Why do I get a linker error when declaring a function in an unnamed namespace in a header?

Question

I try do that:

file.h

namespace {
   void fun();
   const bool nevermind = Register( fun );
}

file.cpp

namespace {
     void fun() {
        do_some_job();
     }
}

Having linking error. Function void fun() is not found by linker.

If I try do that:

file.h

namespace {
    void fun() {
         do_some_job();
    } 
    const bool nevermind = Register( fun );
}

all goes ok.

How do I compile the first case? I don't want to have the function definition in header file.

Answer 1

The purpose of anonymous namespaces is to prevent you from using that function anywhere else. As such, there is little point in having it being defined in a header. I would assume that whenever you add an anonymous namespace, the compiler actually treats it as a namespace with a gibberish unique name. So when you add another anonymous namespace it is not the same namespace.

Also see comment by BoBTFish below that clarifies this a bit.