Possible to wrap or merge separate namespaces?

Question

I seem to recall seeing notes somewhere on a way to combine multiple namespaces into one.

Now, looking for said notes I am not finding them -- even searching using s

Answer 1

The best solution since C++11 is:

namespace c
{
    inline namespace a { using namespace ::a; }
    inline namespace b { using namespace ::b; }
}

This way for names that not conflict you can qualify only by c and you can resolve conflicts by qualifing c::a or c::b.

e.g.:

namespace a
{
    auto foo_a() { cout << "a::foo_a" << endl; }
    auto foo() { cout << "a::foo" << endl; }
}
namespace b
{
    auto foo_b() { cout << "b::foo_b" << endl; }
    auto foo() { cout << "b::foo" << endl; }
}

namespace c
{
    inline namespace a { using namespace ::a; }
    inline namespace b { using namespace ::b; }
}

int main()
{
  c::foo_a();
  c::foo_b();

  c::a::foo();
  c::b::foo();

  return 0;
}

Answer 2

You can wrap the namespaces in a new one like this:

namespace c {
    namespace a { using namespace ::a; }
    namespace b { using namespace ::b; }
}

Answer 3

You can do this:

namespace c
{
    using namespace a;
    using namespace b;
}

But if a and b have elements with the same names, you won't be able to use them from namespace c.