reading bytes using fstream c++ - c++

Question

I created a read / write of bytes into a binary file using fstream object
#include <iostream>
#include <fstream>
using namespace std;
#define COL_WIDTH 20
int writeBinaryFile(char *desPath);
int readBinaryFile(char *desPath);
int age;
char name[COL_WIDTH];
int recsize = sizeof(int)+sizeof(name);
int n;
int main(){
char desPath[MAX_PATH+1];
char input[2];
while(true){
cout << "Main Menu:\n1 Write Binary File\n2 Read Binary File\n3 EXIT" << endl;
cin.getline(input, 2);
if(atoi(input)== 1){
cout << "Enter destination path:" << endl;
cin.getline(desPath, MAX_PATH+1);
for(;;){
int output = writeBinaryFile(desPath);
if(output == 1) break;
else if(output == 2) { *input = '4'; break;}
}
}
else if(atoi(input) == 2){
cout << "Enter destination path:" << endl;
cin.getline(desPath, MAX_PATH+1);
for(;;){
int output = readBinaryFile(desPath);
if(output == 1) break;
else if(output == 2){ *input = '4'; break;}
}
}
else if(atoi(input) == 3)
break;
else cout << "You entered wrong input, enter only 1 or 2." << endl;
if(atoi(input) == 4) continue;
else break;
}
system("pause");
return 0;
}
int writeBinaryFile(char *desPath){
char option[2];
fstream wBin(desPath, ios::binary | ios::out);
if(!wBin){
cout << desPath << " is not good path."<< endl;
return 1;
}
cout << "Enter name: " << endl;
cin.getline(name,COL_WIDTH);
cout << "Enter age: " << endl;
cin >> age;
cout << "Enter record number: " << endl;
cin >> n;
wBin.seekp(n*recsize);
wBin.write(name, sizeof(name));
wBin.write((char*)&age, sizeof(age));
wBin.close();
cout << "Enter record again? Press:" << endl
<< "Enter to continue" << endl
<< "Q to quit" << endl
<< "M to go back to main menu" << endl;
cin.ignore(256,'\n');
cin.getline(option,2);
if(option[0] == 'q' ||option[0] == 'Q') return 1;
else if(option[0] == 'm' ||option[0] == 'M') return 2;
return 0;
}
int readBinaryFile(char *desPath){
char option[2];
fstream rBin(desPath, ios:: binary | ios:: in);
if(! rBin){
cout << "File not found!" << endl;
return 1;
}
cout << "What record number?" <<endl;
cin >> n;
rBin.seekp((n*recsize));
rBin.read(name,sizeof(name));
rBin.read((char*)&age, sizeof(int));
cout << name <<endl;
cout << age << endl;
rBin.close();
cout << "Print more? Press enter. Press Q to QUIT or M to go back." << endl;
cin.clear();
cin.sync();
cin.getline(option, 2);
if(option[0] == 'q'|| option[0] == 'Q'){rBin.close(); return 1;}
else if(option[0] == 'm' || option[0] == 'M'){rBin.close(); return 2;}
return 0;
}
i created a binary file first with name, age, recordnumber ( position of bytes which is 0) then on second loop i input name, age, recordnumber 2.
When i tried to read the first pair(char*, int) of bytes pos(0) but it returns wrong result but when i tried to read the last pair in pos(1) it returns correct result.
I also tried making 3 times the input, but only the last char* and int is correct.
Why is it getting wrong results in first bytes(name(20bytes),age(4bytes)) but getting correct it last bytes?

Answer 1

In each call to writeBinaryFile the output is reopened, destroying the previous contents of the file.
You'll want to use ios::binary | ios::ate | ios::out | ios::in.
But, this bitmask will not create the file if it does not exist. You can work around that problem with this code sequence:
int writeBinaryFile(char *desPath) {
...
// Create the file only if it doesn't exist
fstream(desPath, ios::binary|ios::out|ios::app);
// Now it exists -- open it
fstream wBin(desPath, ios::binary|ios::out|ios::in);
...
}
Credit #JoeFish for the useful part of the answer.