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
vector reside inside the
std namespace. A using directive imports all those names from the
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
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::endl are said to be fully qualified.