C Stream Input/Output

1
C Stream Input/Output

• Chapter 11

2
What You Will Learn

• Using C object oriented stream input and output
• Formatting I/O, creating manipulators
• I/O Class hierarchies
• Determine streamsuccess or failure

3
Introduction

• Many I/O features are object oriented
• Uses function and operation overloading
• We will see type safe I/O
• operations performed sensitive to the data type
• if no I/O function exists for a given type, then
  compiler notes as error

4
Streams

• Defn gt a sequence of bytes
• I/O moves them from/to secondary devices
• Applications associate meaning to the bytes
• I/O is extremely slow compared to processing
  speeds
• C provides both formatted and unformatted I/O

5
IOstream Library Header files

• We use the include ltiostream.hgt
• Contains basic infor required for all stream-I/O
  operations
• cin, cout, cerr, clog
• include ltiomanip.hgt
• formatted I/O with parameterized stream
  manipulators
• include ltfstream.hgt
• file processing operations

6
File Class Hierarchy
ios
istream
ostream
iostream
iostream.h
fstream.h
ifstream
fstream
ofstream
7
Stream I/O Classes Objects

• gtgt and ltlt are right and left bit shift operators
• Have been overloaded for input and output
• cin is an object of class istream
• tied to standard input, keyboard
• cout is an object of class ostream
• tied to standard output, screen

8
Stream-Insertion Operator

• ltlt operator overloaded to output data items of
• built in types
• strings
• pointer values
• When outputting expressions, use parentheses
• avoids confusion of operator precedence with ltlt

9
Cascading Insertion/Extraction Operators

• ltlt associates from left to right
• Overloaded ltlt operator returns a reference to its
  left-operand object (cout)
• Next cascaded operand sent to the result of the
  first function call (which points to cout)

cout ltlt "total " ltlt a b
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Output of char Variables

• Consider data object of type char
• a character stringcout ltlt name//contents of
  array printed
• What if we wish to print the value of the pointer
  the address to which it points?
• Cast the pointer to void cout ltlt
  static_castltvoid gt(name)

11
Char Output with put

• put is a member function of ostream
• Sends a single character value to the output
  stream
• The function returns reference to object which
  called it (cout)
• thus can be cascadedcout.put('W').put('O').put('W
  ')
• Integer value parameter taken in as ASCII
  character number

12
Stream Input

• Performed with extraction operator gtgt
• Normally skips whitespace characters
• blanks, tabs, newlines
• Operator returns zero (false) when eof occurs
• otherwise returns reference to stream object
  (thus can be cascaded)
• Stream has state bits used to control state of
  stream

13
Stream-Extraction Operator

• Stream extraction can be cascaded
• Warnings
• don't try extraction gtgt with cout
• don't try insertion ltlt with cin
• possible need of parentheses due to precedence of
  ltlt and gtgt
• while (cin) specify end of I/O with ctrl-z
  (ctrl-d on Mac or UNIX)

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get and getline Member Functions

• ch cin.get( )// inputs returns one
  character
• even if it is whitespace -- returns the
  whitespace character
• returns the EOF when end of file reached
• cin.eof( )returns true (1) when user enters ctrl-z

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get and getline Member Functions

• cin.get(ch) // new version
• reads value from stream into ch
• returns 0 when eof encountered
• otherwise returns reference to stream object (for
  cascading)
• cin.get (name, 30,'\n') // 3rd version
• 30 specifies max number of characters
• '\n' terminates input
• '\n' not consumed

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get and getline Member Functions

• cin.getline(name,30,'\n')
• Similar to cin.get( )
• Difference is that the getline version will
  "consume" the '\n' (or whatever delimiter is
  specified)
• Need to be aware of using cin.ignore if we know
  '\n' will still be in the stream

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istream Member Functions

• ignore -- skips over designated number of
  characters
• putback -- places character previously obtained
  with get back onto input stream
• good for scanning a stream looking for specific
  character
• peek -- returns next character in stream without
  getting it off the stream

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Type-Safe I/O

• ltlt and gtgt overloaded to accept data items of
  specific types
• that is, there are separate overloaded operators
  for different types of operands (function
  parameters)
• Error flags set if wrong type of data comes
  through the stream

19
Unformatted I/O

• Performed with read and write member functions
• They take two parameters
• an array of char or a buffer
• a number which tells how many characterscin.read(
  buffer,20)cin.write (buffer,15)

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Stream Manipulators

• Provided for formatting I/O
• Capabilities included
• setting field widths
• setting decimal precision
• set/unset format flags
• setting fill in char fields
• flushing streams

21
Integral Stream Base

• Integers normally interpred as base 10 (decimal)
  values
• Make sure to include ltiomanip.hgt
• Specify other bases with
• hex for base 16 (hexadecimal)
• oct for base 8 (octal)cout ltlt n ltlt " " ltlt
  oct ltlt n ltlt " in octal"

22
Floating-Point Precision

• Specify number of decimal digits shown (rounded
  to that digits)
• Make sure to include ltiomanip.hgt
• And to cout.setf (iosfixed) // orcout ltlt
  setiosflags (iosfixed)
• Use cout ltlt setprecision (places) or
  cout.precision (places)
• Author notes -- setting precision to 0 may result
  in default precision setting

23
Field Width

• Two versionscout.width(places) // andcout ltlt
  setw (places)
• If values take up less space, padding added to
  left gt right justify
• If values take up more space, width specification
  is overridden
• thus may push succeeding columns of output to the
  right

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User-Defined Manipulators
// bell manipulator (using escape sequence
\a) ostream bell( ostream output ) return
output ltlt '\a' // ret manipulator (using
escape sequence \r) ostream ret( ostream output
) return output ltlt '\r' // tab
manipulator (using escape sequence \t ) ostream
tab( ostream output ) return output ltlt '\t'
Note use of - return type is ostream - parameter
type is ostream Usage cout ltlt 'a' ltlt tab ltlt
'c' ltlt bell
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Format State Flags

• Note table on pg 621 (Fig. 11.20)
• These flags controlled by
• flags ( )
• cout.setf ( ) member
• cout.unsetf ( ) functions
• setiosflags ( ) // in the ltlt sequence

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Trailing Zeros, Decimals

• Use iosshowpoint
• floating point values print as integers if that
  is what is stored
• showpoint forces decimal points to be printed for
  floats

(Fig 11.21)
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Justification

• Use iosleft iosright iosinternal
• Right justification is default with setw( )
• padded characters added on right when iosleft
  is specified
• iosinternal gt a number's sign left justified,
  magnitude right justified

(Fig. 11.21)
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Padding

• The cout.fill('x') specifies that padding be
  x's instead of spaces
• Alternative version cout ltlt setfill('x') ltlt
  setw(4) ltlt 5
• What would output would be?

(Fig. 11-24)
xxx5
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Integral Stream Base

• Use showbase flag

cout ltlt setiosflags (iosshowbase) ltlt x ltlt
endl ltlt oct ltlt x ltlt endl ltlt hex ltlt x ltlt
endl
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Floating-Point Numbers

• We have used in previous course cout.setf
  (iosfixed, iosfloatfield)
• Also possible to specify scientific notation
  cout.setf (iosscientific,
  iosfloatfield)
• Can be turned off with cout.unsetf
  (iosscientific)

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Forms of setf( )
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Uppercase/Lowercase Control

• Forces uppercase letters for
• scientific notation
• hex notation
• Usecout ltlt setiosflags (iosuppercase) ltlt
  1.23e6

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Setting/Resetting Format Flags

• flags ( ) member function without parameters
  returns long value with current settings of
  format flags
• store in a long variable long setting
  cout.flags( )
• then later on use with argumentcout.flags
  (setting)

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Stream Error States

• Base class ios has bits which tell state of I/O
• eofbit set when end-of-fileuse cin.eof( )
• failbit set when format error with no loss
  of charuse cin.fail( )

35
Stream Error States

• badbit set for error with lost datause cin.bad(
  )
• goodbit set if none of eofbit, failbit or
  badbit are setuse cin.good( )
• cin.rdstate( ) returns errorstate of stream
• cin.clear( ) restores stream state to good

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Tying an Output Stream to an Input Stream

• Interactive programs use istream for input,
  ostream for output
• Prompt must first appear
• Only when output buffer fills or is flushed can
  input proceed
• To explicitly tie these cout and cin
  togethercin.tie (cout)