3.1. Namespace using Declarations

Up to now, our programs have explicitly indicated that each library name we use is in the std namespace. For example, to read from the standard input, we write std::cin. These names use the scope operator (::) (§ 1.2, p. 8), which says that the compiler should look in the scope of the left-hand operand for the name of the right-hand operand. Thus, std::cin says that we want to use the name cin from the namespace std.

Referring to library names with this notation can be cumbersome. Fortunately, there are easier ways to use namespace members. The safest way is a using declaration. § 18.2.2 (p. 793) covers another way to use names from a namespace.

A using declaration lets us use a name from a namespace without qualifying the name with a namespace_name:: prefix. A using declaration has the form

using namespace::name;

Once the using declaration has been made, we can access name directly:

#include <iostream>
// using declaration; when we use the name cin, we get the one from the namespace std
using std::cin;
int main()
{
    int i;
    cin >> i;       // ok: cin is a synonym for std::cin
    cout << i;      // error: no using declaration; we must use the full name
    std::cout << i; // ok: explicitly use cout from namepsace std
    return 0;
}