Question

Today, while seeing preprocessed code of <iostream> (g++ 4.4.1), I saw a line which is quite unusual for me syntactically.

namespace std __attribute__ ((__visibility__ ("default"))) {
// ...
}

I have questions for that particular line:

1. What’s __attribute__ and what it’s
   doing after std ? (new syntax !)
2. How the double braces are coming ((
... )) in place and is it a new syntax ?
3. What’s __visibility__ and what is
   it setting as "default" ?

Answer 1

__attribute__ is a gcc-specific language extension. It lets you alter the declaration of a function, namespace, or other entity in ways that aren’t directly supported by the standard C++ language.

C++0x adds language support for attributes, though the syntax is different and most attributes are still implementation-specific.