what is namespace in c 2b 2b

1A namespace is a declarative region that provides a scope to the 
2  identifiers (the names of types, functions, variables, etc) inside 
3  it. Namespaces are used to organize code into logical groups and to
4  prevent name collisions that can occur especially when your code base
5  includes multiple libraries

1Namespaces avoids name collisions bacause of large libraray in c++.
2This feature was not supported in C

1//namespace is a declarative region to provide scope for identifiers
2#include <bits/stdc++.h>
3
4using namespace std; //including namespace std for cin and cout
5//my custom namespace for variables and functions
6namespace abc
7{
8  void fun()
9  {
10    cout<<"Hello world"<<endl;
11  }
12  int x=10;
13}
14using namespace abc;
15int main()
16{
17  cout<<10;
18  fun();
19  return 0;
20}

1namespace Parent
2{
3    inline namespace new_ns
4    {
5         template <typename T>
6         struct C
7         {
8             T member;
9         };
10    }
11     template<>
12     class C<int> {};
13}
14

1//Header.h
2#include <string>
3
4namespace Test
5{
6    namespace old_ns
7    {
8        std::string Func() { return std::string("Hello from old"); }
9    }
10
11    inline namespace new_ns
12    {
13        std::string Func() { return std::string("Hello from new"); }
14    }
15}
16
17#include "header.h"
18#include <string>
19#include <iostream>
20
21int main()
22{
23    using namespace Test;
24    using namespace std;
25
26    string s = Func();
27    std::cout << s << std::endl; // "Hello from new"
28    return 0;
29}
30