C++ Primer Plus

The function corresponding to that prototype then reads the stream of characters being sent to the program—say, the characters 2, 3, 1, 8, and 4. For a system using a 2-byte int, the function then converts these characters to the 2-byte binary representation of the integer 23184. If, on the other hand, staff_size were type double, cin would use operator>>(double &) to convert the same input into the 8-byte floating-point representation of the value 23184.0.

Incidentally, you can use the hex, oct, and dec manipulators with cin to specify that integer input is to be interpreted as hexadecimal, octal, or decimal format. For example, the following statement causes an input of 12 or 0x12 to be read as hexadecimal 12, or decimal 18, and it causes ff or FF to be read as decimal 255:

cin >> hex;

The istream class also overloads the >> extraction operator for character pointer types:

• signed char *

• char *

• unsigned char *

For this type of argument, the extraction operator reads the next word from input and places it at the indicated address, adding a null character to make a string. For example, suppose you have this code:

cout << "Enter your first name:\n";

char name[20];

cin >> name;

If you respond to the request by typing Liz, the extraction operator places the characters Liz\0 in the name array. (As usual, \0 represents the terminating null character.) The name identifier, being the name of a char array, acts as the address of the array’s first element, making name type char * (pointer-to-char).

The fact that each extraction operator returns a reference to the invoking object lets you concatenate input, just as you can concatenate output:

char name[20];

float fee;

int group;

cin >> name >> fee >> group;

Here, for example, the cin object returned by cin >> name becomes the object that handles fee.

How cin >> Views Input

The various versions of the extraction operator share a common way of looking at the input stream. They skip over white space (blanks, newlines, and tabs) until they encounter a non-white-space character. This is true even for the single-character modes (those in which the argument is type char, unsigned char, or signed char), which is not true of C’s character input functions (see Figure 17.5). In the single-character modes, the >> operator reads that character and assigns it to the indicated location. In the other modes, the operator reads in one unit of the indicated type. That is, it reads everything from the initial non-white-space character up to the first character that doesn’t match the destination type.

Figure 17.5. cin >> skips over white space.

For example, consider the following code:

int elevation;

cin >> elevation;

Suppose you type the following characters:

-123Z

The operator will read the -, 1, 2, and 3 characters because they are all valid parts of an integer. But the Z character isn’t valid, so the last character accepted for input is the 3. The Z remains in the input stream, and the next cin statement will start reading at that point. Meanwhile, the operator converts the character sequence -123 to an integer value and assigns it to elevation.

It can happen that input fails to meet a program’s expectation. For example, suppose you enter Zcar instead of -123Z. In that case, the extraction operator leaves the value of elevation unchanged and returns the value 0. (More technically, an if or while statement evaluates an istream object as false if it’s had an error state set; we’ll discuss this in more depth later in this chapter.) The false return value allows a program to check whether input meets the program requirements, as Listing 17.11 shows.

Listing 17.11. check_it.cpp

// check_it.cpp -- checking for valid input

#include

int main()

{

    using namespace std;

    cout << "Enter numbers: ";

    int sum = 0;

    int input;

    while (cin >> input)

    {

        sum += input;

    }

    cout << "Last value entered = " << input << endl;

    cout << "Sum = " << sum << endl;

    return 0;

}

Here’s the output of the program in Listing 17.11 when some inappropriate input (-123Z) sneaks into the input stream:

Enter numbers: 200

10 -50 -123Z 60

Last value entered = -123

Sum = 37

Because input is buffered, the second line of keyboard input values didn’t get sent to the program until you pressed Enter at the end of the line. But the loop quit processing input at the Z character because it didn’t match any of the floating-point formats. The failure of input to match the expected format, in turn, caused the expression cin>> input to evaluate to false, thus terminating the while loop.

Stream States