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This isachieved by enclosing the entire file in a preprocessor conditionalwhich evaluates to false after the first time it has been seen by thepreprocessor. Traditionally, the macro used is in all upper case, andnamed after the installation path without the installation prefix.Imagine a header that will be installed to<TT>`/usr/local/include/sys/foo.h'</TT>, for example. The preprocessorcode would be as follows:</P><P><TABLE width=100%><tr><td> </td><td class=example bgcolor=#6688aa><br><pre>#ifndef SYS_FOO_H#define SYS_FOO_H 1...#endif /* !SYS_FOO_H */</pre></td></tr></table></P><P>Apart from comments, the entire content of the rest of this header filemust be between these few lines. It is worth mentioning that inside theenclosing <CODE>ifndef</CODE>, the macro <CODE>SYS_FOO_H</CODE> must be definedbefore any other files are <CODE>#include</CODE>d. It is a common mistake tonot define that macro until the end of the file, but mutual dependencycycles are only stalled if the guard macro is defined before the<CODE>#include</CODE> which starts that cycle<A NAME="DOCF6" HREF="autobook_fot.html#FOOT6">(6)</A>.</P><P>If a header is designed to be installed, it must <CODE>#include</CODE> otherinstalled project headers from the local tree using angle-brackets.There are some implications to working like this:</P><P><UL><LI>You must be careful that the names of header file directories in thesource tree match the names of the directories in the install tree. Forexample, when I plan to install the aforementioned <TT>`foo.h'</TT> to<TT>`/usr/local/include/project/foo.h'</TT>, from which it will be includedusing <SAMP>`#include <project/foo.h>'</SAMP>, then in order for the sameinclude line to work in the source tree, I must name the sourcedirectory it is installed from <TT>`project'</TT> too, or other headers whichuse it will not be able to find it until after it has been installed.<P><LI>When you come to developing the next version of a project laid out inthis way, you must be careful about finding the correct header.Automake takes care of that for you by using <SAMP>`-I'</SAMP> options thatforce the compiler to look for uninstalled headers in the current sourcedirectory before searching the system directories for installed headersof the same name.<P><LI>You don't have to install all of your headers to <TT>`/usr/include'</TT> --you can use subdirectories. And all without having to rewrite theheaders at install time.</UL><P><A NAME="C++ Compilers"></A></TR></TABLE><BR> <FONT SIZE="-1">This document was generatedby <I>Gary V. Vaughan</I> on <I>September, 12 2004</I>using <A HREF="http://www.mathematik.uni-kl.de/~obachman/Texi2html"><I>texi2html</I></A></BODY></HTML>⌨️ 快捷键说明
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