Question

Consider the following two statements:

namespace foo = bar;

and

namespace foo {
  using namespace bar;
}

Are those two statements equivalent, or are there some subtle differences I’m not aware of?

(Please note that this is not a question about coding style – I’m just interested in C++ parsing).

Answer 1

namespace foo=bar;

This does not affect any name lookup rules. The only affect is to make ‘foo’ an alias to ‘bar’. for example:

namespace bar
{
  void b();
}

void f () {
  bar::b ();  // Call 'b' in bar
  foo::b ();  // 'foo' is an alias to 'bar' so calls same function
}

The following does change lookup rules

namespace NS
{
  namespace bar
  {
  }

  namespace foo {
    using namespace bar;

    void f () {
      ++i;
    }
  }
}

When lookup takes place for ‘i’, ‘foo’ will be searched first, then ‘NS’ then ‘bar’.