别名声明范围

别名声明受先前使用声明的影响

namespace boost
{
    namespace multiprecision
    {
        class Number ...
    }
}

using namespace boost;

//   Both Namespace are equivalent 
namespace Name1 = boost::multiprecision;
namespace Name2 = multiprecision;

但是，当你有这样的东西时，更容易混淆你是哪个名称空间别名：

namespace boost
{
    namespace multiprecision
    {
        class Number ...
    }
}

namespace numeric
{
    namespace multiprecision
    {
        class Number ...
    }
}

using namespace numeric;
using namespace boost;

//    Not recommended as 
//    its not explicitly clear whether Name1 refers to
//    numeric::multiprecision or boost::multiprecision
namespace Name1 = multiprecision;

//    For clarity, its recommended to use absolute paths
//    instead
namespace Name2 = numeric::multiprecision;
namespace Name3 = boost::multiprecision;