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

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.