Why “using namespace X;” is not allowed inside class/struct level?

Question

class C {
  using namespace std;  // error
};
namespace N {
  using namespace std; // ok
}
int main () {
  using namespace std; // ok
}

Edi

Answer 1

Because the C++ standard explicitly forbids it. From C++03 §7.3.4 [namespace.udir]:

using-directive:
    using namespace ::opt nested-name-specifieropt namespace-name ;

A using-directive shall not appear in class scope, but may appear in namespace scope or in block scope. [Note: when looking up a namespace-name in a using-directive, only namespace names are considered, see 3.4.6. ]

Why does the C++ standard forbid it? I don't know, ask a member of the ISO committee that approved the language standard.