What are the complete legal entities that can be put in a namespace?

I'm learning C++ now. What are the complete legal entities that can be put in a namespace?

Legal entities here means valid members of a namespace

Oh, this is a real question. I'm coming from .net and I have the .net mindset.

Any code can be put inside namespace.

However main() function must be at global namespace. It cannot be put inside user-defined namespace.

namespace userns
{
  int main() 
  {
     return 0; 
  }
}

This program wouldn't compile link : http://www.ideone.com/k6SPc

Its because userns::main() will not be considered entry-point of the program; it became just like any other user function, not the standard main(). To compile it successfull, you've to add main() at global namespace:

namespace userns
{
  int main() 
  {
     return 0; 
  }
}

int main() 
{
     return 0; 
}

This will compile link now : http://www.ideone.com/76Ynu

Anything can be put in a namespace (which is legal for C++, of course).

Actually, everything is in some namespace - the global namespace, if not specified.

Everything can be put in namespace except few "entities", which will not compile.

(1) Globally overloaded operator new and operator delete

namespace N
{
  void* operator new (size_t size) // error
  { ... }
}

(2) Definition of the constructs which are declared in outer scope of the namespace; for example you have a class A declared globally then you cannot define its method inside your namespace N. In the same way, if you have method declared in a namespace N then you cannot put its definition inside namespace N::Nested (i.e. Nested is a namespace inside N).

//file
struct A {
  void foo ();
  static int i;
};

namespace N
{
  int A::i = 0; // error
  void A::foo()  // error
  {}
}

Demo: this is not allowed.

I remember at least these 2 restrictions from my experience. Don't know about specs.