C++ Primer Plus

The problem lies with how cin determines when you’ve finished entering a string. You can’t enter the null character from the keyboard, so cin needs some other means for locating the end of a string. The cin technique is to use whitespace—spaces, tabs, and newlines—to delineate a string. This means cin reads just one word when it gets input for a character array. After it reads this word, cin automatically adds the terminating null character when it places the string into the array.

The practical result in this example is that cin reads Alistair as the entire first string and puts it into the name array. This leaves poor Dreeb still sitting in the input queue. When cin searches the input queue for the response to the favorite dessert question, it finds Dreeb still there. Then cin gobbles up Dreeb and puts it into the dessert array (see Figure 4.4).

Figure 4.4. The cin view of string input.

Another problem, which didn’t surface in the sample run, is that the input string might turn out to be longer than the destination array. Using cin as this example did offers no protection against placing a 30-character string in a 20-character array.

Many programs depend on string input, so it’s worthwhile to explore this topic further. We’ll have to draw on some of the more advanced features of cin, which are described in Chapter 17, “Input, Output, and Files.”

Reading String Input a Line at a Time

Reading string input a word at a time is often not the most desirable choice. For instance, suppose a program asks the user to enter a city, and the user responds with New York or Sao Paulo. You would want the program to read and store the full names, not just New and Sao. To be able to enter whole phrases instead of single words as a string, you need a different approach to string input. Specifically, you need a line-oriented method instead of a word-oriented method. You are in luck, for the istream class, of which cin is an example, has some line-oriented class member functions: getline() and get(). Both read an entire input line—that is, up until a newline character. However, getline() then discards the newline character, whereas get() leaves it in the input queue. Let’s look at the details, beginning with getline().

Line-Oriented Input with getline()

The getline() function reads a whole line, using the newline character transmitted by the Enter key to mark the end of input. You invoke this method by using cin.getline() as a function call. The function takes two arguments. The first argument is the name of the target (that is, the array destined to hold the line of input), and the second argument is a limit on the number of characters to be read. If this limit is, say, 20, the function reads no more than 19 characters, leaving room to automatically add the null character at the end. The getline() member function stops reading input when it reaches this numeric limit or when it reads a newline character, whichever comes first.

For example, suppose you want to use getline() to read a name into the 20-element name array. You would use this call:

cin.getline(name,20);

This reads the entire line into the name array, provided that the line consists of 19 or fewer characters. (The getline() member function also has an optional third argument, which Chapter 17 discusses.)

Listing 4.4 modifies Listing 4.3 to use cin.getline() instead of a simple cin. Otherwise, the program is unchanged.

Listing 4.4. instr2.cpp

// instr2.cpp -- reading more than one word with getline

#include

int main()

{

    using namespace std;

    const int ArSize = 20;

    char name[ArSize];

    char dessert[ArSize];

    cout << "Enter your name:\n";

    cin.getline(name, ArSize);  // reads through newline

    cout << "Enter your favorite dessert:\n";

    cin.getline(dessert, ArSize);

    cout << "I have some delicious " << dessert;

    cout << " for you, " << name << ".\n";

    return 0;

}

Here is some sample output for Listing 4.4:

Enter your name:

Dirk Hammernose

Enter your favorite dessert:

Radish Torte

I have some delicious Radish Torte for you, Dirk Hammernose.

The program now reads complete names and delivers the user his just desserts! The getline() function conveniently gets a line at a time. It reads input through the newline character marking the end of the line, but it doesn’t save the newline character. Instead, it replaces it with a null character when storing the string (see Figure 4.5).

Figure 4.5. getline() reads and replaces the newline character.

Line-Oriented Input with get()