What are the differences between typedef and using?

Question

What are the differences between using

typedef Some::Nested::Namespace::TypeName TypeName;

or

using Some::Nested::Namespace         


        

Answer 1

Using just brings declaration into the local scope, while typedef introduces a typedef-name. One difference between them is elaborated type specifiers, e.g.:

namespace n
{
  class foo
  {
  };
}

typedef n::foo n_foo;

using n::foo;

int main()
{
  class foo f1; // ok, declares f1 variable of type n::foo.
  class n_foo f2; // error, typedef-name can't be used in elaborated-type-specifier.
}