cpp_namespace

using and using namespace have validity only in the same block in which they are stated or in the entire source code file if they are used directly in the global scope. For example, it would be possible to first use the objects of one namespace and then those of another one by splitting the code in different blocks:

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// using namespace example
#include <iostream>
using namespace std;

namespace first
{
  int x = 5;
}

namespace second
{
  double x = 3.1416;
}

int main () {
  {
    using namespace first;
    cout << x << ‘\n‘;
  }
  {
    using namespace second;
    cout << x << ‘\n‘;
  }
  return 0;
}

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cpp_namespace

原文：http://www.cnblogs.com/hackerma/p/3738662.html