Lesson 166 +10 XP

Using Declarations and Directives

Writing std:: everywhere gets tiresome. C++ has two tools to shorten it - and they work quite differently.

A using declaration brings a single name into the current scope:

using std::cout;    // now cout works without std::
using std::endl;

cout << "Hello" << endl;

Only the names you list are imported. The rest of std stays qualified.

A using directive pulls in an entire namespace at once:

using namespace std;

cout << "Hello";    // works, but see below

Now every name in std is visible without qualification. Convenient - and risky.

A using directive dumps many names into your scope, and you can't always predict which ones will clash with your own or with another library:

using namespace std;
using namespace alice;   // both might define the same name!

string data;             // is this std::string or alice::string?

When two imported namespaces define the same name, you get ambiguity - and it can be a real headache to untangle.

Prefer using declarations (using std::cout;) over directives when you only need a few names.
In header files, never use using namespace - it leaks into every file that includes the header.
Many projects avoid using namespace std; entirely and just write std:: explicitly.

using std::cout;    // good: precise and limited
using namespace std; // works, but use with care