Namespace in c++

Consider following C++ program.

// A program to demonstrate need of namespace int main() {     int value;     value = 0;     double value; // Error here     value = 0.0; }Output :

Compiler Error:
'value' has a previous declaration as 'int value'

In each scope, a name can only represent one entity. So, there cannot be two variables with the same name in the same scope. Using namespaces, we can create two variables or member functions having the same name.

// Here we can see that more than one variables // are being used without reporting any error. // That is because they are declared in the // different namespaces and scopes. #include <iostream> using namespace std; // Variable created inside namespace namespace first {     int val = 500; } // Global variable int val = 100; int main() {     // Local variable     int val = 200;     // These variables can be accessed from     // outside the namespace using the scope     // operator ::     cout << first::val << '\n';     return 0; }Output:

500

Definition and Creation:

Namespaces allow us to group named entities that otherwise would have global scope into narrower scopes, giving them namespace scope. This allows organizing the elements of programs into different logical scopes referred to by names.

• Namespace is a feature added in C++ and not present in C.
• A namespace is a declarative region that provides a scope to the identifiers (names of the types, function, variables etc) inside it.
• Multiple namespace blocks with the same name are allowed. All declarations within those blocks are declared in the named scope.

A namespace definition begins with the keyword namespace followed by the namespace name as follows:

namespace namespace_name 
{
   int x, y; // code declarations where 
             // x and y are declared in 
             // namespace_name's scope
}

• Namespace declarations appear only at global scope.
• Namespace declarations can be nested within another namespace.
• Namespace declarations don’t have access specifiers. (Public or private)
• No need to give semicolon after the closing brace of definition of namespace.
• We can split the definition of namespace over several units.

// Creating namespaces #include <iostream> using namespace std; namespace ns1 {     int value()    { return 5; } } namespace ns2 {     const double x = 100;     double value() {  return 2*x; } } int main() {     // Access value function within ns1     cout << ns1::value() << '\n';     // Access value function within ns2     cout << ns2::value() << '\n';     // Access variable x directly     cout << ns2::x << '\n';           return 0; }Output:

5
200
100

Classes and Namespace:

Following is a simple way to create classes in a name space

// A C++ program to demonstrate use of class // in a namespace #include <iostream> using namespace std; namespace ns {     // A Class in a namespace     class geek     {     public:         void display()         {             cout << "ns::geek::display()\n";         }     }; } int main() {     // Creating Object of student Class     ns::geek obj;     obj.display();     return 0; }Output:

ns::geek::display()

Class can also be declared inside namespace and defined outside namespace using the following syntax

// A C++ program to demonstrate use of class // in a namespace #include <iostream> using namespace std; namespace ns {     // Only declaring class here     class geek; } // Defining class outside class ns::geek { public:     void display()     {         cout << "ns::geek::display()\n";     } }; int main() {     //Creating Object of student Class     ns::geek obj;     obj.display();     return 0; }Output:

ns::geek::display()

We can define methods also outside the namespace. Following is an example code.

// A C++ code to demonstrate that we can define // methods outside namespace. #include <iostream> using namespace std; // Creating a namespace namespace ns {     void display();     class geek     {     public:        void display();     }; } // Defining methods of namespace void ns::geek::display() {     cout << "ns::geek::display()\n"; } void ns::display() {     cout << "ns::display()\n"; } // Driver code int main() {     ns::geek obj;     ns::display();     obj.display();     return 0; }Output:

ns::display()
ns::geek::display()

Can namespaces be nested in C++?

Extending namespace and Unnamed namespace

It is also possible to create more than one namespaces in the global space. This can be done in two ways.

• namespaces having different names

  // A C++ program to show more than one namespaces // with different names. #include <iostream> using namespace std; // first name space namespace first {    int func() {  return 5; } } // second name space namespace second {    int func() { return 10; } } int main() {    // member function of namespace    // accessed using scope resolution operator    cout << first::func() <<"\n";           cout << second::func() <<"\n";    return 0; }Output:

  5
  10
  

• Extending namespaces (Using same name twice)
  It is also possible to create two namespace blocks having the same name. The second namespace block is nothing but actually the continuation of the first namespace. In simpler words, we can say that both the namespaces are not different but actually the same, which are being defined in parts.

  // C++ program to demonstrate namespace exntension #include <iostream> using namespace std; // first name space namespace first {    int val1 = 500;  } // rest part of the first namespace namespace  first {    int val2 = 501;  } int main() {    cout << first::val1 <<"\n";           cout << first::val2 <<"\n";    return 0; }Output:

  500
  501
  

Unnamed Namespaces

• They are directly usable in the same program and are used for declaring unique identifiers.
• In unnamed namespaces, name of the namespace in not mentioned in the declaration of namespace.
• The name of the namespace is uniquely generated by the compiler.
• The unnamed namespaces you have created will only be accessible within the file you created it in.
• Unnamed namespaces are the replacement for the static declaration of variables.

// C++ program to demonstrate working of unnamed // namespaces #include <iostream> using namespace std; // unnamed namespace declaration namespace {    int rel = 300; } int main() {    cout << rel << "\n"; // prints 300    return 0; }Output:

300