C++ Primer Plus

As you can see, cout works with both strings and integers. This might not seem particularly remarkable to you, but keep in mind that the integer 25 is something quite different from the string "25". The string holds the characters with which you write the number (that is, a 2 character and a 5 character). The program internally stores the numeric codes for the 2 character and the 5 character. To print the string, cout simply prints each character in the string. But the integer 25 is stored as a numeric value. Rather than store each digit separately, the computer stores 25 as a binary number. (Appendix A, “Number Bases,” discusses this representation.) The main point here is that cout must translate a number in integer form into character form before it can print it. Furthermore, cout is smart enough to recognize that carrots is an integer that requires conversion.

Perhaps the contrast with old C will indicate how clever cout is. To print the string "25" and the integer 25 in C, you could use C’s multipurpose output function printf():

printf("Printing a string: %s\n", "25");

printf("Printing an integer: %d\n", 25);

Without going into the intricacies of printf(), note that you must use special codes (%s and %d) to indicate whether you are going to print a string or an integer. And if you tell printf() to print a string but give it an integer by mistake, printf() is too unsophisticated to notice your mistake. It just goes ahead and displays garbage.

The intelligent way in which cout behaves stems from C++’s object-oriented features. In essence, the C++ insertion operator (<<) adjusts its behavior to fit the type of data that follows it. This is an example of operator overloading. In later chapters, when you take up function overloading and operator overloading, you’ll learn how to implement such smart designs yourself.

cout and printf()

If you are used to C and printf(), you might think cout looks odd. You might even prefer to cling to your hard-won mastery of printf(). But cout actually is no stranger in appearance than printf(), with all its conversion specifications. More importantly, cout has significant advantages. Its capability to recognize types reflects a more intelligent and foolproof design. Also, it is extensible. That is, you can redefine the << operator so that cout can recognize and display new data types you develop. And if you relish the fine control printf() provides, you can accomplish the same effects with more advanced uses of cout (see Chapter 17, “Input, Output, and Files”).

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

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;

}

Program Adjustments

If you found that you had to add a cin.get() statement in the earlier listings, you will need to add two cin.get() statements to this listing to keep the program output visible onscreen. The first one will read the newline generated when you press the Enter or Return key after typing a number, and the second will cause the program to pause until you hit Return or Enter again.

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

How many carrots do you have?

12

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.)