Advanced Input and Output

1. 10 Advanced Input and Output Object-Oriented Programming Using C++ Second Edition

2. 10 Objectives • In this chapter, you will learn: • How cout and cin possess the same traits as other C++ objects • How to use istream member functions, particularly get(), ignore(), and getline() • How to use ostream member functions, particularly setf(), unsetf(), width(), and precision() • How to create your own manipulator functions

3. 10 Objectives • In this chapter, you will learn: • How to use built-in manipulators • How to create a manipulator that takes an argument • About computer files and the data hierarchy • How to perform file output • How to read a file from within a program • How to write class objects to files • How to read a data file into class objects

4. 10 Understanding CIN and COUT as Class Objects • You can think of cout and cin as real-world objects • Like other C++ objects you have created, cout and cin are members of a class • Their class is derived from another class (which is derived from yet another class), so they use inheritance • The cout and cin objects can take advantage of overloaded operators such as << and >> which are used for shifting bits in other contexts

5. 10 Understanding CIN and COUT as Class Objects • When you include iostream.h in a program, you are including a file that contains the definition for a derived class named iostream • In C++, a stream is a sequence of characters used to perform input and output operations • The name iostream is short for “Input and Output” stream

6. 10 Using istream Member Functions • In C++, the easiest way to read in a character is to use cin with the extraction operator, for example, cin>>someVariable; • The extraction operator is actually an overloaded function named operator>>()

7. 10 Using the get() Function • Another member function of the istream class is get() • The get() function takes a character argument and returns a reference to the object (the istream class) that invoked the get() function • Therefore more than one get() function can be included in a statement • Its prototype has the following form: istream& get(char &c);

8. 10 Using the get() Function • Most compilers overload get() so that, in addition to taking a character reference as an argument, it also can take no argument • The following version of the get() function returns the character being read in as an integer • Its prototype is: int get( ); • The third argument is the character that terminates the entry, often called the delimiter character • The default value to stop data entry, as you can see from the prototype, is the Enter key, which coded as ‘\n’

9. 10 Using the get() Function • The second argument of the get() function—the number of characters to be stored—is very important • Without the second argument, a user could destroy memory by entering a string of characters that was longer than the area prepared to receive it • The output in Figure 10-3 and 10-4 illustrate one benefit of using the get() function instead of the extraction operator (>>) for keyboard data entry

10. 10 Using the get() Function • One unfortunate side effect of the get() function is that it leaves unused characters in the input stream • A subsequent call to get() retrieves the next (unused) character, whether or not that retrieval was intended • As shown in Figure 10-6, the program doesn’t stop to obtain your grade, because the second call to get() already has been satisfied with the newline character

11. 10 The UserNameAndGrade Program

12. 10 Using the get() Function • The program output shown in Figure 10-6 includes your first name and a newline (instead of a grade) for the letter grade • To allow the user to enter a grade, you could add a third cin.get() statement to the program, as shown in Figure 10-7

13. 10 Using the get() Function • Programmers say this call to the get() function absorbs or consumes the extra character

14. 10 Using the get() Function • If you run the program in Figure 10-7 and enter a name that is too long, the output looks like Figure 10-9

15. 10 Using the ignore() Function • It is impossible to guess how many cin.get() statements are needed to consume all the potential additional letters of a name if you use the UserNameAndGrade2 program—imagine the challenge of a name such as Barbara Penelope • A superior alternative is to use the ignore() function to ignore or skip any additional characters left in the input stream • The prototype of the ignore() function is: istream& ignore(int length = 1, char c = ‘\n’);

16. 10 The UserNameAndGrade Program Using ignore()

17. 10 Using the ignore() Function

18. 10 Using the getline() Function • As an alternative to using an extra call to get() to absorb the Enter key after character data entry, or using the ignore() function to absorb any number of characters, you can include another istream member, getline() • Its prototype is: istream& getline(char *str,int len, char c=‘\n’); • The getline() function reads a line of text at the address represented by str • It reads text until it reaches either the length used as the second argument or the character used as the third argument

19. 10 Using the getline() Function • The program in Figure 10-13, and the output in Figure 10-14, show how the getline() function correctly accepts characters up to and including the default newline delimiter

20. 10 Using the getline() Function • When you use a delimiter other than ‘\n’ with getline(), the getline() consumes the delimiter but leaves he subsequent Enter key in the input stream, so you still must account for it • The program in Figure 10-15 and its output in Figure 10-16 illustrate the use of get() to consume the Enter key by getline()

21. 10 Using the getline() Function

22. 10 Other istream Member Functions • Most compilers support other istream member functions with names such as eof(), bad(), and good() • The istream class is not mysterious • It is just a class, and cin is just an object that already has been instantiated for you

23. 10 Using ostream Member Functions • The concepts you have learned while studying the cin object apply to the cout object as well • The ostream class supports member functions and overloaded operators just like the istream class — or any other class, for that matter

24. 10 Using Format Flags with setf() and unsetf() • Many of the states of the cout object are contained in a single long integer field, in which each bit represents some condition of the object • The arguments that determine the state of the cout object are called format flags or state flags • All format flags begin with ios:: • One member function of the ios class, the setf() function, takes arguments that set the bits of cout; that is, the arguments turn the bits in the flag on or off • Another member function, unsetf(), can be used to deselect the bit: cout.unsetf(ios::showpos); • Using the setf() function, you also can combine format flags using the bitwise OR operator (|)

25. 10 Using Format Flags with setf() and unsetf() • Some commonly used format flags are: • ios::left—left-justifies output within the field size, which may be set by the width() function (described in a following section) • ios::right—right-justifies output within the field size • ios::dec—formats numbers in decimal (base 10) • ios::hex—formats numbers in hexadecimal (base 16) • ios::oct—formats numbers in octal (base 8) • ios::showpos—inserts a + before positive numbers • ios::showpoint—displays the decimal point and six decimal positions for all floating-point numbers

26. 10 Using the width() Function • You can change the output field width with the iostream member width() function • This function defines the size of the output field in which the passed argument will be displayed • The width() function applies only to the first subsequent field to be output

27. 10 Using the width() Function

28. 10 Using the precision() Function • You can use the precision() function to control the number of significant digits you see in the output • If you want to control the number of positions shown to the right of the decimal point (instead of simply the number of significant positions visible), you must combine cout.setf(ios::fixed); and cout.precision();

29. 10 Using the precision() Function • In the steps outlined on pages 373 and 374 of the textbook, you declare an array of doubles, assign values with different numbers of significant digits, and then display all the values using the same precision

30. 10 Creating Manipulator Functions • If you need to display a variable named amountMoney in currency format with a dollar sign, in base 10, in a field size of eight, you might write the code shown in Figure 10-20 • When you create a manipulator function, the desired results become much clearer • A manipulator function is used to manipulate, or change, the state of the cout object

31. 10 Creating Manipulator Functions • You could write a manipulator to format output as currency • The function contains statements that display the dollar sign, set some ios flags, and set the width • In the function shown in Figure 10-21, a reference to ostream is passed into the function as an argument

32. 10 Using Built-In Manipulators • Some manipulators are so useful that they are already coded and placed in libraries included with your C++ compiler • You already have used the endl manipulator to output a newline character and flush the output stream

33. 10 Using the setprecision() Manipulator • You use the setprecision() manipulator to specify the number of decimals that will print • The setprecision() manipulator works like the precision() function—it specifies the number of significant digits to display • It is considered a manipulator instead of a member function because you chain a call to setprecision() along with other output and the insertion operator, rather than using an object and a dot operator, as you do with cout.precision()

34. 10 The DemoPrecision2 Program

35. 10 Using the setprecision() Manipulator • Any C++ manipulator, such as setprecision(), that takes an argument requires the inclusion of the iomanip.h file in your program • The program in Figure 10-22 produces the output in Figure 10-23

36. 10 Using the setw() Manipulator • The setw() manipulator allows you to set the width of a field for output • Use of the setw() manipulator requires inclusion of the iomanip.h file, because setw() requires an argument that represents the width of the output field • The setw() manipulator works like the width() member function you can use with the cout objects; the advantage of using setw() is its chaining capability in a cout statement

37. 10 Using the setiosflags() and resetiosflags() Manipulators • Two additional manipulators, setiosflags() and resetiosflags(), each perform several manipulations, depending on the flags (such as ios::dec or ios::showpoint) they receive as arguments • The setiosflags() manipulator turns on bit codes for the attributes named as arguments; the resetiosflags() manipulator turns off those bit codes

38. 10 Using the setiosflags() and resetiosflags() Manipulators • In the set of steps shown on pages 379 and 380 of the textbook, you combine the setprecision() and setiosflags() manipulators to produce output that displays decimal values with a fixed number of decimal places in a column

39. 10 Creating Manipulators that Require an Argument • To create a manipulator that does not require an argument, you must write a function that receives and returns an ostream reference argument • To create a manipulator that takes an argument, you must write two functions • When you use a manipulator that requires no argument, such as endl or the currency manipulator developed above, the address of the output or input stream is passed to the function • With manipulators, such as setw(), that take an argument both the address of the stream and the argument itself must be passed to the manipulator function • This task is handled by the omanip() function, which is defined in the file iomanip.h

40. 10 The Currency Manipulator with an Argument

41. 10 Creating Manipulators that Require an Argument • In Figure 10-25, the main() program calls the currency function, passing a 7 as the argument that indicates field width • In the steps referred to on pages 382 and 383 of the textbook, you create a manipulator that formats a part number for a manufacturing company

42. 10 Creating Manipulators that Require an Argument

43. 10 Understanding Computer Files • When you store data items in a computer system, you use a permanent storage device, such as a disk or a reel of magnetic tape • The term permanent is used to contrast this type of data storage with the temporary data storage that exists in computer memory • Data items typically exist in memory for only a short time • It is common practice to store data in a data hierarchy, which represents the relationships between the sizes of data units that business professionals most often use

44. 10 Understanding Computer Files • A data field represents one piece of data, such as a first or last name, phone or Social Security number, or salary • A data record consists of a number of data fields that are logically connected because they pertain to the same entity • A data file contains records that are logically related • Often, records within a data file are stored with a space or other delimiting character between fields, and a newline between each record

45. 10 A Data File Containing Employee Data

46. 10 Simple File Output • You have used a descendant of the ios class— and its descendant iostream to produce all sorts of screen output • In C++, when you write to a disk file rather than to the screen, you use a class named fstream, which, like ostream, is ultimately derived from ios • Figure 10-30 shows the relationship between fstream and some other input and output classes

47. 10 The fstream Family Tree

48. 10 Simple File Output • The fstream class is defined in the fstream.h file, which must be included in any C++ program that writes to or reads from a disk • You instantiate an ofstream output file object as you do any other object—by calling its constructor

49. 10 Simple File Output • In the steps provided on pages 387 to 389 of the textbook, you open a file write your name to it, and close it

50. 10 Simple File Output