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File I/O Finished off

File I/O Finished off. Colin Cherry standing in for Dr. Lin. What happens if your input is bigger than your character array?. char arr[10]; cin >> arr; The characters are written to memory starting at the spot pointed to by arr

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File I/O Finished off

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  1. File I/O Finished off Colin Cherry standing in for Dr. Lin

  2. What happens if your input is bigger than your character array? char arr[10]; cin >> 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!

  3. 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

  5. Overflow Demo! !

  6. 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()

  7. So where were we? • Went over opening & closing files • Using the << and >> for input & output • Various stream flags and options: • Precision, width, setf • Manipulators • cout << setw(6) << myNumber << endl;

  8. 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

  9. Saving Flag Settings Example • void outputStuff(ofstream& outStream){ int precisionSetting = outStream.precision(); long flagSettings = outStream.flags(); outStream.setf(ios::fixed | ios::showpoint); outStream.precision(2); outStream << “Do stuff here!”; outStream.precision(precisionSetting); outStream.flags(flagSettings);}

  10. Restoring Default setf Settings • Can also restore default settings: cout.setf(0, ios::floatfield); • Not necessarily the ‘last’ setting! • Default values are implementation-dependent • Does not reset precision settings • Only setf settings

  11. 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

  12. 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

  13. 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

  14. Random Access Tools • Opens same as istream or ostream • Adds second argument • fstream rwStream;rwStream.open(“stuff”, ios::in | 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

  15. Random Access Sizes • To move about  must know sizes • sizeof() operator determines number of bytesrequired for an object:sizeof(s) //Where s is string s = “Hello”sizeof(10)sizeof(double)sizeof(myObject) • Position put-pointer at 100th record of objects:rwStream.seekp(100*sizeof(myObject) – 1);

  16. Opening for binary fstream obStream, ibStream; obstream.open(“file”, ios::binary| ios::out); // OR ibstream.open(“file”, ios::binaray | ios::in); • ios::binary, ios::in and ios::out • Other constants that tell C++ how to open files • Like ios::app

  17. Writing bytes • Characters are one byte each, so you’ve 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));

  18. 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. Lin’s load on demand hash table

  19. 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

  20. To the example! !

  21. 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

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