使用具体输出操作符声明(带有具体签名)

namespace nm
{
  class C1 {};
  class C2 {};
  inline std::ostream& operator << (std::ostream& lhs, std::vector<C1> const&) { return lhs; }
  inline std::ostream& operator << (std::ostream& lhs, std::vector<C2> const&) { return lhs; }
}

using nm::operator<<;
namespace nm
{
  class C1 {};
  class C2 {};