Difference between 'using' and 'using namespace'

Question

In the boost libraries, there are often examples of including the library like:

#pragma once #include <boost/property_tree/ptree.hpp> using boost::property_tree::ptree; 

Throughout my program I have been importing namespaces like this:

#include "../MyClass.h" using namespace MyClassNamespace; 

Can someone please explain:

1. The difference between using and using namespace;
2. What the advantage of negating the use of using namespace in favour of using;
3. The differences in forward declaring using and using namespace;

Thanks

Answer 1

using namespace makes visible all the names of the namespace, instead stating using on a specific object of the namespace makes only that object visible.