Writing Data Randomly to a Random-Access File
Figure 17.13 writes data to the file credit.dat and uses the combination of fstream functions seekp and write to store data at exact locations in the file. Function seekp sets the "put" file-position pointer to a specific position in the file, then write outputs the data. Note that line 19 includes the header file ClientData.h defined in Fig. 17.10, so the program can use ClientData objects.
Figure 17.13. Writing to a random-access file.
(This item is displayed on pages 862 - 864 in the print version)
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1 // Fig. 17.13: Fig17_13.cpp 2 // Writing to a random-access file. 3 #include 4 using std::cerr; 5 using std::cin; 6 using std::cout; 7 using std::endl; 8 using std::ios; 9 10 #include 11 using std::setw; 12 13 #include 14 using std::fstream; 15 16 #include 17 using std::exit; // exit function prototype 18 19 #include "ClientData.h" // ClientData class definition 20 21 int main() 22 { 23 int accountNumber; 24 char lastName[ 15 ]; 25 char firstName[ 10 ]; 26 double balance; 27 28 fstream outCredit( "credit.dat", ios::in | ios::out | ios::binary ); 29 30 // exit program if fstream cannot open file 31 if ( !outCredit ) 32 { 33 cerr << "File could not be opened." << endl; 34 exit( 1 ); 35 } // end if 36 37 cout << "Enter account number (1 to 100, 0 to end input) ? "; 38 39 // require user to specify account number 40 ClientData client; 41 cin >> accountNumber; 42 43 // user enters information, which is copied into file 44 while ( accountNumber > 0 && accountNumber <= 100 ) 45 { 46 // user enters last name, first name and balance 47 cout << "Enter lastname, firstname, balance ? "; 48 cin >> setw( 15 ) >> lastName; 49 cin >> setw( 10 ) >> firstName; 50 cin >> balance; 51 52 // set record accountNumber, lastName, firstName and balance values 53 client.setAccountNumber( accountNumber ); 54 client.setLastName( lastName ); 55 client.setFirstName( firstName ); 56 client.setBalance( balance ); 57 58 // seek position in file of user-specified record 59 outCredit.seekp( ( client.getAccountNumber() - 1 ) * 60 sizeof( ClientData ) ); 61 62 // write user-specified information in file 63 outCredit.write( reinterpret_cast< const char * >( &client ), 64 sizeof( ClientData ) ); 65 66 // enable user to enter another account 67 cout << "Enter account number ? "; 68 cin >> accountNumber; 69 } // end while 70 71 return 0; 72 } // end main
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Lines 5960 position the "put" file-position pointer for object outCredit to the byte location calculated by
( client.getAccountNumber() - 1 ) * sizeof( ClientData )
Because the account number is between 1 and 100, 1 is subtracted from the account number when calculating the byte location of the record. Thus, for record 1, the file-position pointer is set to byte 0 of the file. Note that line 28 uses the fstream object outCredit to open the existing credit.dat file. The file is opened for input and output in binary mode by combining the file-open modes ios::in, ios::out and ios::binary. Multiple fileopen modes are combined by separating each open mode from the next with the bitwise inclusive OR operator (|). Opening the existing credit.dat file in this manner ensures that this program can manipulate the records written to the file by the program of Fig. 17.12, rather than creating the file from scratch. Chapter 22, Bits, Characters, Strings and structs, discusses the bitwise inclusive OR operator in detail.