What does a leading :: mean in “using namespace ::X” in C++

Question

can somebody explain me the difference between the following namespace usages:

using namespace ::layer::module;

and

using namespa

Answer 1

There would be a difference if it was used in a context such as:

namespace layer {
    namespace module {
        int x;
    }
}

namespace nest {
    namespace layer {
        namespace module {
            int x;
        }
    }
    using namespace /*::*/layer::module;
}

With the initial :: the first x would be visible after the using directive, without it the second x inside nest::layer::module would be made visible.