C++ Primer Plus

Enter a string for getline() processing:

Please pass

me a #3 melon!

Here is your input:

Please pass

me a

Done with phase 1

The next input character is 3

Enter a string for get() processing:

I still

want my #3 melon!

Here is your input:

I still

want my

Done with phase 2

The next input character is #

Note that the getline() function discards the # termination character in the input, and the get() function does not.

Unexpected String Input

Some forms of input for get(char *, int) and getline() affect the stream state. As with the other input functions, encountering end-of-file sets eofbit, and anything that corrupts the stream, such as device failure, sets badbit. Two other special cases are no input and input that meets or exceeds the maximum number of characters specified by the function call. Let’s look at those cases now.

If either method fails to extract any characters, the method places a null character into the input string and uses setstate() to set failbit. When would a method fail to extract any characters? One possibility is if an input method immediately encounters end-of-file. For get(char *, int), another possibility is if you enter an empty line:

char temp[80];

while (cin.get(temp,80))  // terminates on empty line

      ...

Interestingly, an empty line does not cause getline() to set failbit. That’s because getline() still extracts the newline character, even if it doesn’t store it. If you want a getline() loop to terminate on an empty line, you can write it this way:

char temp[80];

while (cin.getline(temp,80) && temp[0] != '\0') // terminates on empty line

Now suppose the number of characters in the input queue meets or exceeds the maximum specified by the input method. First, consider getline() and the following code:

char temp[30];

while (cin.getline(temp,30))

The getline() method will read consecutive characters from the input queue, placing them in successive elements of the temp array, until (in order of testing) end-of-file is encountered, until the next character to be read is the newline character, or until 29 characters have been stored. If end-of-file is encountered, eofbit is set. If the next character to be read is a newline character, that character is read and discarded. And if 29 characters were read, failbit is set, unless the next character is a newline. Thus, an input line of 30 characters or more will terminate input.

Now consider the get(char *, int) method. It tests the number of characters first, end-of-file second, and for the next character being a newline third. It does not set the failbit flag if it reads the maximum number of characters. Nonetheless, you can tell if too many input characters caused the method to quit reading. You can use peek() (see the next section) to examine the next input character. If it’s a newline, then get() must have read the entire line. If it’s not a newline, then get() must have stopped before reaching the end. This technique doesn’t necessarily work with getline() because getline() reads and discards the newline, so looking at the next character doesn’t tell you anything. But if you use get(), you have the option of doing something if less than an entire line is read. The next section includes an example of this approach. Meanwhile, Table 17.6 summarizes these behaviors.

Table 17.6. Input Behavior

Other istream Methods

Other istream methods besides the ones discussed so far include read(), peek(), gcount(), and putback(). The read() function reads a given number of bytes and stores them in the specified location. For example, the following statements read 144 characters from the standard input and place them in the gross array:

char gross[144];

cin.read(gross, 144);

Unlike getline() and get(), read() does not append a null character to input, so it doesn’t convert input to string form. The read() method is not intended for keyboard input. Instead, it is most often used in conjunction with the ostream write() function for file input and output. The method’s return type is istream &, so it can be concatenated as follows:

char gross[144];

char score[20];

cin.read(gross, 144).read(score, 20);

The peek() function returns the next character from input without extracting from the input stream. That is, it lets you peek at the next character. Suppose you want to read input up to the first newline or period, whichever comes first. You can use peek() to peek at the next character in the input stream in order to judge whether to continue:

char great_input[80];

char ch;

int i = 0;

while ((ch = cin.peek()) != '.' && ch != '\n')

    cin.get(great_input[i++]);

great_input [i] = '\0';