Question

Look at the program in Display 4.17 . The main function contains the using directive: using namespace std; Why doesn't the method unitprice contain this using directive?

Short answer

Answer: The "unitprice" method doesn't contain the "using namespace std;" directive because it doesn't utilize any functions, classes, or variables from the standard namespace. The inclusion of the standard namespace in the main function doesn't affect the scope of the "unitprice" method.

Step-by-step solution

Understanding using Namespace

Before diving into why "unitprice" doesn't contain "using namespace std;", it's important to understand the concept of namespaces in C++. Namespace is a feature that groups entities such as classes, functions, and variables, under a unique name to avoid naming conflicts. The keyword "using" is followed by "namespace" and then the namespace name (like "std" for standard namespace). Including a namespace ensures that the whole code can access the functions and variables defined under that namespace. For example, "using namespace std;" allows you to use the names defined in the standard namespace without using their fully qualified names, like "std::".

Recognizing the Scope of an Included Namespace

When a namespace is included in a portion of code, it applies to that specific scope only. In the provided exercise, "using namespace std;" is included in the main function. This means the entire standard namespace is only accessible within the main function. However, when dealing with separate functions or methods, like "unitprice", including the namespace is not necessary if it is not being utilized within that particular function/method.

Analysing the "unitprice" method

As the exercise asks about the "unitprice" method, it's critical to observe if it requires the standard namespace or any of its components for its functionality. If the method doesn't use any function, class or variable from the standard namespace, then there's no need to include "using namespace std;" in the method.

Deduction

Based on the previous steps, we can deduce that the reason for the "unitprice" method not containing "using namespace std;" is because it neither uses any functions, variables, or classes from the standard namespace nor is affected by the scope of the main function, where the standard namespace is already included. In conclusion, the "unitprice" method doesn't need the "using namespace std;" directive because it doesn't utilize any functions, classes or variables from the standard namespace.