Why can't we declare namespace aliases inside a class?

Question

It looks like it's impossible to declare a namespace alias inside a class; however we can do so at function-level (tested with g++ 4.3.4) :

namespace A
{
}

class C
{
  namespace N = A; // error: expected unqualified-id before `namespace'
};

class D
{
  void f();
};

void D::f()
{
  namespace N = A; // OK
}

Any idea why such a restriction exists ? This doesn't seem very consistent with typedefs which can be declared inside a class.

Answer 1

According C++ standard 3.3.6

The following rules describe the scope of names declared in classes.

1) The potential scope of a name declared in a class consists not only of the declarative region following the name’s declarator, but also of all function bodies, default arguments, and constructor ctor- initializers in that class (including such things in nested classes). ...........

So you can declare only things from this list in class scope. Declaring anything else in class scope isn't valid. Not only namespace alliance, but also namespace. For example

class myClass
{
    //compilation error !!!
    namespace myNamespace
    {
    }
    using namespace std;//another compilation error
}

Edit:

Any idea why such a restriction exists ? This doesn't seem very consistent with typedefs which can be declared inside a class.

Because using typedefs in classes are very usefull (for example vector<int>::iterator), while for namespaces it's useless. Consider following code

class myClass
{
   namespce N=std;
};

//now let's use N
MyClass::N::vector<int> v;//don't you think, that this syntax is horrible, and useless?????

For comparison see what it is doing in function

void f()
{
    namespace bnu= boost::numeric::ublas;
    bnu::matrix<int> m;//and now we can use short name bnu
}

For class we can declare namespace alliance in cpp file and there is NO NEED to daclare it in class declaration.