where should using namespace std be code example

Example 1: using namespace std in c++

#include <iostream>

using namespace std;

int main()
{
    cout << "Hello World";
    system("pause");
    return 0;
    
}

Example 2: why to use std:: in c++

When you are using "std::" it means that you will grap the following command
from the "std" namespace.
The std namespace is an abbreviation for standard

If you use "std::" you hvae to include "#include <iostream>"