问题描述

class C {
  using namespace std;  // error
};
namespace N {
  using namespace std; // ok
}
int main () {
  using namespace std; // ok
}

我想知道它背后的动机。

推荐答案

我不太清楚，但我猜在类范围内允许此操作可能会导致念力：

namespace Hello
{
    typedef int World;
}

class Blah
{
    using namespace Hello;
public:
    World DoSomething();
}

//Should this be just World or Hello::World ?
World Blah::DoSomething()
{
    //Is the using namespace valid in here?
}

由于没有明显的方法可以做到这一点，所以标准只是说您不能这样做。

现在，当我们讨论命名空间作用域时，这不那么令人困惑的原因是：

namespace Hello
{
    typedef int World;
}

namespace Other
{
    using namespace Hello;
    World DoSomething();
}

//We are outside of any namespace, so we have to fully qualify everything. Therefore either of these are correct:

//Hello was imported into Other, so everything that was in Hello is also in Other. Therefore this is okay:
Other::World Other::DoSomething()
{
    //We're outside of a namespace; obviously the using namespace doesn't apply here.
    //EDIT: Apparently I was wrong about that... see comments. 
}

//The original type was Hello::World, so this is okay too.
Hello::World Other::DoSomething()
{
    //Ditto
}

namespace Other
{
    //namespace Hello has been imported into Other, and we are inside Other, so therefore we never need to qualify anything from Hello.
    //Therefore this is unambiguiously right
    World DoSomething()
    {
        //We're inside the namespace, obviously the using namespace does apply here.
    }
}

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