C++ Tutorial/Language Basics/namespace

A namespace

<source lang="cpp">namespace MyNameSpace {

 int start; 
 int end; 

 class counter { 
    int count; 
  public: 
    counter(int n) {  
      count = n; 
    } 

    void reset(int n) { 
      count = n; 
    } 
 }; 

}</source>

A nested namespace

<source lang="cpp">#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;

}</source>

19 10
190

function in a namespace reference variables in the same namespace

<source lang="cpp">#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;

}</source>

98
(global) integer1 = 98
Iinteger1 = 1992

Reference global variable, variable in the nested namespace

<source lang="cpp">#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;

}</source>

doubleInUnnamed = 88.22
(global) integer1 = 98100
Iinteger1 = 99

Reference variables in namespaces

<source lang="cpp">#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;

}</source>

5
3.1416

Reuse namespace

<source lang="cpp">#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;

}</source>

100
100

Use a namespace

<source lang="cpp">#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; 

}</source>

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

<source lang="cpp">/* 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.
*/

1. include <iostream>
2. 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;

}

1. 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;
   }

}</source>

a
I/O exception: input error in readAndProcessSum()

Use Namespace to organize functions

<source lang="cpp">#include <ios>

1. include <iostream>
2. 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);

}</source>

double: 3.00000
int: 4

Using namespace aliases

<source lang="cpp">#include <iostream>

1. 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";

}</source>

42
42
42

using namespace in program block

<source lang="cpp">#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;

}</source>

5
3.1416

using namespace std;

<source lang="cpp">#include <iostream> using namespace std; int main() {

  // Print hello world on the screen
  cout << "Hello World";
  return 0;

}</source>

Hello World"

Using the using directive

<source lang="cpp">#include <iostream>

1. 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";    

}</source>

60
2
2
5

Using unnamed namespaces.

<source lang="cpp">#include <iostream>

1. 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";

}</source>

10
20
10