how to disable C++ namespace indentation

[chun]

In C++, I want to keep other indentations intact except for namespace
brackets, e.g.
namespace a {
struct A {
  int b;
};

void c {
  int d;
}
}

The above is what I want.  I tried (c-set-offset 'innamespace 0).
However it disables automatic indentation at every line when I hit
return.