C++: Visibility of parent namespace to child namespace

Question

If you have ns2 as a child namespace of ns1, and you use ns1 classes inside a header for an ns2 class, do you need to explicitly decalre it as you would when ns1 & ns2 are unrelated?

e.g

button.h

namespace ns1
{
 class Button
 {
  ...
 };
}

dialog.h

include "button.h"
namespace ns1
{
 namespace ns2
 {
  class TestDialog
  {
   Button *pButton;
  };
 }
}

Should that be right? It seems I have to change dialog.h to be:

namespace ns1
{
 ----->class Button;
 namespace ns2
 {

But I'm not quite sure why. Do namespaces not inherit? If I don't make this change, I get linker errors about "unresolved symbol ns1::ns2:Button::...".

Answer 1

All functions, classes (types), vars, etc. declared in a namespace will be available (without prefix) in all sub namepaces and so on. So when namespace n2 is defined in n1 all code in n2 can use n1 code without prefix.