using namespace std in c 2b 2b

1#include <iostream>
2
3using namespace std;
4
5int main()
6{
7    cout << "Hello World";
8    system("pause");
9    return 0;
10    
11}

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

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

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}

1#include <iostream>
2using namespace std;
3namespace square{
4	int x;
5	int y;
6}
7int main(){
8	using namespace square;
9	x = 10;
10	y = 0;
11	cout << x << y << endl;
12}

1Namespace std::cout or cout <<