Example for namespaces for discussion in cppreference.com wiki

namespace_example.cpp

namespace A {
    constexpr int i = 2;
}
namespace B {
    constexpr int i = 3;
    int j;
    namespace C {
        namespace D {
            using namespace A; // all names from A injected into D
            int j;
            int k;
            int a = i; // i is B::i, because A::i is hidden by B::i
        }
        using namespace D; // names from D are injected into C
                           // names from A are also injected into C
        int k = 89; // OK to declare name identical to one introduced by a using
        //int l = k;  // ambiguous: C::k or D::k
        int m = i;  // ok: B::i hides A::i
        int n = j;  // ok: D::j hides B::j
        static_assert(C::i == 2); // this is ok, takes `i` from `A` via `C`
        static_assert(i == 3); // this is ok (takes 'i' from B)
        static_assert(::i == 2 || ::i == 3); // Compile failure; `i` not in global namespace
    }
}
static_assert(i == 2 || i == 3); // Compile failure; `i` not in global namespace