Declaring fstream Objects

1. Declaring fstream Objects • An istream object named cin connects program and keyboard • An ostream object named cout connects the program and the screen These streams are constructed automatically.

2. Declaring fstream Objects • For doing I/O from/to a file a program must explicitly open a stream • Creates a connection between a program in memory and a text file

3. Basic fstream Operations • Must use #include <fstream> • open() Establishes connection program to file • is_open() Returns true/false • >> Operator, inputs value from file • getline() Reads line of text into string object • << Operator, outputs value to file • eof() Returns true/false, end of file • close() Terminates connection between program, file

4. The open() Operation • Given:ifstream inStream;inStream.open( "pressure.dat"); • Parameter can also be a variable • If it is a string variable ( string fileName ) must use fileName.data() for correct parameter type • When input file is opened, read position pointer set to beginning of sequence of characters in the file

5. The open() Operation (for output) • When output file is opened, file is created on the disk, with write-position pointer pointing at the eof marker • Opening an ofstream to a file will create a new file • If file existed before, it is now (by default) destroyed • Otherwise, new file is created

6. The open() Operation (omit) • Possible to open the file with a mode argument as a second parameter

7. Initialization at Declaration • Possible to open at declaration of variableofstream outStream ("pressure.out");ifstream inStream ("pressure.in"); ExecutingProgram

8. Programming Defensively • The success or failure of a call to open a file should always be tested • Use inStream.open() • Us in an assert( ) mechanism • Call before proceeding with additional operations on the file

9. The Input Operator • We have used cin >> x; • Value entered via the keyboard • C++ uses the same operator to bring values into variables from a streaminStream >> reading; • The reading pointer keeps track of where in the stream the program is currently reading

10. The getline() Function • Requires an istream object, a string objectgetline (nameStream, name); • Reads entire name into variable • Reads until it hits a newline character • Newline character read, not added to variable Note: the >> operator does not read the newline. The next >> skips it as white space. But if a getline is used next, it sees the newline and terminates. Think about what happens if you mix >> and getline calls.

11. The eof() Message • Can be used as a sentinel value to control an input loopfor ( ; ; ) { inStream >> reading; if (inStream.eof() ) break; // . . . process the input } • inStream.eof() returns true following execution of the input statement at this point

12. The Output Operator << • Overloaded to perform with ostream, ofstream objectsoutStream <<"\n--> There were a total of" << count << "values."; • Note that the write pointer is pushed forward, keeps pointing at the eof marker.

13. The close() Message • The file stream is disconnected when • The program leaves the scope of the fstream object (implicitly) • The close() message is executed (explicitly) • It is good practice to explicitly close a file when the program is done using it • If many files are accessed, the operating system may place a limit on how many files are open simultaneously