blocks|key|1611866|text|该语言允许：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1611867|extern+"C"+{
++#include+"foo.h"
}|code-block|syntax|javascript|1611868|如果foo.h包含需要C%2B%2B链接的内容怎么办？|1611869|++++void+f_plain(const+char+*);
++++extern+"C%2B%2B"+void+f_fancy(const+std::string+&);|1611870|这就是你保持链接器快乐的方法。|1611871|entityMap^0|0|0|0|0|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]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

The language permits:

<pre><code>extern "C" {
 #include "foo.h"
}
</code></pre>

What if foo.h contains something which requires C++ linkage? 

<pre><code> void f_plain(const char *);
 extern "C++" void f_fancy(const std::string &amp;);
</code></pre>

That's how you keep the linker happy.

blocks|key|1397379|text|没有使用extern+"C%2B%2B"的真正理由。它只是显式的链接，即隐式缺省。如果您有一个类，其中一些成员具有外部"C“链接，您可能希望显式声明其他成员是外部"C%2B%2B”。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1397380|请注意，C%2B%2B标准在语法上定义了extern+"anystring"。它只给extern+"C"和extern+"C%2B%2B"以形式上的含义。编译器供应商可以根据自己的喜好自由定义extern+"Pascal"，甚至extern+"COM%2B"。|1397381|entityMap^0|4|C|0|G|I|12|A|1D|C|2G|F|2Y|D|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|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]|$9|R|A|S|B|C]|$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|X|8|@]|D|@]|E|$]]]|I|$]]

There is no real reason to use <code>extern "C++"</code>. It merely make explicit the linkage that is the implicit default. If you have a class where some members have extern "C" linkage, you may wish the explicit state that the others are extern "C++".

Note that the C++ Standard defines syntactically <code>extern "anystring"</code>. It only give formal meanings to <code>extern "C"</code> and <code>extern "C++"</code>. A compiler vendor is free to define <code>extern "Pascal"</code> or even <code>extern "COM+"</code> if they like.

blocks|key|1397461|text|我不知道为什么需要这样做，但根据来自Sun的this文章，您可以在外部"C“块中使用外部"+C%2B%2B”来指定一组具有本机C%2B%2B链接的"C“函数中的某些函数。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1397462|extern+"C"+{
++++void+f();+++++++++++++//+C+linkage
++++extern+"C%2B%2B"+{
++++++++void+g();+++++++++//+C%2B%2B+linkage
++++++++extern+"C"+void+h();+//+C+linkage
++++++++void+g2();++++++++//+C%2B%2B+linkage
++++}
++++extern+"C%2B%2B"+void+k();//+C%2B%2B+linkage
++++void+m();+++++++++++++//+C+linkage
}|code-block|syntax|javascript|1397463|entityMap|0|LINK|mutability|MUTABLE|url|http://www.astro.louisville.edu/moore/software/xmccd/archive/xmccd-4.2.1/xmccd-4.2.1/docs/cpp/mixing_c_and_cpp.txt^0|M|4|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@$A|R|B|S|1|T]]|C|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|V|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]]]

I'm not sure why you would need to do it, but according to <a href="http://www.astro.louisville.edu/moore/software/xmccd/archive/xmccd-4.2.1/xmccd-4.2.1/docs/cpp/mixing_c_and_cpp.txt" rel="nofollow noreferrer">this</a> article from Sun, you can use extern "C++" inside a block of extern "C" to specify certain functions in a group of "C" functions have the native C++ linkage.

<pre><code>extern "C" {
 void f(); // C linkage
 extern "C++" {
 void g(); // C++ linkage
 extern "C" void h(); // C linkage
 void g2(); // C++ linkage
 }
 extern "C++" void k();// C++ linkage
 void m(); // C linkage
}
</code></pre>

blocks|key|1612249|text|外部"C“得到了很多人的回答。外部"+C%2B%2B“的用例是在C函数中调用C%2B%2B库函数时。子用例，也就是相关的用例，是将C%2B%2B库与具有主函数的C源代码相链接。有关详细信息，请查看此wiki+page：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1612250|entityMap|0|LINK|mutability|MUTABLE|url|http://en.wikipedia.org/wiki/Compatibility_of_C_and_C%2B%2B#Linking_C_and_C.2B.2B_code^0|2F|9|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

Extern "C" is answered by many. The use case for extern "C++" is when calling C++ library function in a C function. The sub-use case, that is relevant, is when linking a C++ library with a C source code with main function. Check this <a href="http://en.wikipedia.org/wiki/Compatibility_of_C_and_C++#Linking_C_and_C.2B.2B_code" rel="nofollow">wiki page</a> for more details:

blocks|key|1397236|text|C和C%2B%2B使用不同的name+mangling+rules。从本质上讲，外部"C“告诉C%2B%2B编译器按照C的命名方式命名函数。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1397237|entityMap|0|LINK|mutability|MUTABLE|url|http://en.wikipedia.org/wiki/Name_mangling^0|A|J|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

C and C++ use different <a href="http://en.wikipedia.org/wiki/Name_mangling" rel="nofollow noreferrer">name mangling rules</a>. Essentially, extern "C" tells the C++ compiler to name the function as C would name it.

blocks|key|1612141|text|我使用外部"C“的第一个原因是避免C%2B%2B的名字混乱规则。如果你在.Net语言中工作并且想要PInvoke到一个特定的本机函数中，这是非常重要的。唯一的方法是禁用名字混乱。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1612142|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

The #1 reason I use extern "C" is to avoid C++'s name mangling rules. This is very important if you are working in a .Net language and want to PInvoke into a particular native function. The only way to do this is with name mangling disabled.

blocks|key|1397599|text|回答第二个问题，“它实用吗？”：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1397600|在像<cmath>这样的标准头文件中，它是实用的，而且几乎是不可避免的。|offset|length|style|CODE|1397601|设想一个仅包含头文件的库X.h，它是用C%2B%2B和C的公共子集编写的，并且打算在这两种语言中使用。为了C%2B%2B用户的利益，X.h包括<cmath>，而不是<math.h>。但是，如果<cmath>的作者不是把所有东西都夹在extern+"C%2B%2B"+{+...+}中，这对C用户就不起作用了。|1397602|entityMap^0|0|2|7|0|1R|7|22|8|2G|7|30|E|3J|1|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|M|8|@$D|N|E|O|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|P|8|@$D|Q|E|R|F|G]|$D|S|E|T|F|G]|$D|U|E|V|F|G]|$D|W|E|X|F|G]|$D|Y|E|Z|F|G]]|9|@]|A|$]]|$1|J|3|-4|5|6|7|10|8|@]|9|@]|A|$]]]|K|$]]

To answer the second question, &quot;is it practical?&quot;:
It is practical, and practically unavoidable, in standard headers like <code>&lt;cmath&gt;</code>.
Imagine a header-only library X.h, written in a common subset of C++ and C, and intended to be used from both languages. For the benefit of C++ users, X.h includes <code>&lt;cmath&gt;</code>, not <code>&lt;math.h&gt;</code>. This would not work for C users though, hadn't the authors of <code>&lt;cmath&gt;</code> sandwiched everything in <code>extern &quot;C++&quot; {</code> ... <code>}</code>.

blocks|key|1397136|text|简而言之，您可以使用extern+C来告诉编译器不要使用名称损坏。这意味着您可以将同一项目中的C和C%2B%2B代码链接在一起。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1397137|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Short answer is that you can use extern C to tell the compiler not to use name-mangling. This means you can link together bits of C and C++ code in the same project.

blocks|key|1612041|text|外部"C“用于表示C%2B%2B函数应该有C链接。这意味着依赖于实现，但通常它会关闭C%2B%2B名称损坏(以及重载和严格的类型检查)。当你有一个想要从C代码中调用的C%2B%2B函数时，你可以使用它：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1612042|extern+"C"+void+Foo();+++//+can+be+called+easily+from+C|code-block|syntax|javascript|1612043|至于外部"+C%2B%2B+"，我从来没有在真正的代码中见过它，尽管C%2B%2B标准允许它。我猜这是个禁区。|1612044|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|$]]

extern "C" is used to say that a C++ function should have C linkage. What this means is implementation dependant, but normally it turns off C++ name-mangling (and so overloading and strict type checking). You use it when you have a C++ function you want to be called from C code:

<pre><code>extern "C" void Foo(); // can be called easily from C
</code></pre>

As for extern "C++", I've never seen it in real code, though the C++ Standard allows it. I guess it is a no-op.

In this <a href="http://msdn.microsoft.com/en-us/library/0603949d(VS.80).aspx" rel="noreferrer">article</a> the keyword extern can be followed by "C" or "C++". Why would you use 'extern "C++"'? Is it practical?

Why would you use 'extern "C++"'?

编译器

在这个中，关键字extern后面可以跟"C“或"C++”。为什么要使用'extern“C++‘？它实用吗？

问为什么要使用'extern“C++‘？
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