C++ Primer Plus

Namespace support is a C++ feature designed to simplify the writing of large programs and of programs that combine pre-existing code from several vendors and to help organize programs. One potential problem is that you might use two prepackaged products that both have, say, a function called wanda(). If you then use the wanda() function, the compiler won’t know which version you mean. The namespace facility lets a vendor package its wares in a unit called a namespace so that you can use the name of a namespace to indicate which vendor’s product you want. So Microflop Industries could place its definitions in a namespace called Microflop. Then Microflop::wanda() would become the full name for its wanda() function. Similarly, Piscine::wanda() could denote Piscine Corporation’s version of wanda(). Thus, your program could now use the namespaces to discriminate between various versions:

Microflop::wanda("go dancing?");       // use Microflop namespace version

Piscine::wanda("a fish named Desire"); // use Piscine namespace version

In this spirit, the classes, functions, and variables that are a standard component of C++ compilers are now placed in a namespace called std. This takes place in the h-free header files. This means, for example, that the cout variable used for output and defined in iostream is really called std::cout and that endl is really std::endl. Thus, you can omit the using directive and, instead, code in the following style:

std::cout << "Come up and C++ me some time.";

std::cout << std::endl;

However, many users don’t feel like converting pre-namespace code, which uses iostream.h and cout, to namespace code, which uses iostream and std::cout, unless they can do so without a lot of hassle. This is where the using directive comes in. The following line means you can use names defined in the std namespace without using the std:: prefix:

using namespace std;

This using directive makes all the names in the std namespace available. Modern practice regards this as a bit lazy and potentially a problem in large projects. The preferred approaches are to use the std:: qualifier or to use something called a using declaration to make just particular names available:

using std::cout;   // make cout available

using std::endl;   // make endl available

using std::cin;    // make cin available

If you use these directives instead of the following, you can use cin and cout without attaching std:: to them:

using namespace std;  // lazy approach, all names available

But if you need to use other names from iostream, you have to add them to the using list individually. This book initially uses the lazy approach for a couple reasons. First, for simple programs, it’s not really a big issue which namespace management technique you use. Second, I’d rather emphasize the more basic aspects about learning C++. Later, the book uses the other namespace techniques.

C++ Output with cout

Now let’s look at how to display a message. The myfirst.cpp program uses the following C++ statement:

cout << "Come up and C++ me some time.";

The part enclosed within the double quotation marks is the message to print. In C++, any series of characters enclosed in double quotation marks is called a character string, presumably because it consists of several characters strung together into a larger unit. The << notation indicates that the statement is sending the string to cout; the symbols point the way the information flows. And what is cout? It’s a predefined object that knows how to display a variety of things, including strings, numbers, and individual characters. (An object, as you might remember from Chapter 1, is a particular instance of a class, and a class defines how data is stored and used.)

Well, using objects so soon is a bit awkward because you won’t learn about objects for several more chapters. Actually, this reveals one of the strengths of objects. You don’t have to know the innards of an object in order to use it. All you must know is its interface—that is, how to use it. The cout object has a simple interface. If string represents a string, you can do the following to display it:

cout << string;

This is all you must know to display a string, but now take a look at how the C++ conceptual view represents the process. In this view, the output is a stream—that is, a series of characters flowing from the program. The cout object, whose properties are defined in the iostream file, represents that stream. The object properties for cout include an insertion operator (<<) that inserts the information on its right into the stream. Consider the following statement (note the terminating semicolon):

cout << "Come up and C++ me some time.";

It inserts the string “Come up and C++ me some time.” into the output stream. Thus, rather than say that your program displays a message, you can say that it inserts a string into the output stream. Somehow, that sounds more impressive (see Figure 2.2).