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Why would you use 'extern “C++”'?

Posted by: admin November 29, 2017 Leave a comment

Questions:

In this article the keyword extern can be followed by “C” or “C++”. Why would you use ‘extern “C++”‘? Is it practical?

Answers:

The language permits:

extern "C" {
  #include "foo.h"
}

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

    void f_plain(const char *);
    extern "C++" void f_fancy(const std::string &);

That’s how you keep the linker happy.

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There is no real reason to use extern "C++". 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 extern "anystring". It only give formal meanings to extern "C" and extern "C++". A compiler vendor is free to define extern "Pascal" or even extern "COM+" if they like.

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I’m not sure why you would need to do it, but according to this 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.

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
}

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Two guesses:

  • If you are in a extern "C" block, you can get C++ language linkage again by specifying a nested extern "C++".
  • It reserves C++ linkage, because it’s the document defining C++. Who is in a better position for defining C++ language linkage than it itself. It also provides for completeness. Same deal as with signed/unsigned.

Read this answer that explains extern "LanguageName" (i.e GCC has extern "Java") aswell.

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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 wiki page for more details:

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C and C++ use different name mangling rules. Essentially, extern “C” tells the C++ compiler to name the function as C would name it.

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This specify which link convention to use. Most languages know how to link with a “C” style function.

You need this in two cases :

  • A C – or other languages for that matter- program calling a function written in C++
  • A C++ program calling a function written in C

Example :

// declared in function.h
void f1(void);

Your C code – actually other languages are able to link with C function – will not be able to link to it because the name in the object table will use C++ convention.

If you write

extern "C" void f1(void);

Now the linking works because it uses C convention.

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extern “C” is used to say that a C++ function should have C linkage. What this means is implementation dependant, but nptmally 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:

extern "C" void Foo();   // can be called easily from C

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.

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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.

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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.