using namespace code example

Example 1: what is namespace in c++

//namespace is a declarative region to provide scope for identifiers
#include <bits/stdc++.h>

using namespace std; //including namespace std for cin and cout
//my custom namespace for variables and functions
namespace abc
{
  void fun()
  {
    cout<<"Hello world"<<endl;
  }
  int x=10;
}
using namespace abc;
int main()
{
  cout<<10;
  fun();
  return 0;
}

Example 2: access the namespace members using namespace member function

//Header.h
#include <string>

namespace Test
{
    namespace old_ns
    {
        std::string Func() { return std::string("Hello from old"); }
    }

    inline namespace new_ns
    {
        std::string Func() { return std::string("Hello from new"); }
    }
}

#include "header.h"
#include <string>
#include <iostream>

int main()
{
    using namespace Test;
    using namespace std;

    string s = Func();
    std::cout << s << std::endl; // "Hello from new"
    return 0;
}

Example 3: access the namespace members using namespace member function

namespace Parent
{
    inline namespace new_ns
    {
         template <typename T>
         struct C
         {
             T member;
         };
    }
     template<>
     class C<int> {};
}