Programming for Beginners

Counter, Arithmetic Mean and Division by Zero

using Directive and using Declaration

When using standard input, standard output or vectors in our programs so far, we have been obliged to insert a using directive like this:

using namespace std;

The names in libraries are organized by namespaces. The names cin, cout, endl, setw and vector reside inside the std namespace. A using directive imports all those names from the std namespace into the global namespace.

Another way to import names into the global namespace is by utilizing a using declaration, instead of a using directive, like this:

using std::cout;
using std::endl;
using std::vector;

That allows the names cout, endl and vector to be used in the program. A using declaration imports names one by one, while a using directive imports all the names in a namespace at once.

The third possibility is to prefix names from included libraries by an appropriate namespace name. For example:

    std::vector<double> array;
    std::cout << "namespace std contains the name endl, too!" 
              << std::endl;
    std::cout << "It also contains names cin and setw." << std::endl;

The names like std::vector, std::cout and std::endl are said to be fully qualified.