Namespace and its significance

Namespace

A namespace is a named region that holds a group of identifiers (variables, functions, classes) so that their names do not conflict with the same names declared elsewhere. The standard library places all its names in the namespace std.

How to use it

Defining

#include <iostream>
using namespace std;

namespace first {
    int value = 10;
    void show() { cout << "First: " << value << endl; }
}
namespace second {
    int value = 20;
    void show() { cout << "Second: " << value << endl; }
}

Accessing members in three ways:

1. Scope resolution operator: first::show();
2. using declaration for one name: using first::value;
3. using directive for the whole namespace: using namespace second;

int main() {
    first::show();                   // First: 10
    cout << second::value << endl;   // 20
    using first::value;              // using declaration
    cout << value << endl;           // 10 (first::value)
    using namespace second;          // using directive
    show();                          // Second: 20
    return 0;
}

Namespaces can be nested and can also be extended by declaring the same namespace again in another place. An unnamed namespace limits names to the current file.

Significance

• Avoids name collisions when the same name is used in different libraries or by different programmers.
• Organises a large program into logical groups.
• Allows code from several sources to be combined safely.
• Keeps global scope clean.