A Separate using Declaration Is Required for Each Name

Each using declaration introduces a single namespace member. This behavior lets us be specific about which names we’re using. As an example, we’ll rewrite the program from § 1.2 (p. 6) with using declarations for the library names it uses:

#include <iostream>
// using declarations for names from the standard library
using std::cin;
using std::cout; using std::endl;
int main()
{
    cout << "Enter two numbers:" << endl;
    int v1, v2;
    cin >> v1 >> v2;
    cout << "The sum of " << v1 << " and " << v2
         << " is " << v1 + v2 << endl;
    return 0;
}

The using declarations for cin, cout, and endl mean that we can use those names without the std:: prefix. Recall that C++ programs are free-form, so we can put each using declaration on its own line or combine several onto a single line. The important part is that there must be a using declaration for each name we use, and each declaration must end in a semicolon.