namespace A {
int f1();
}
  
namespace A {
int f2();
}

class A {
    int f1();
};
  
class A { // illegal
    int f2();
};

namespace { // fine
  
// some code....
}
  
class { // illegal
}

#include <iostream>
  
namespace foo {
    namespace bar {
        namespace baz {
            int qux = 42;
        }
    }
}
  
namespace fbz = foo::bar::baz;
  
int main()
{
    std::cout << fbz::qux << '\n';
}

42 

class Car {
public:
    typedef std::vector<Wheel> WheelCollection;
    WheelCollection wheels;
};