CSE 2341 Object Oriented Programming with C++ Note Set #15

1. CSE 2341Object Oriented Programming with C++Note Set #15

2. Overview • Review of Binary File I/O • Random File I/O

3. File I/O • 3 steps • open the file • read/write using the file • close the file • Binary File I/O • Open the file using ios::binary • use the read and write functions to write char pointer that points to first byte of structure to the read/written

4. Quick Example #include <iostream> #include <fstream> using namespace std; struct personInfo { char name[15]; int age; }; int main() { personInfo pi[4] = { {"Bob", 22}, {"Sally", 23}, {"Sam", 24}, {"Pat", 25} }; fstream fOut ("myFile.dat", ios::out|ios::binary);

5. Quick Example if(!fOut) { cout << "Error" << endl; return 0; } for (int i = 0; i < 4; i++) { fOut.write(reinterpret_cast<char*>(pi + i), sizeof (personInfo)); } fOut.close(); return 0; }

6. Problem • You want to read the 3rd record out of the file without having to read the first two. • Remember – there are “pointers” in the file object that maintain state • it knows you are currently reading and writing in the file • you can manipulate these via member functions called using your file objects

7. seekp • fObj.seekp(32L, ios::beg) • sets the write position to the 33rd byte (byte 32) from the beginning of the file • fObj.seekp(-10L, ios::end) • sets the write pos to the 11th byte (byte 10) from the end of the file • fObj.seekp(120L, ios::curr) • sets the write pos to the 121st byte (byte 120) from the current position

8. seekg • fObj.seekg(2L, ios::beg) • sets the read position to the 3rd (byte 2) from the beginning of the file • fObj.seekg(-100L, ios::end) • sets the read position to the 101st byte (byte 100) from the end of the file • fObj.seekg(40L, ios::curr) • sets the read position to the 41st byte (byte 40) from the current position • fObj.seekg(0L, ios::end) • sets the read position to the end of the file

9. Simple Write #include <iostream> #include <fstream> using namespace std; int main() { fstream fout ("data.dat", ios::out|ios::binary); char c = 'A'; while (c < 'K') { fout.write(&c, sizeof(c)); c++; } fout.close(); return 0; } data.dat ABCDEFGHIJ

10. Reading with Random Functions int main() { fstream fin("data.dat", ios::in|ios::binary); char a; fin.get(a); cout << a << endl; fin.seekg(3, ios::beg); fin.get(a); cout << a << endl; fin.seekg(-4, ios::end); fin.get(a); cout << a << endl; fin.seekg(2, ios::cur); fin.get(a); cout << a << endl; return 0; } OUTPUT: A D G J data.dat ABCDEFGHIJ

11. See Lecture 15 Handout for a More Robust example

12. Fini ?