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 } 

I want to know the motivation behind it.

Answer 1

I don't know exactly, but my guess is that allowing this at class scope could cause confusion:

namespace Hello {     typedef int World; }  class Blah {     using namespace Hello; public:     World DoSomething(); }  //Should this be just World or Hello::World ? World Blah::DoSomething() {     //Is the using namespace valid in here? } 

Since there is no obvious way of doing this, the standard just says you can't.

Now, the reason this is less confusing when we're talking namespace scopes:

namespace Hello {     typedef int World; }  namespace Other {     using namespace Hello;     World DoSomething(); }  //We are outside of any namespace, so we have to fully qualify everything. Therefore either of these are correct:  //Hello was imported into Other, so everything that was in Hello is also in Other. Therefore this is okay: Other::World Other::DoSomething() {     //We're outside of a namespace; obviously the using namespace doesn't apply here.     //EDIT: Apparently I was wrong about that... see comments.  }  //The original type was Hello::World, so this is okay too. Hello::World Other::DoSomething() {     //Ditto }  namespace Other {     //namespace Hello has been imported into Other, and we are inside Other, so therefore we never need to qualify anything from Hello.     //Therefore this is unambiguiously right     World DoSomething()     {         //We're inside the namespace, obviously the using namespace does apply here.     } } 

Answer 2

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

 using-directive:     using namespace ::optnested-name-specifieroptnamespace-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.