I/O Streams

1. I/O Streams • C++ I/O occurs in streams that are simply a sequence of bytes. • Low-level I/O transfers some number of bytes where the individual byte is the item of interest. • High-level I/O transfers information as meaningful units such as integers, floating-point numbers, characters, strings, etc.

2. istream • The istream library supports stream-input operations including cin. • Input from the istream object is called extraction because characters are being pulled out of the stream into the program.

3. istream • Each istream object has an error state and several formatting values. • Errors include reaching the end of file, formatting errors such getting letters when digits are expected, and serious errors such as file not found.

4. istream • The extraction steps are: • Check the stream’s error state. • If it is non-zero then there is some error. • If the extraction is from cin then the cout stream is flushed. • Any leading whitespace is skipped. • The characters are extracted to obtain the value. • Whitespace terminates the extraction.

5. istream errors • eofbit - end of file was reached • EOF is set when the program attempts to read one character past the last character. • If EOF is found while skipping leading white space the failbit is set. • failbit – extraction failed • EOF reached while skipping white space or wrong type of characters encountered.

6. istream Errors • badbit – the stream is bad and can not be used. • cin.good() – none of error bits are set. • cin.eof() – eofbit is set. • cin.fail()- failbit or badbit is set. • cin.bad()- badbit is set. • Note: Test the stream after completing an extraction, not before.

7. Formatted istream Extraction • The >> operator is used for formatted extractions. • Extraction operators are overloaded for the following types: • char – single character • char * - A sequence of characters • short, long, int – integer values • float, double – floating point values.

8. Formatted istream Extraction • Format manipulators • If a value begins with 0 (zero), it is assumed to be an octal number. • If a value begins with 0X or 0x, then the number is assumed to be a hexadecimal number. • Otherwise, the value is assumed to be a decimal number.

9. Formatted istream Extraction • What is the issue with the following code that reads dates in the form mm/dd/yy? int month, day, year; char slash; cin >> month >> slash >> day >> slash >> year;

10. Formatted istream Extraction • The correct code would be: cin >> dec >> month >> slash >> day >> slash >> year;

11. Unformatted istream Extraction • Unformatted extraction means that: • The bytes are copied. There is no conversion to data types. • Leading spaces are not skipped. • Calls are made to member functions to extract data rather than overloaded operators.

12. Unformatted istream Extraction • The get() function extracts one character at a time from the input stream. • char letter; • cin.get(letter); • while (letter != EOF) { • cin.get(letter); • } • char letter; • while (cin.get(letter)) { • }

13. Unformatted istream Extraction • The get(bufferName, BUFFER_LENGTH) reads characters up to (but not including) a particular delimiter. • Input is read until the buffer is full or the delimiter character is found. • const int BUFF_LEN = 80; • char inputData; • cin.get(inputData, BUFF_LEN, ’,’);

14. Unformatted istream Extraction • The getline(bufferName, BUFFER_LENGTH) function extracts one line at a time. • This is similar to: • get(bufferName, BUFFER_LENGTH, ‘\n’); • getline discards the ‘\n’ character.

15. istream functions • The ignore function skips over a designated number of characters or terminates upon reaching a designated delimiter. • The default number of characters to skip is 1. • The putback function places the previous character obtained by a get back into the stream.

16. istream Functions • The peek function returns the next character from the input stream but does not remove the character from the stream.

17. ostream • The ostream library supports stream-output operations including cout. • Output to the ostream object is called insertion because characters are being inserted into the stream by the program.

18. ostream • Each ostream object has an error state and several formatting values. • Errors include reaching the end of file, and serious errors such as file not found. • Formatting state controls the details of the conversions for formatted insertions.

19. ostream • The insertion steps are: • The stream’s error is checked. • The value is converted to the appropriate characters, possibly padded, and inserted into the stream.

20. ostream Format State • The format state consists of flags and three additional values: the fill character, precision, and width. • By default, no flags are set, the fill character is space, the width is 0, and the precision is 6.

21. ostream Format State • The format flags can be manipulated using the following: • The flags() function returns the current flag settings. • The flags(int f) replaces the current flag settings with f and returns the previous flag settings.

22. ostream Format State • The setf(int f) turns on all flags given in f and returns the previous flag settings. • Any flags not indicated in f are left in their current state. • This is a bitwise or of the current flag settings and f. • The unset(int f) turns off all flags given in f and returns the previous flag settings. • Any flags not indicated in f are left in their current state. • This is a bitwise and of the current flag settings and the complement of f.

23. ostream Format State • Other values can be manipulated with the following functions: • The fill() function returns the current fill character. • The fill(char c) function sets the fill character to c and returns the previous value. • The precision() function returns the current precision.

24. ostream Format State • The precision(int p) function sets the precision to p and returns the previous value. • The width() function returns the current width. • The width(int w) sets the width to w and returns the previous value.

25. ostream Format State • The following functions are used to manipulate the output: • cout << dec - sets all output to be decimal values. • cout << oct - sets all output to be octal. • cout << hex - sets all output to be hexadecimal. • cout << flush - flushes the output buffer.

26. ostream Format State • Additional manipulator functions are available in the iomanip.h file. • cout << setw(w) - sets the width of the output to w. • cout << setfill(c) – sets the fill character to c. • cout << setprecision(p)- sets the precision to p.

27. ostream Format State • cout << setiosflags(f) – sets the flags just as the setf(f) function does. • Cout << resetiosflags(f) - turns off all flags in f in the same manner as unsetf(f).

28. ostream Example • Formatting integer output. • Formatting floating point output.

29. Unformatted ostream Insertion • Unformatted insertions only transfer bytes with no conversion. • Member functions are used to insert data rather than overloaded operators. • The cout.put(ch) function writes a single byte. • The cout.write(buffer, length) function writes an array of bytes.

30. Tying an istream to an ostream • By default, cout is synchronized to cin. • This ensures that the cout statements appear before the subsequent input.

31. Tying an istream to an ostream • The tie function permits the programmer to synchronize an ostream with an istream. inputStream.tie(outputStream); • Ties the outputStream to the inputStream. inputStream.tie(0); • Unties the outputStream from the inputStream.