Namespace scope of ANSI C++ Standard

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We have been defining variables in different scopes in c++ programs, such as classes, function, blocks, etc. ANSI C++ Standard has added a new keyword namespace to define a scope that could hold global identifiers. The best example of namespace scope is the C++ Standard Library. That is why we have been using the directive

                                using namespace std;


The using namespace statement specifies that the members defined in std namespace will be used frequently throughout the program.

Defining a Namespace

We can define our own namespace in our programs. The syntax for defining a namespace is similar to the syntax for defining a class. The general form of namespace is:

namespace namespace_name
{
              // Declaration of
             // variables, function, classes, etc.
}


Example:

namespace TestSpace
{
            int m;
            void display(int n)
            {
                   cout<<n;
             }
}                                                                 // No semicolon here

Here, the variable m and the function display are inside the scope defined by the TestSpace namespace. If we want to assign a value to m, we must use the scope resolution operator as shown below.

TestSpace::m=100;

Note that m is qualified using the namespace name.

This approach becomes cumbersome if the members of a namespace are frequently used. In such cases, we can use a using directive to simplify their access. This can be done in two ways:

using namespace namespace_name;                       // using directive

using namespace_name::member_name;               // using declaration

In the first form, all the members declared within the specified namespace may be accessed with-out using qualification. In the second form, we can access only the specified member in the program.

Example:

using namespace TestSpace;
m=100;                                // OK
display(200);                       // OK

using TestSpace::m;
m=100;                                // OK
display(200);                       // Not ok, display not visible