Chapte t r r 9

Transcription

1 Chapter 9

2 Session Objectives Stream Class Stream Class Hierarchy String I/O Character I/O Object I/O File Pointers and their manipulations Error handling in Files Command Line arguments OOPS WITH C++ Sahaj Computer Solutions 2

3 Introduction A stream is a general name given to a flow of data. There are two types of streams Input stream : The input stream is used to read data from a disk file. Output stream: The output stream is used to write data into the file. Input stream Disk File Program Output Stream OOPS WITH C++ Sahaj Computer Solutions 3

4 The Stream Class Hierarchy The stream classes are arranged in a rather complex hierarchy. The extraction operator >> is a member of the istream class, and the insertion operator << is a member of the ostreamclass. Both of these classes are derived from the iosclass. The coutobject, representing the standard output stream, which is usually directed to the video display. Similarly cin is an object of the istream_withassign class, which is derived from istreamand is the standard input stream. OOPS WITH C++ Sahaj Computer Solutions 4

5 The Stream Class Hierarchy The classes used for input and output to the video display and keyboard are declared in the header file IOSTREAM. The classes used specifically for disk file I/O are declared in the file FSTREAM. The istreamand ostreamclasses are derived from ios and are dedicated to input and output, respectively. The istreamclass contains such functions as get(), getline(), read(), and the overloaded extraction (>>) operators, while ostreamcontains put() and write(), and the overloaded insertion (<<) operators. OOPS WITH C++ Sahaj Computer Solutions 5

6 The Stream Class Hierarchy The iostreamclass is derived from both istreamand ostreamby multiple inheritance. Classes derived from it can be used with devices, such as disk files, that may be opened for both input and output at the same time. Figure below shows the stream class hierarchy OOPS WITH C++ Sahaj Computer Solutions 6

7 The Stream Class Hierarchy OOPS WITH C++ Sahaj Computer Solutions 7

8 The istreamclass The istreamclass, which is derived from ios, performs input-specific activities, or extraction. Table below lists the functions you ll most commonly use Function Purpose >> Formatted extraction for all basic (and overloaded) types. get(ch); get(str) get(str, MAX) Extract one character into ch. Extract characters into array str, until \n.from the istream class. Extract up to MAX characters into array. OOPS WITH C++ Sahaj Computer Solutions 8

9 The ostreamclass The ostreamclass handles output or insertion activities. Table below shows the most commonly used member functions of this class. Function Purpose << Formatted insertion for all basic (and overloaded) types. put(ch) write(str, SIZE) flush() Insert character ch into stream. Insert SIZE characters from array str into file. Flush buffer contents and insert newline. OOPS WITH C++ Sahaj Computer Solutions 9

10 Character I/O The put() and get() functions, which are members of ostreamand istream, respectively, can be used to output and input single characters. OOPS WITH C++ Sahaj Computer Solutions 10

11 Writing Character data to a file // file output with characters #include <fstream.h> //for file functions #include <iostream.h> #include <string.h> intmain() string str = Time is a great teacher, but unfortunately it kills all its pupils. Berlioz ; ofstream outfile( TEST.TXT ); //create file for output for(int j=0; j<strlen(str); j++) //for each character, outfile.put( str[j] ); //write it to file cout << File written\n ; return 0; } OOPS WITH C++ Sahaj Computer Solutions 11

12 Reading character data from a file // file input with characters #include <fstream.h> //for file functions #include <iostream.h> intmain() char ch; //character to read ifstream infile( TEST.TXT ); //create file for input while( infile) //read until EOF or error infile.get(ch); //read character cout<< ch; //display it } cout << endl; return 0; } OOPS WITH C++ Sahaj Computer Solutions 12

13 Object I/O Since C++ is an object-oriented language, it s reasonable to wonder how objects can be written to and read from disk. Writing an Object to Disk When writing an object, we generally want to use binary mode. This writes the same bit configuration to disk that was stored in memory, and ensures that numerical data contained in objects is handled properly. OOPS WITH C++ Sahaj Computer Solutions 13

14 Writing an Object to Disk // saves person object to disk #include <fstream.h> //for file streams #include <iostream.h> //////////////////////////////////////// class person //class of persons protected: char name[80]; //person s name short age; //person s age public: void getdata() //get person s data cout<< Enter name: ; cin >> name; cout<< Enter age: ; cin >> age; } }; /////////////////////////////////////////// intmain() person pers; //create a person pers.getdata(); //get data for person //create ofstream object ofstream outfile( PERSON.DAT ); //write to it outfile.write((char*)(&pers), sizeof(pers)); return 0; } OOPS WITH C++ Sahaj Computer Solutions 14

15 Reading an Object from Disk // reads person object from disk #include <fstream.h> //for file streams #include <iostream.h> ///////////////////////////////////////// class person //class of persons protected: char name[80]; //person s name short age; //person s age public: void showdata() //display person s data cout<< Name: << name << endl; cout<< Age: << age << endl; } }; ////////////////////////////////////////// intmain() person pers; //create person variable ifstreaminfile( PERSON.DAT ); //create stream //read stream infile.read((char *)&pers, sizeof(pers) ); pers.showdata(); //display person return 0; } OOPS WITH C++ Sahaj Computer Solutions 15

16 I/O with Multiple Objects Our next example opens a file and writes as many objects as the user wants. Then it reads and displays the entire contents of the file. // reads and writes several objects to disk #include <fstream> //for file streams #include <iostream> using namespace std; /////////////////////////////////////////// class person //class of persons protected: char name[80]; //person s name int age; //person s age public: void getdata() cout<< \n Enter name: ; cin>> name; cout<< Enter age: ; cin>> age; } void showdata() cout<< \n Name: << name; cout<< \n Age: << age; } }; OOPS WITH C++ Sahaj Computer Solutions 16

17 I/O with Multiple Objects intmain() char ch; person pers; //create person object fstream file; //create input/output file file.open( GROUP.DAT, ios::app ios::out ios::in ios::binary); do //data from user to file cout << \nenter person s data: ; pers.getdata(); //get one person s data //write to file file.write((char*)(&pers), sizeof(pers) ); cout << Enter another person (y/n)? ; cin>> ch; } while(ch== y ); //quit on n file.seekg(0); //reset to start of file //read first person file.read( (char*)(&pers), sizeof(pers) ); while(!file.eof() ) //quit on EOF cout << \nperson: ; //display person pers.showdata(); //read another person file.read( reinterpret_cast<char*>(&pers), sizeof(pers) ); } cout << endl; return 0; } OOPS WITH C++ Sahaj Computer Solutions 17

18 The fstreamclass So far the file objects we have created have been for either input or output. In some cases we want to create a file that can be used for both input and output. This requires an object of the fstreamclass, which is derived from iostream, which is derived from both istreamand ostreamso it can handle both input and output. OOPS WITH C++ Sahaj Computer Solutions 18

19 The open() Function The open() function is used to open multiple files that use the same stream object. Syntax: Example: file-stream-class stream-object; stream-object. open( filename ); ofstream outfile; outfile.open( DATA1 );. outfile.close(); OOPS WITH C++ Sahaj Computer Solutions 19

20 The Mode Bits In the open() function we include several new mode bits. The mode bits, defined in ios, specify various aspects of how a stream object will be opened. Parameters ios::app ios::ate ios::binary ios::in ios::nocreate ios::noreplace Description Append to end-of-file Go to end of file on opening Binary file Open file for reading only Open fails if the file does not exist Opens fails if the file already exists OOPS WITH C++ Sahaj Computer Solutions 20

21 Detecting End-of-File When we read a file little by little, we will eventually encounter an end-of-file (EOF) condition. The EOF is a signal sent to the program from the operating system when there is no more data to read. Example: while(!infile.eof() ) // until eof encountered OOPS WITH C++ Sahaj Computer Solutions 21

22 File Pointers Each file has two associated pointers known as thefile pointers. One of them is called the input pointer(or get pointer ) and the other is called the output pointer( or the put pointer) We can use these pointers to move through the file while reading or writing. The input pointer is used to read the contents of the given file location and the output pointer is used for writing to a given file location and the output pointer is used for writing to a given file location. OOPS WITH C++ Sahaj Computer Solutions 22

23 Actions on file pointers while opening a file Open for reading only H E L L O W O R L D Input pointer Open in append mode (for writing more data) H E L L O W O R L D Output pointer Open for Writing only Output pointer OOPS WITH C++ Sahaj Computer Solutions 23

24 Manipulation of File Pointers The file stream classes support the following functions to manage such situations: Seekg(): moves get pointer(input) to a specified location Seekp(): Moves put pointer (outout) to a specified location. Tellg(): Gives the current position of the get pointer. Tellp():Gives the current position of the put pointer. OOPS WITH C++ Sahaj Computer Solutions 24

25 Error Handling During File Operations The class iossupports several member functions that can be used to read the status recorded in a file stream. The list of functions are as follows: eof(): Returns true(non-zero) if end-of-file is encountered while reading. Otherwise returns false(zero) fail(): Returns true(non-zero) when an input or output operation has failed. OOPS WITH C++ Sahaj Computer Solutions 25

26 Error Handling During File Operations Bad(): Returns true if an invalid operation is attempted or any unrecoverable error has occurred. However if it is false, it may be possible to recover from any other error reported, and continue operation. Good() return true if no error has occurred. This means, all the above functions are false. For instance, if filegood() is true, all is well with the stream file and we can proceed to perform I/O operations. When it returns false, no further operations can be carried out. OOPS WITH C++ Sahaj Computer Solutions 26

27 Command Line Arguments Like C, C++ too supports a feature that facilitates the supply of arguments to the main() function. These arguments are supplied at the time of invoking the program. Example: c> exam data results Here exam is the name of the file containing the program. Data and results are two arguments passed from the command line. OOPS WITH C++ Sahaj Computer Solutions 27

28 Command Line Arguments The command line arguments are typed by the user and are delimited by a space. The first argument is always the file name and contains the program to be executed. The main() function which we have been using up to now without arguments can take two arguments as shown main(int argc, char* argv[]) The first argument argc(also known as argument counter) represents the number of arguments in the command line. The second argument argv(also known as argument vector) is an array of a char type pointer that points to the command line arguments. OOPS WITH C++ Sahaj Computer Solutions 28

29 Command Line Arguments The size of argument array will be equal to the value of argc. For example: C>exam data result argv[0] exam argv[1] data argv[2] result OOPS WITH C++ Sahaj Computer Solutions 29