Neither fwrite nor write work in my C++ program

Question

I'm writing a c++ program for my school project which involves lots of binary files. My problem is that whenever the compiler encounters a write statement (by this I mean writing data from an object of a class to a binary file using fstream::write), execution stops and I get an abnormal termination message. I'm sure the 'write' statement is the problem because every statement before it gets executed, while the next statements do not. The problem seemed to have been solved when I used fwrite instead, but then I discovered that data was not actually being written into the file. I've tried writing the object into the file in another function or in methods using 'this' pointer, but the computer doesn't seem to accept any of it.

The program was working just fine until a few days ago, but seemingly out of the blue, it's stopped working. I can't attach the source code because the problem is spread throughout the program, which is 5000+ lines long. However, here's a sample of what causes problems:

bottle b; //bottle is a class
fstream file ("Bottle.dat",ios::out|ios::binary);
file.write ((char*)&b,sizeof(b));

As an alternate, I've also tried

FILE*fptr=fopen ("Bottle.dat","w");
fwrite (&b, sizeof(b), 1, fptr);

The former results in abnormal termination, the latter does not work. There are no error messages during compilation. Can anybody help me? I'm a total newbie at programming, though (still in high school), so please try to keep it simple. Thanks!

EDIT: Maybe this will work as better example.

class irrigation
{   int n,i,itime[20],t[20];
    float amt[20],water[20];
protected:
    float irr;
public:
    float getirr()
    {   return irr;
    }
    void irrigate();
};

void irrigate_calc()
{   fstream file ("Irrigate.dat",ios::in|ios::binary);
    irrigation i;
    file.read ((char*)&i,sizeof(i));
    file.close();
    i.irrigate();
    fstream file1 ("Irrigate.dat",ios::out|ios::binary);
    file1.write ((char*)&i,sizeof(i));
    file1.close();
}

As far as I can tell, the problem arises when I open a file for reading, close it again and then open it for writing. Of course, I did try directly opening the file in ios::in, ios::out and ios::ate mode together, but that messed things up, big time.

EDIT 2: Another issue the same program is facing is that some normal cout statements intermittently spew out garbage: lots of special characters and little bits from other, totally unrelated cout statements. Any particular reason this is happening?

That would only work if bottle is a POD type. How is bottle defined? — K-ballo
Oh, really? Bottle is definitely not a POD: it has private and protected data members as well as public member functions. So I can't use fwrite then? — user1951594
You can, but you wont get anything meaningful from it... You shouldn't be getting a crash though — K-ballo
Can you provide a small but complete example that demonstrates the problem? — NPE
I can't attach the source code because <excuse> Always means you didn't create a testcase during your debugging, which means you should not be here asking yet. :) — Lightness Races in Orbit

RageD RageD · Accepted Answer · 2013-01-06T02:13:30

Since your object is not POD, in a simple way, you can try to write out all of its members. For instance,

#include <fstream>

using namespace std;

class B
{
public:
    // Simple constructor
    B(int mem1, char mem2) : member1(mem1), member2(mem2) {}

    // Have some methods here
    bool write() const {
      fstream file("Bottle.dat", ios::out|ios::binary);
      if(!file.is_open())
        return false;
      file.write(reinterpret_cast<const char*>(&member1), sizeof(int));
      file.write(&member2, sizeof(char));
      file.close();
      return true;
    }

private:
    int member1;
    char member2;
};

int main()
{
  B sample(52, 'c');
  sample.write();
  return 0;
}

Now you can call yourobj.write() and it can write out to Bottle.dat. You can write out/encode your data in anyway you wish. But this is a simple direct output. Your first 4 bytes (well, sizeof(int) bytes) will be member1 and your next byte is member2. Thus, you can easily read in this format if you would like as well.

EDIT

I have updated my code to make it a compilable sample. You can observe the output file with a hex editor to verify everything is correct. You should be able (in any editor) to be able to visibly see the char ('c') which corresponds to member2. If you open this in a hex editor, you will see that the first 4-bytes are equal to the number 0x34. Depending on the endianess of your machine, it may look like 00000034 or 34000000 this is decimal 52 which is what we expect based on my sample code.

Neither fwrite nor write work in my C++ program

1 Answers