File I/O Finished off

1
File I/O Finished off

• Colin Cherry standing in for
• Dr. Lin

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What happens if your input is bigger than your
character array?

• char arr10 cin gtgt arr
• The characters are written to memory starting at
  the spot pointed to by arr
• Depending on whether or the memory is being used,
  different things could happen
• Harmless
• Could be used to execute malicious code
• Segfault!

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Solutions (1)

• Use the string class!
• Will resize automatically if too small
• Should work for most situations

4
Solutions (2)

• Use cin.getline(arr,size)
• Will read in up to size-1 characters
• Will add the c-string delimiter for you
• Will fail if buffer overflows, but gracefully
• Have to check cin.fail()
• Good for situations where an agreement exists
• Any violation is undefined

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Overflow Demo!

• !

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Stream object flags

• When a file operation fails or hits EOF, it sets
  a flag
• Two from our example, fin.fail(), fin.eof()
• These flags stay with the stream object
• fin.eof() returns true, even after we close the
  current file and open a new one
• To reset stream object, call clear()

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So where were we?

• Went over opening closing files
• Using the ltlt and gtgt for input output
• Various stream flags and options
• Precision, width, setf
• Manipulators
• cout ltlt setw(6) ltlt myNumber ltlt endl

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Saving Flag Settings

• Flag settings stay until changed
• Precision and setf flags can be savedand
  restored
• Function precision() returns current settingif
  called with no arguments
• Member function flags() provides
  similarcapability

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Saving Flag Settings Example

• void outputStuff(ofstream outStream) int
  precisionSetting outStream.precision() long
  flagSettings outStream.flags() outStream.setf(
  iosfixed iosshowpoint) outStream.precision
  (2)
• outStream ltlt Do stuff here!
• outStream.precision(precisionSetting) outStrea
  m.flags(flagSettings)

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Restoring Default setf Settings

• Can also restore default settings cout.setf(0,
  iosfloatfield)
• Not necessarily the last setting!
• Default values are implementation-dependent
• Does not reset precision settings
• Only setf settings

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A function that takes a stream

• void outputStuff(ofstream outStream)
• But this also works
• Void outputStuff(ostream outStream)
• Stream always need to be passed by reference

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Stream Hierarchies

• Input file streams class is derived from classof
  all input streams
• It then adds open and close member functions
• ifstream is derived from istream
• ofstream is derived from ostream

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Random Access to Files

• Sequential Access
• Most commonly used
• Random Access
• Rapid access to records
• Perhaps very large database
• Access randomly to any part of file
• Use fstream objects
• input and output

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Random Access Tools

• Opens same as istream or ostream
• Adds second argument
• fstream rwStreamrwStream.open(stuff, iosin
  ios out)
• Opens with read and write capability
• Move about in file
• rwStream.seekp(1000)
• Positions put-pointer at 1000th byte
• rwStream.seekg(1000)
• Positions get-pointer at 1000th byte

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Random Access Sizes

• To move about ? must know sizes
• sizeof() operator determines number of
  bytesrequired for an objectsizeof(s) //Where s
  is string s Hellosizeof(10)sizeof(double)si
  zeof(myObject)
• Position put-pointer at 100th record of
  objectsrwStream.seekp(100sizeof(myObject)
  1)

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Opening for binary

• fstream obStream, ibStream
• obstream.open(file, iosbinary iosout)
• // OR
• ibstream.open(file, iosbinaray iosin)
• iosbinary, iosin and iosout
• Other constants that tell C how to open files
• Like iosapp

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Writing bytes

• Characters are one byte each, so youve been
  doing it all along!
• You can write other things, though
• Base data types (int, float)
• Classes and structs
• Arrays
• Just cast to character first
• obstream.write( (char) obj, sizeof(obj))

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Why would I do that?

• Say you were writing software for a video store
• Each movie is a class with title, running time,
• status (rented or no), and rented by field
• Inventory is an array of classes
• Just write inventory to file, makes it easier to
  save
• and load before and after shut-down
• Why not just build over a database?
• Well, if you want to do it the easy way, sure
• Valid example Dr. Lins load on demand hash table

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Reading bytes

• Same deal as before
• ibstream.read( (char) obj, sizeof(obj))
• Make sure your object is a pointer!
• Tricky bit
• You need to know the size of an object in advance
• Solutions
• Fixed width objects (easy, not always possible)
• Header

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To the example!

• !

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Summary

• Streams connect to files with openoperation
• Member function fail() checks successes
• Stream member functions format output
• e.g. width, setf, precision
• Same usage for cout (screen) or files
• Member function eof
• Used to test for end of input file