blocks|key|1738850|text|您可以有一个预编译步骤运行，生成一个包含文件的包含文件，该文件包含表示当前目录中存在的文件名称的#定义列表：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1738851|#define+EXISTS_FILE1_C
#define+EXISTS_FILE1_H
#define+EXISTS_FILE2_C|code-block|syntax|javascript|1738852|然后，将该文件包含在源代码中，然后源代码可以测试EXISTS_*定义，以查看文件是否存在。|offset|length|style|CODE|1738853|entityMap^0|0|0|O|8|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@$I|R|J|S|K|L]]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

You could have a pre-build step run that generates an include file that contains a list of #defines that represent the names of the files existing in the current directory:

<pre><code>#define EXISTS_FILE1_C
#define EXISTS_FILE1_H
#define EXISTS_FILE2_C
</code></pre>

Then, include that file from within your source code, and then your source can test the <code>EXISTS_*</code> defines to see whether a file exists or not.

blocks|key|1738911|text|通常，这是通过使用脚本来完成的，该脚本在尝试包含该文件时尝试运行预处理器。根据预处理器是否返回错误，脚本使用适当的#define+(或#undef)更新生成的.h文件。在bash中，脚本可能大致如下所示：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1738912|cat+>+.test.h+<<'EOM'
#include+<asdf.h>
EOM
if+gcc+-E+.test.h
+then
++echo+'#define+HAVE_ASDF_H+1'+>>+config.h
+else+
++echo+'#ifdef+HAVE_ASDF_H'+>>+config.h
++echo+'#+undef+HAVE_ASDF_H'+>>+config.h
++echo+'#endif'+>>+config.h
+fi|code-block|syntax|javascript|1738913|对于像这样的可移植性检查(以及其他数千种检查)来说，一个相当全面的框架是autoconf。|offset|length|1738914|entityMap|0|LINK|mutability|MUTABLE|url|http://www.gnu.org/software/autoconf/^0|0|0|10|8|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@]|9|@$I|V|J|W|1|X]]|A|$]]|$1|K|3|-4|5|6|7|Y|8|@]|9|@]|A|$]]]|L|$M|$5|N|O|P|A|$Q|R]]]]

Generally this is done by using a script that tries running the preprocessor on an attempt at including the file. Depending on if the preprocessor returns an error, the script updates a generated .h file with an appropriate #define (or #undef). In bash, the script might look vaguely like this:

<pre><code>cat &gt; .test.h &lt;&lt;'EOM'
#include &lt;asdf.h&gt;
EOM
if gcc -E .test.h
 then
 echo '#define HAVE_ASDF_H 1' &gt;&gt; config.h
 else 
 echo '#ifdef HAVE_ASDF_H' &gt;&gt; config.h
 echo '# undef HAVE_ASDF_H' &gt;&gt; config.h
 echo '#endif' &gt;&gt; config.h
 fi
</code></pre>

A pretty thorough framework for portably working with portability checks like this (as well as thousands others) is <a href="http://www.gnu.org/software/autoconf/" rel="noreferrer">autoconf</a>.

blocks|key|1523954|text|预处理器本身不能识别文件的存在，但您肯定可以使用构建环境来识别。我最熟悉的是make，它允许你在你的makefile中做类似这样的事情：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1523955|ifdef+$(test+-f+filename+&&+echo+"present")
++DEFINE=-DFILENAME_PRESENT
endif|code-block|syntax|javascript|1523956|当然，您必须在其他构建环境(如VisualStudio+)中找到与此类似的工具，但我确信它们确实存在。|1523957|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

The preprocessor itself cannot identify the existence of files but you certainly can use the build environment to do so. I'm mostly familiar with make, which would allow you to do something like this in your makefile:

<pre><code>ifdef $(test -f filename &amp;&amp; echo "present")
 DEFINE=-DFILENAME_PRESENT
endif
</code></pre>

Of course, you'd have to find an analog to this in other build environments like VisualStudio, but I'm sure they exist.

blocks|key|1739026|text|据我所知，cpp没有关于文件存在的指令。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1739027|如果您正在跨平台使用相同的make，那么借助Makefile的一些帮助，您也许能够实现这一点。您可以检测Makefile中是否存在文件：|1739028|foo.o:+foo.c
++++if+[+-f+header1.h+];+then+CFLAGS%2B=-DHEADER1_INC|code-block|syntax|javascript|1739029|正如@Greg+Hewgill提到的，你可以让你的#includes成为有条件的：|1739030|#ifdef+HEADER1_INC
#include+<header1.h>
#endif|1739031|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|P|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|Q|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|R|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|S|8|@]|9|@]|A|$G|H]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

So far as I know cpp does not have a directive regarding the existence of a file.

You might be able to accomplish this with a bit of help from the Makefile, if you're using the same make across platforms. You can detect the presence of a file in the Makefile:

<pre><code>foo.o: foo.c
 if [ -f header1.h ]; then CFLAGS+=-DHEADER1_INC
</code></pre>

As @Greg Hewgill mentions, you can then make your #includes be conditional:

<pre><code>#ifdef HEADER1_INC
#include &lt;header1.h&gt;
#endif
</code></pre>

blocks|key|1739080|text|与这里和互联网上的一些声明相反，Visual+Studio2015不支持__has_include功能-至少根据我的经验。已使用更新3进行测试。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1739081|传言可能是因为VS+2017也被称为“版本15"；VS+2015被称为”版本14“。对该功能的支持似乎是在"Visual+Studio+2017版本15.3“中正式引入的。|1739082|entityMap^0|10|D|0|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|M|8|@]|D|@]|E|$]]|$1|H|3|-4|5|6|7|N|8|@]|D|@]|E|$]]]|I|$]]

Contrary to some claims here and on the internet, Visual Studio 2015 does NOT support the <code>__has_include</code> feature - at least according to my experience. Tested with Update 3.

The rumors may have arisen from the fact that VS 2017 is also referred to as "Version 15"; VS 2015 is instead referred to as "Version 14". Support for the feature seems to have been officially introduced with "Visual Studio 2017 Version 15.3".

I have a very large codebase (read: thousands of modules) that has code shared across numerous projects that all run on different operating systems with different C++ compilers. Needless to say, maintaining the build process can be quite a chore. 

There are several places in the codebase where it would clean up the code substantially if only there were a way to make the pre-processor ignore certain <code>#includes</code> if the file didn't exist in the current folder. Does anyone know a way to achieve that?

Presently, we use an <code>#ifdef</code> around the <code>#include</code> in the shared file, with a second project-specific file that #defines whether or not the <code>#include</code> exists in the project. This works, but it's ugly. People often forget to properly update the definitions when they add or remove files from the project. I've contemplated writing a pre-build tool to keep this file up to date, but if there's a platform-independent way to do this with the preprocessor I'd much rather do it that way instead. Any ideas?

Can the C preprocessor be used to tell if a file exists?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我有一个非常大的代码库(阅读:数千个模块)，其中的代码在许多项目中共享，这些项目都运行在使用不同C++编译器的不同操作系统上。不用说，维护构建过程可能是一件相当繁琐的事情。在代码库中有几个地方，如果有一种方法可以让预处理器忽略当前文件夹中不存在的某些#includes，那么它就会在很大程度上清理代码。有谁知道实现这一目...

问C预处理器可以用来判断文件是否存在吗？
EN

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问C预处理器可以用来判断文件是否存在吗？EN