C++ Primer Plus

This doesn’t translate a number to the correct characters; instead, it transmits the bit representation as stored in memory. For example, a 4-byte long value such as 560031841 would be transmitted as 4 separate bytes. An output device such as a monitor would then try to interpret each byte as if it were ASCII (or whatever) code. So 560031841 would appear onscreen as some 4-character combination, most likely gibberish. (But maybe not; try it and see.) However, write() does provide a compact, accurate way to store numeric data in a file. We’ll return to this possibility later in this chapter.

Flushing the Output Buffer

Consider what happens as a program uses cout to send bytes on to the standard output. Because the ostream class buffers output handled by the cout object, output isn’t sent to its destination immediately. Instead, it accumulates in the buffer until the buffer is full. Then the program flushes the buffer, sending the contents on and clearing the buffer for new data. Typically, a buffer is 512 bytes or an integral multiple thereof. Buffering is a great time-saver when the standard output is connected to a file on a hard disk. After all, you don’t want a program to access the hard disk 512 times to send 512 bytes. It’s much more effective to collect 512 bytes in a buffer and write them to a hard disk in a single disk operation.

For screen output, however, filling the buffer first is less critical. Indeed, it would be inconvenient if you had to reword the message “Press any key to continue” so that it consumed the prerequisite 512 bytes to fill a buffer. Fortunately, in the case of screen output, the program doesn’t necessarily wait until the buffer is full. Sending a newline character to the buffer, for example, normally flushes the buffer. Also as mentioned before, most C++ implementations flush the buffer when input is pending. That is, suppose you have the following code:

cout << "Enter a number: ";

float num;

cin >> num;

The fact that the program expects input causes it to display the cout message (that is, flush the "Enter a number: " message) immediately, even though the output string lacks a newline character. Without this feature, the program would wait for input without prompting the user with the cout message.

If your implementation doesn’t flush output when you want it to, you can force flushing by using one of two manipulators. The flush manipulator flushes the buffer, and the endl manipulator flushes the buffer and inserts a newline character. You use these manipulators the way you would use a variable name:

cout << "Hello, good-looking! " << flush;

cout << "Wait just a moment, please." << endl;

Manipulators are, in fact, functions. For example, you can flush the cout buffer by calling the flush() function directly:

flush(cout);

However, the ostream class overloads the << insertion operator in such a way that the following expression gets replaced with the flush(cout) function call:

cout << flush

Thus, you can use the more convenient insertion notation to flush with success.

Formatting with cout

The ostream insertion operators convert values to text form. By default, they format values as follows:

• A type char value, if it represents a printable character, is displayed as a character in a field one character wide.

• Numeric integer types are displayed as decimal integers in a field just wide enough to hold the number and, if present, a minus sign.

• Strings are displayed in a field equal in width to the length of the string.

The default behavior for floating-point types has changed. The following are the differences between ancient and current C++ implementations:

• New style— Floating-point types are displayed with a total of six digits, except that trailing zeros aren’t displayed. (Note that the number of digits displayed has no connection with the precision to which the number is stored.) The number is displayed in fixed-point notation or else in E notation (see Chapter 3, “Dealing with Data”), depending on the value of the number. In particular, E notation is used if the exponent is 6 or larger or -5 or smaller. Again, the field is just wide enough to hold the number and, if present, a minus sign. The default behavior corresponds to using the standard C library function fprintf() with a %g specifier.

• Old style— Floating-point types are displayed with six places to the right of the decimal, except that trailing zeros aren’t displayed. (Note that the number of digits displayed has no connection with the precision to which the number is stored.) The number is displayed in fixed-point notation or else in E notation (see Chapter 3), depending on the value of the number. Again, the field is just wide enough to hold the number and, if present, a minus sign.

Because each value is displayed in a width equal to its size, you have to provide spaces between values explicitly; otherwise, consecutive values would run together.