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Setting Out to C++

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This chapter is from the book

This chapter is from the book

More C++ Statements

Let’s look at a couple more examples of statements. The program in Listing 2.3 expands on the preceding example by allowing you to enter a value while the program is running. To do so, it uses cin (pronounced “see-in”), the input counterpart to cout. Also the program shows yet another way to use that master of versatility, the cout object.

Listing 2.3. getinfo.cpp

// getinfo.cpp -- input and output
#include <iostream>

int main()
    using namespace std;

    int carrots;

    cout << "How many carrots do you have?" << endl;
    cin >> carrots;                // C++ input
    cout << "Here are two more. ";
    carrots = carrots + 2;
// the next line concatenates output
    cout << "Now you have " << carrots << " carrots." << endl;
    return 0;

Here is an example of output from the program in Listing 2.3:

How many carrots do you have?
Here are two more. Now you have 14 carrots.

The program has two new features: using cin to read keyboard input and combining four output statements into one. Let’s take a look.

Using cin

As the output from Listing 2.3 demonstrates, the value typed from the keyboard (12) is eventually assigned to the variable carrots. The following statement performs that wonder:

cin >> carrots;

Looking at this statement, you can practically see information flowing from cin into carrots. Naturally, there is a slightly more formal description of this process. Just as C++ considers output to be a stream of characters flowing out of the program, it considers input to be a stream of characters flowing into the program. The iostream file defines cin as an object that represents this stream. For output, the << operator inserts characters into the output stream. For input, cin uses the >> operator to extract characters from the input stream. Typically, you provide a variable to the right of the operator to receive the extracted information. (The symbols << and >> were chosen to visually suggest the direction in which information flows.)

Like cout, cin is a smart object. It converts input, which is just a series of characters typed from the keyboard, into a form acceptable to the variable receiving the information. In this case, the program declares carrots to be an integer variable, so the input is converted to the numeric form the computer uses to store integers.

Concatenating with cout

The second new feature of getinfo.cpp is combining four output statements into one. The iostream file defines the << operator so that you can combine (that is, concatenate) output as follows:

cout << "Now you have " << carrots << " carrots." << endl;

This allows you to combine string output and integer output in a single statement. The resulting output is the same as what the following code produces:

cout << "Now you have ";
cout << carrots;
cout << " carrots";
cout << endl;

While you’re still in the mood for cout advice, you can also rewrite the concatenated version this way, spreading the single statement over four lines:

cout << "Now you have "
     << carrots
     << " carrots."
     << endl;

That’s because C++’s free format rules treat newlines and spaces between tokens interchangeably. This last technique is convenient when the line width cramps your style.

Another point to note is that

Now you have 14 carrots.

appears on the same line as

Here are two more.

That’s because, as noted before, the output of one cout statement immediately follows the output of the preceding cout statement. This is true even if there are other statements in between.

cin and cout: A Touch of Class

You’ve seen enough of cin and cout to justify your exposure to a little object lore. In particular, in this section you’ll learn more about the notion of classes. As Chapter 1 outlined briefly, classes are one of the core concepts for object-oriented programming (OOP) in C++.

A class is a data type the user defines. To define a class, you describe what sort of information it can represent and what sort of actions you can perform with that data. A class bears the same relationship to an object that a type does to a variable. That is, a class definition describes a data form and how it can be used, whereas an object is an entity created according to the data form specification. Or, in noncomputer terms, if a class is analogous to a category such as famous actors, then an object is analogous to a particular example of that category, such as Kermit the Frog. To extend the analogy, a class representation of actors would include definitions of possible actions relating to the class, such as Reading for a Part, Expressing Sorrow, Projecting Menace, Accepting an Award, and the like. If you’ve been exposed to different OOP terminology, it might help to know that the C++ class corresponds to what some languages term an object type, and the C++ object corresponds to an object instance or instance variable.

Now let’s get a little more specific. Recall the following declaration of a variable:

int carrots;

This creates a particular variable (carrots) that has the properties of the int type. That is, carrots can store an integer and can be used in particular ways—for addition and subtraction, for example. Now consider cout. It is an object created to have the properties of the ostream class. The ostream class definition (another inhabitant of the iostream file) describes the sort of data an ostream object represents and the operations you can perform with and to it, such as inserting a number or string into an output stream. Similarly, cin is an object created with the properties of the istream class, also defined in iostream.

You have learned that classes are user-defined types, but as a user, you certainly didn’t design the ostream and istream classes. Just as functions can come in function libraries, classes can come in class libraries. That’s the case for the ostream and istream classes. Technically, they are not built in to the C++ language; instead, they are examples of classes that the language standard specifies. The class definitions are laid out in the iostream file and are not built into the compiler. You can even modify these class definitions if you like, although that’s not a good idea. (More precisely, it is a truly dreadful idea.) The iostream family of classes and the related fstream (or file I/O) family are the only sets of class definitions that came with all early implementations of C++. However, the ANSI/ISO C++ committee added a few more class libraries to the Standard. Also most implementations provide additional class definitions as part of the package. Indeed, much of the current appeal of C++ is the existence of extensive and useful class libraries that support Unix, Macintosh, and Windows programming.

The class description specifies all the operations that can be performed on objects of that class. To perform such an allowed action on a particular object, you send a message to the object. For example, if you want the cout object to display a string, you send it a message that says, in effect, “Object! Display this!” C++ provides a couple ways to send messages. One way, using a class method, is essentially a function call like the ones you’ll see soon. The other way, which is the one used with cin and cout, is to redefine an operator. Thus, the following statement uses the redefined << operator to send the “display message” to cout:

cout << "I am not a crook."

In this case, the message comes with an argument, which is the string to be displayed. (See Figure 2.5 for a similar example.)

Figure 2.5

Figure 2.5 Sending a message to an object.

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