c++ - Calling a static method by repeating the object name?

Question

I have a singleton:

struct foo {
  static foo& instance() {
    static foo f;
    return f;
  }
};

When re-arranging some code I ended up with this statement "by error":

foo::foo::instance()

But this is deemed correct by my compiler (gcc 4.7). In fact, even foo::foo::foo::instance() compiles. Why?

Answer 1

It is due to "injected-name" — which means if foo is a class-name, and the same name "foo" is also injected into the class-scope which is why your code works. It is 100% Standard-conformant.

Here is one interesting example which shows the benefits of this feature:

namespace N
{
   //define a class here
   struct A 
   { 
       void f() { std::cout << "N::A" << std::endl; }
   };
}

namespace M
{
   //define another class with same name!
   struct A 
   { 
       void f() { std::cout << "M::A" << std::endl; }
   };

   struct B : N::A  //NOTE : deriving from N::A
   {
         B()
         {
            A a;
            a.f(); //what should it print?
         }
   };
}

What should a.f() call? What is the type of a? Is it M::A or N::A? The answer is, N::A, not M::A.

• Online Demo

It is because of name-injection, N::A is available inside the constructor of B without qualification. It also hides M::A, which remains outside the scope of B. If you want to use M::A, then you've to write M::A (or better ::M::A).