Namespace

Namespaces in C++ are used to organize code into logical groups and to prevent name collisions that can occur especially when your code base includes multiple libraries. We can have namespace inside namespace(Sub-namespace). In order to declare namespace we need to use namespace keyword and we need to give name for it.

Syntax
namespace namespaceName{
    //Sub-namespaces , classes, methods
}

In order to access members of a class which is present inside difference namespace. We need to first call the outer most namespace then we need to call sub namespaces then we need to call class then only we can access members of that particular class. We'll see this with code example below

NOTE:
so far in all our previous codes we used something called using namespace std, where std is a namespace where cout, cin are present. If we didn't refer that std namespace, each time we need to use std::cout and std::cin

CODE

EG 1 . Creating a namespace with Sub-namespace.

#include <iostream>
using namespace std;

namespace NewlyCreatedNamespace {
    namespace subNamespace1 {
        namespace space {
            class rectangle {
              public:
                double length, width, height;
                double Area() {
                    return length * width;
                }
                double Perimeter() {
                    return 2 * (length + width);
                }
                double Volume() {
                    return Area() * height;
                }
            };
        }
    }
}

//Either we can refer namespace here or we can each time call individual namespaces and access its member(Explained clearly in CODE EXPLANATION).
using namespace NewlyCreatedNamespace::subNamespace1::space;

int main() {
    //We need to give entire directory of namespaces using :: then only we can access class and its members.
    NewlyCreatedNamespace::subNamespace1::space::rectangle wall;
    wall.length = 5;
    wall.width = 6;
    wall.height = 7;
    cout << wall.Area() << endl;
    cout << wall.Volume() << endl;
    cout << wall.Perimeter() << endl;
}

CODE EXPLANATION
In the above code, we created namespace called NewlyCreatedNamespace . Inside that we created 1 sub-namespace called subNamespace1. That subNamespace1 is having one more sub-namespace called space. Inside that we created 1 class called rectangle. If we want to access rectangle class, we need to first call NewlyCreatedNamespace then subNamespace1 then space namespace then only we can access rectangle class. Apart from this, function wise above code is same as that of what we studied before.

Here each time calling individual namespace is not required, we can use using namespace keyword to refer namespaces.
using namespace NewlyCreatedNamespace::subNamespace1::space;
once we declare above codes in the starting, we no need to each time call namespaces we can directly access the class present inside it. If we want to access members directly we can refer namespace by using namespace keyword or we can each time call individual namespace then access members present inside it.


Where to use Namespace?
The only use of namespace is to organize code elements into a group. Namespace also solves the problem of naming conflict. Two or more classes when put into different namespaces can have same name. Namespaces are not mandatory in a C++ program, but they do play an important role in writing cleaner codes and managing larger projects.

Exercises
1. Create a namespace with 5 sub-namespaces and write mixture of classes inside each sub-namespace. Try to invoke all class members inside main method.

(Click Here, For more exercises)