Is a using-directive in a detail namespace problematic?

问题

Consider this library header:

#include<vector>
#include<algorithm>
#include<iostream>

namespace Lib {
  namespace detail {
    using namespace std;

    template<class T>
    void sort_impl(istream &in,ostream &out) {
      vector<T> v;
      {
        int n;
        in >> n;
        v.resize(n);
      }
      for(auto &i : v) cin >> i;

      sort(v.begin(),v.end());
      for(auto i : v) out << i << endl;
    }
  }

  inline void sort_std() {
    detail::sort_impl<int>(std::cin,std::cout);
  }
}

Does the detail namespace successfully isolate the clients of the library (and the rest of library's implementation) from the using-directive in this example? I'm not interested in the discussion at Why is "using namespace std" considered bad practice?, even though some of the arguments apply even to "well contained" using-directives.

Note that there are two existing questions concerning the same situation but with using-declarations:

Using declarations in private namespaces in header files
Elegant way to prevent namespace poisoning in C++ (whose one answer is really an answer to the "bad practice" question above)

This could be combined with either of them, but the editing would be severe.

回答1:

You pollute your own detail namesapce, but not the Lib or global namespaces. So assuming a responsible adult is using your library, they won't have unintentional name collisions:

#include <vector>

namespace Lib {
  namespace detail {
    using namespace std;
  }
}

using namespace Lib;

int main() {
    vector<int> v; // This is an error, vector not declared in this scope
}

回答2:

No, the detail namespace will not isolate clients from the nested using directive. [namespace.udir] is quite explicit about that

A using-directive specifies that the names in the nominated namespace can be used in the scope in which the using-directive appears after the using-directive. During unqualified name lookup, the names appear as if they were declared in the nearest enclosing namespace which contains both the using-directive and the nominated namespace. [ Note: In this context, “contains” means “contains directly or indirectly”. — end note ]

A little example

#include <iostream>

namespace foo {
    namespace detail {
        using namespace std;
    }
}

int main()
{
    foo::detail::cout << "Hello World!\n";

    // nothing is stopping me from doing that
    using namespace foo::detail;
    cout << "Hello World!\n";
}

STL gives a nice explanation of how name lookup works in his video Core C++, 1 of n.

来源：https://stackoverflow.com/questions/46168188/is-a-using-directive-in-a-detail-namespace-problematic

标签

c++

namespaces

using-directives