How to create a namespace in C++

Posted on November 30, 2022 • 2 minutes • 399 words

In C++ programming, a namespace is a collection of names or identifiers related to one another.

Syntax:

namespace namespace_name {
   // code here
}

Example:

namespace anooptube_namespace
{
    int getName()
    {
      std::cout << "#anooptube" << std::endl; 
    }
}

How to create a arithmetic namespace

Step-1:

Create a header file named arithmetic.h
Add header guards.

// arithmetic.h

#ifndef ANOOPTUBE_ARITHMETIC_H
#define  ANOOPTUBE_ARITHMETIC_H


#endif

Add namespace declaration.

// arithmetic.h

#ifndef ANOOPTUBE_ARITHMETIC_H
#define  ANOOPTUBE_ARITHMETIC_H

namespace arithmetic
{
    
}

#endif

Add required functions.

// arithmetic.h

#ifndef ANOOPTUBE_ARITHMETIC_H
#define  ANOOPTUBE_ARITHMETIC_H

#include <iostream>

namespace arithmetic
{
    int getSum(int x, int y)
    {
        std::cout << "Called arithmetic getSum()" << std::endl;
        return x+y;
    }

    int getDiff(int x, int y)
    {
        std::cout << "Called arithmetic getSum()" << std::endl;
        return x-y;
    }
}

#endif

you have successfully added a header file with arithmetic namespace.

Step-2:

Create a cpp file named main.cpp

// main.cpp
#include <iostream>

int main() {
  std::cout << "Hello Welcome to Anooptube!\n";
}

include arithmetic.h

// main.cpp
#include <iostream>
#include "arithmetic.h"

int main() {
  std::cout << "Hello Welcome to Anooptube!\n";
}

We must use scope-resolution operator :: to access functions in the arithmetic namespace.

// main.cpp
#include <iostream>
#include "arithmetic.h"

int main() {
  std::cout << "Hello Welcome to Anooptube!\n";
  std::cout << arithmetic::getSum(100,200) << std::endl;
  std::cout << arithmetic::getDiff(200,50) << std::endl;
}

Nested namespaces

It is possible to add namespaces in another namespace. It is called nested namespaces.

We can add sum and diff namespaces in arithmetic namespace.

// arithmetic.h

#ifndef ANOOPTUBE_ARITHMETIC_H
#define  ANOOPTUBE_ARITHMETIC_H

#include <iostream>

namespace arithmetic
{
    namespace sum
    {
        int getSum(int x, int y)
        {
            std::cout << "Called arithmetic getSum()" << std::endl;
            return x+y;
        }
    }

    namespace diff
    {
        int getDiff(int x, int y)
        {
            std::cout << "Called arithmetic getSum()" << std::endl;
            return x-y;
        }
    }
}

#endif

Access of getSum and getDiff will be

// main.cpp
#include <iostream>
#include "arithmetic.h"

int main() {
  std::cout << "Hello Welcome to Anooptube!\n";
  std::cout << arithmetic::sum::getSum(100,200) << std::endl;
  std::cout << arithmetic::diff::getDiff(200,50) << std::endl;
}

using keyword

we can use using keyword to simplify the namespace access.

Syntax:

using namespace namespace_name;

Example 1:

// main.cpp
#include <iostream>
#include "arithmetic.h"

using namespace arithmetic;

int main() {
  std::cout << "Hello Welcome to Anooptube!\n";
  std::cout << sum::getSum(100,200) << std::endl;
  std::cout << diff::getDiff(200,50) << std::endl;
}

Example 2:

// main.cpp
#include <iostream>
#include "arithmetic.h"

using namespace arithmetic::sum;
using namespace arithmetic::diff;

int main() {
  std::cout << "Hello Welcome to Anooptube!\n";
  std::cout << getSum(100,200) << std::endl;
  std::cout << getDiff(200,50) << std::endl;
}