One way to get input into a program or to display output from a program is to use standard input and standard output, respectively.All that means is that to read in data, we use cin (or a few other functions) and to write out data, we use cout. When we need to take input from a file (instead of having the user type data at the keyboard) we can use input redirection.
One of the difficulties with C++ in moving to stream objects for output is learning how to format the output. There are actually two ways to format-stream I/O.
The first technique that programmers normally learn is through direct method calls. For example, the following call sets the output stream in hexadecimal mode so that integers appear onscreen as hexadecimal values rather than as decimal values:
Here the setf() method sets the format flags by first masking off the bits in the second argument and then setting the bits indicated by the first argument. The function returns the old value so that the calling function can restore the format flags before returning.
In practice, this function appears as follows:
The program starts by setting the variable n to a hexadecimal value. It then displays the value to cout. It then calls the displayHex() function to display the value in hexadecimal format. The output from the program appears as follows:
The value in decimal format is not recognizable but the hexadecimal display is clearly the value used to initialize n.
However, C++ provides a second way to control I/O format: I/O manipulators.
What is an I/O manipulator?
Dev C Outputting Io Free
An I/O manipulator is an object that the programmer can insert into the output stream to invoke the format control functions. The following code snippet demonstrates the user of I/O manipulators. It performs the same function as the prior snippet, except this version uses an I/O manipulator to invoke the setf() function:
Here main() first outputs n as a decimal value. It then uses the setbase(16) manipulator to change the display base from decimal (base 10) to hexadecimal (base 16) before displaying the value again. The output from this version is indistinguishable from the earlier version:
The setbase(16) object appears just like a call to a function in the middle of the output stream. The fact that an object is being created is invisible to the user.
For hexadecimal, octal and decimal, C++ provides the shortcuts hex, oct and dec meaning that the code could take the following even shorter form:
What I/O manipulators does C++ offer?
Have a look at this table, which contains a list of I/O manipulators available to C++ 2011.
| I/O Manipulator | Function |
|---|---|
| boolalpha noboolalpha | Switches between textual and numeric display of a Boolean. When boolalpha is set, true and false appear onscreen as ‘true‘ and ‘false‘. When it’s not set, they appear as ‘1‘ and ‘0’. |
| showbase noshowbase | When showbase is set, octal values are preceded by a 0 and hexadecimal values are preceded by ‘0x‘ or ‘0X‘. |
| showpoint noshowpoint | A decimal point is always displayed when showpoint is set. |
| showpos noshowpos | Displays a ‘+‘ sign in front of positive values when showpos is set. Displays nothing when noshowpos is set. |
| skipws noskipws | Skips leading white space on input. |
| uppercase nouppercase | If uppercase is set, then use uppercase for output formats, for example, ‘0X‘ for hexadecimal output. If nouppercase is set, use lowercase, for example, ‘0x‘ for hexadecimal output. |
| unitbuf nounitbuf | If set, then output is not buffered but flushed after each insert. If not set, output is buffered. |
| fixed scientific | Set output of floating-point values to either fixed or scientific notation. |
| ws | This object eats white space from an input stream. |
| ends | Inserts a null character (‘‘) into an output stream. Useful for null-terminating a string when using ostrstream. |
| flush | Flushes any buffer characters to the output stream. |
| endl | Inserts a newline character . |
| setiosflags(n) resetiosflags(n) | Set or clear the ios flags en masse. |
| setbase(n) | Sets the base for integer input and output to 8, 10, 16 or 0. See text for explanation of 0. |
| dec oct hex | Set the base to decimal, octal or hexadecimal. Shorthand for setbase(10), setbase(8), and setbase(16), respectively. |
| setfill(c) | Sets the character used to fill space when the number of characters to display is less than the field width. The default character is a space. |
| setprecision(n) | Sets the number of digits to output when displaying a floating-point number. |
| setw(n) | Sets the minimum width of the next output field. The field is expanded with setfill() characters as necessary. |
| setfill(c) | Sets the character used to fill space when the number of characters to display is less than the field width. The default character is a space. |
internal left right | Set the placement of fill characters. If the fill character were ‘*’ (for example), then: internal→ “$**123.00“ left→“**$123.00“ right→“$123.00**“ |
| get_money() put_money() | Read or display a monetary value, using the local rules as set by locale. (This is not implemented in the current version of gcc.) |
| get_time() put_time() | Read or display a time or date, using the local rules as set by locale. (This is not implemented in the current version of gcc.) |
Most of the manipulators are straightforward. A few require some explanation, however. For example, regarding the setbase() manipulator, not all values of base are supported. In fact, C++ supports only decimal, octal, and hexadecimal I/O — which is okay, as that’s pretty much all that’s required. You can, however, set the base to 0 — in fact, 0 is the default value.
Setting the base to 0 is the same as setting it to decimal for output. For input, however, setting the base to 0 means “extract the base from the number itself”. The rules for doing so are as follows:
*If a number begins with 0x or 0X, it is assumed to be hexadecimal.
*If a number begins with just 0, then it is assumed to be octal.
*Otherwise the number is assumed to be decimal.
The following simple program demonstrates these rules:
The first line sets the base for the cin object to 0 and then extracts three integers. The second line displays these three integers in hexadecimal format. The showbase manipulator causes the leading ‘0x‘ to be displayed for hexadecimal values. The following shows an example execution of this program, where the three numbers in bold are values that I input from the keyboard:
You can see that the first number was interpreted as 10 in base 10, which is 0xA in hexadecimal. The second number was interpreted as 10 in base 8, which is 8 in hexadecimal. The third and final number was interpreted as 10 in hexadecimal.
Another manipulator that deserves special mention is the setw(), short for “set width”. This sets the minimum display width of the next field displayed. The following simple program shows how setw()interacts with padding and the fill character:
This program sets the fill character to ‘*‘ (the default is space). It then reads the width to be used for all numbers from the keyboard before entering a loop that reads and displays values. Within this loop, the program first displays the number with left-hand fill, then with right-hand fill, and finally with internal fill.
The following is a sample run of the program with the input bolded:
Dev C++ Outputting Ion
Here 5 is entered as the width of the field. However, the first value entered, 100,000, requires more than 5 columns to display so none of the display fields had any effect. The set width only sets the minimum width to display.
The next value entered, 100, requires only three columns to display, so the program displayed the value first with two * characters on the right, then on the left, and again on the left.
The next value entered, –100, demonstrates the difference between right fill and internal fill. Right fill put the fill character on the left of the value, including the minus sign. Internal fill put the fill character between the value and the minus sign. This also works for a plus sign when plus is forced on with the showpos manipulator and between the number and the currency sign (when currency is supported by the compiler).
Dev C++ Outputting Iowa
The manipulators don’t add any capability that isn’t already present. After all, each of these manipulators ends up calling a public method on the stream object. However, they do provide a convenient means of formatting I/O.