C++ Tutorial/Language Basics/namespace

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A namespace

namespace MyNameSpace { 
  int start; 
  int end; 
 
  class counter { 
     int count; 
   public: 
     counter(int n) {  
       count = n; 
     } 
 
     void reset(int n) { 
       count = n; 
     } 
  }; 
}

A nested namespace

#include <iostream>
using namespace std;
namespace MyNamespace1 {
  int i;
  namespace MyNamespace2 { // a nested namespace
    int j;
  }
}
int main()
{
  MyNamespace1::i = 19;
  MyNamespace1::MyNamespace2::j = 10; // this is right
  cout << MyNamespace1::i << " "<<  MyNamespace1::MyNamespace2::j << "\n";
  // use MyNamespace1
  using namespace MyNamespace1;
  cout << i * MyNamespace2::j;
  return 0;
}
19 10
190

function in a namespace reference variables in the same namespace

#include <iostream>
using namespace std;
int integer1 = 98;
namespace Example
{
   const double PI = 3.14159;
   void printValues();
   namespace Inner
   {
      enum Years { FISCAL1 = 1990, FISCAL2, a };
   }
}
namespace
{
   double doubleInUnnamed = 88.22;
}
int main()
{
   Example::printValues();
   return 0;
}
void Example::printValues()
{
   cout << integer1 << "\n(global) integer1 = " << ::integer1
      << "\nIinteger1 = " << Inner::a << endl;
}
98
(global) integer1 = 98
Iinteger1 = 1992

Reference global variable, variable in the nested namespace

#include <iostream>
using namespace std;
int integer1 = 98;
namespace
{
   double doubleInUnnamed = 88.22;
}
namespace Example
{
   const double PI = 3.14159;
   void printValues();
   int integer1 = 100;
   namespace Inner
   {
      int integer1 = 99;
   }
}
int main()
{
   cout << "doubleInUnnamed = " << doubleInUnnamed;
   cout << "\n(global) integer1 = " << integer1;
   cout << Example::integer1 << "\nIinteger1 = "
      << Example::Inner::integer1 << endl;

   return 0;
}
doubleInUnnamed = 88.22
(global) integer1 = 98100
Iinteger1 = 99

Reference variables in namespaces

#include <iostream.h>
namespace first
{
  int var = 5;
}
namespace second
{
  double var = 3.1416;
}
int main () {
  cout << first::var << endl;
  cout << second::var << endl;
  return 0;
}
5
3.1416

Reuse namespace

#include <iostream>
using namespace std;
namespace MyNamespace {
  int i;
}
namespace MyNamespace {
  int j;
}
int main()
{
  MyNamespace::i = MyNamespace::j = 10;
  // refer to MyNamespace specifically
  cout << MyNamespace::i * MyNamespace::j << "\n";
  // use MyNamespace namespace
  using namespace MyNamespace;
  cout << i * j;
  return 0;
}
100
100

Use a namespace

#include <iostream> 
using namespace std; 
 
namespace MyNameSpace { 
  int start; 
  int end; 
 
  class counter { 
     int count; 
   public: 
     counter(int n) {  
       if(n <= start) count = n; 
       else count = start; 
     } 
 
     void reset(int n) { 
       if(n <= start) count = n; 
     } 
 
     int run() { 
       if(count > end) return count--; 
       else return end; 
     } 
  }; 
} 
 
int main() 
{ 
  MyNameSpace::start = 100; 
  MyNameSpace::end = 0; 
 
  MyNameSpace::counter ob1(10); 
  int i; 
 
  do { 
    i = ob1.run(); 
    cout << i << " "; 
  } while(i > MyNameSpace::end); 
  cout << endl; 
 
  MyNameSpace::counter ob2(20); 
 
  do { 
    i = ob2.run(); 
    cout << i << " "; 
  } while(i > MyNameSpace::end); 
  cout << endl; 
 
  ob2.reset(100); 
  MyNameSpace::end = 90; 
  do { 
    i = ob2.run(); 
    cout << i << " "; 
  } while(i > MyNameSpace::end); 
 
  return 0; 
}
10 9 8 7 6 5 4 3 2 1 0
20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
100 99 98 97 96 95 94 93 92 91 90

Use function defined in a namespace

/* The following code example is taken from the book
 * "The C++ Standard Library - A Tutorial and Reference"
 * by Nicolai M. Josuttis, Addison-Wesley, 1999
 *
 * (C) Copyright Nicolai M. Josuttis 1999.
 * Permission to copy, use, modify, sell and distribute this software
 * is granted provided this copyright notice appears in all copies.
 * This software is provided "as is" without express or implied
 * warranty, and with no claim as to its suitability for any purpose.
 */
#include <iostream>
#include <cstdlib>
namespace MyLib {
    double readAndProcessSum (std::istream&);
}
int main()
{
    using namespace std;
    double sum;
    try {
        sum = MyLib::readAndProcessSum(cin);
    }
    catch (const ios::failure& error) {
        cerr << "I/O exception: " << error.what() << endl;
        return EXIT_FAILURE;
    }
    catch (const exception& error) {
        cerr << "standard exception: " << error.what() << endl;
        return EXIT_FAILURE;
    }
    catch (...) {
        cerr << "unknown exception" << endl;
        return EXIT_FAILURE;
    }
    // print sum
    cout << "sum: " << sum << endl;
}
#include <istream>
namespace MyLib {
    double readAndProcessSum (std::istream& strm)
    {
        double value, sum;
        /* while stream is OK
         * - read value and add it to sum
         */
        sum = 0;
        while (strm >> value) {
            sum += value;
        }
        if (!strm.eof()) {
            throw std::ios::failure
                    ("input error in readAndProcessSum()");
        }
        // return sum
        return sum;
    }
}
a
I/O exception: input error in readAndProcessSum()

Use Namespace to organize functions

#include <ios>
#include <iostream>
#include <ostream>
void f(int i)
{
  std::cout << "int: " << i << "\n";
}
namespace MyNamespace {
  void f(double d)
  {
    std::cout << "double: " << std::showpoint << d << "\n";
  }
  void call_f()
  {
    // finds MyNamespace::f(double) first.
    f(3);
  }
}
int main()
{
  MyNamespace::call_f();
  using MyNamespace::f;
  using ::f;
  f(4);
}
double: 3.00000
int: 4

Using namespace aliases

#include <iostream>
#include <ostream>
namespace original {
  int f();
}
namespace ns = original;     
int ns::f() { return 42; }   
using ns::f;                 
int g() { return f(); }
int main()
{
  std::cout << ns::f() << "\n";
  std::cout << original::f() << "\n";
  std::cout << g() << "\n";
}
42
42
42

using namespace in program block

#include <iostream.h>
namespace first
{
  int var = 5;
}
namespace second
{
  double var = 3.1416;
}
int main () {
  {
    using namespace first;
    cout << var << endl;
  }
  {
    using namespace second;
    cout << var << endl;
  }
  return 0;
}
5
3.1416

using namespace std;

#include <iostream>
using namespace std;
int main()
{
   // Print hello world on the screen
   cout << "Hello World";
   return 0;
}
Hello World"

Using the using directive

#include <iostream>
#include <ostream>
using namespace std;
namespace A {
  int x = 1;
}
namespace B {
  int y = 2;
}
namespace C {
  int z = 3;
  using namespace B;
}
namespace D {
  int z = 4;
  using namespace B;
  int y = 5;       
}
int main()
{
  int x = 60;
  using namespace A;
  using namespace C;
  
  cout << x << "\n";  
  cout << y << "\n";  
  cout << C::y << "\n";    
  cout << D::y << "\n";    
}
60
2
2
5

Using unnamed namespaces.

#include <iostream>
#include <ostream>
namespace {
  int i = 10;
}
namespace {
  int j;          // same unnamed namespace
  namespace X {
    int i = 20;
  }
  namespace Y = X;
  int f() { return i; }
}

int main()
{
  std::cout << ::f() << "\n";
  std::cout << Y::i << "\n";
  std::cout << f() << "\n";
}
10
20
10
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