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 Depending on whether or the memory is

being used, different things could happen:– Harmless– Could be used to execute malicious code– Segfault!

Solutions (1):

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

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

Overflow Demo!

!

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()

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;

Saving Flag Settings Flag settings ‘stay’ until changed Precision and setf flags can be saved

and restored Function precision() returns current setting

if called with no arguments Member function flags() provides similar

capability

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);

}

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

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

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

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

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

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);

Opening for binary

fstream obStream, ibStream;obstream.open(“file”, ios::binary| ios::out);// ORibstream.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

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));

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

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

To the example!

!

Summary Streams connect to files with open

operation 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