error: namespace ‘foo::bar’ not allowed in using-declaration

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#include "boost/filesystem.hpp"   // includes all needed Boost.Filesystem declarations
using boost::filesystem; 

int main()
{
        return 0;
}

compiling:

g++ boost_example.cpp -o run
boost_example.cpp:2: error: namespace ‘boost::filesystem’ not allowed in using-declaration

The error comes from the fact that filesystem is a namespace and not a class.

Header  synopsis
  namespace boost
  {
    namespace filesystem
    {
    }
  }

So:

using boost::filesystem;

should be:

using namespace boost::filesystem;