Notes

Groups names together so two libraries can both have a function called render() without colliding.

namespace physics {
    void update();
}
namespace graphics {
    void update();
}

physics::update();    // no ambiguity - fully qualified
graphics::update();

using namespace — convenient, and a known risk

using namespace std;   // now cout, string, vector, etc. are all unqualified

string s = "hi";   // works, but so does anything else's "string" - if two
                    // namespaces both have one, this becomes ambiguous or
                    // silently picks the wrong one

Fine in a small .cpp file. Never do this in a header — it forces the pollution onto every file that includes it, with no way to opt out.

Anonymous namespace — the modern static

namespace {
    void helperOnlyUsedHere() { ... }
}

Same effect as marking something static at file scope (see static) — internal linkage, invisible outside this translation unit. Preferred for non-function things (classes, variables) since static doesn’t apply to those the same way.

Related

static