BORDER="0"></A></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228">10.11. Prototyping Functions</TD></TR></TABLE><HRALIGN="LEFT"WIDTH="684"TITLE="footer"><FONTSIZE="-1"></DIV<!-- LIBRARY NAV BAR --> <img src="../gifs/smnavbar.gif" usemap="#library-map" border="0" alt="Library Navigation Links"><p> <a href="copyrght.htm">Copyright © 2002</a> O'Reilly & Associates. All rights reserved.</font> </p> <map name="library-map"> <area shape="rect" coords="1,0,85,94" href="../index.htm"><area shape="rect" coords="86,1,178,103" href="../lwp/index.htm"><area shape="rect" coords="180,0,265,103" href="../lperl/index.htm"><area shape="rect" coords="267,0,353,105" href="../perlnut/index.htm"><area shape="rect" coords="354,1,446,115" href="../prog/index.htm"><area shape="rect" coords="448,0,526,132" href="../tk/index.htm"><area shape="rect" coords="528,1,615,119" href="../cookbook/index.htm"><area shape="rect" coords="617,0,690,135" href="../pxml/index.htm"></map> </BODY></HTML>⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -