Why would you use 'extern “C++”'?

Question

In this article the keyword extern can be followed by \"C\" or \"C++\". Why would you use \'extern \"C++\"\'? Is it practical?

Answer 1

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.

Answer 2

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.

Answer 3

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
}

Answer 4

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