Need some advice on binary editing

Hi there. I've got a problem which I have been stuck for a good whole week already. Tried alot of different methods but to no avail. I'm still new at this, hopefully someone can help me with this? I've got a binary file which I have created in a previous program. So this second program I'm working on is suppose to allow me to edit the binary file which I created. Struct is still the same, I'm still able to open and read the binary file too. Now the problem is that I cant seem to edit and save the updates back into the file. Is there something I'm doing wrong?

This is just part of the program, in this part I want to increase the variable "size" by 1. size DOES increase by 1, but when I open the binary file again, it goes back to its default value

Code:

void appendStaff (fstream& afile, char nameDat[])
{
    ABC staff[MAX];
    int size;
    
    afile.open (nameDat, ios::in | ios::out | ios::app | ios::binary);
    afile.read (reinterpret_cast <char *> (&staff), sizeof (staff));
    afile.read (reinterpret_cast <char *> (&size), sizeof (size));
    
    size = size + 1;
    afile.write (reinterpret_cast < const char *> (&size), sizeof (staff));
    cout << size <<endl;
        
    afile.close();
}

You're giving it the address of size and then telling it to write sizeof(staff) bytes. That sure as heck isn't going to work, in fact I expect it to be somewhat likely to crash!
You also appear to be writing the new size after the rest of the data. Perhaps you meant to seek back to the start before writing?

Ahh.. I messed up on that part. I copied from an older version. yes, it is suppose to be sizeof(size).
I'm a little confused still. So you're saying that I should use a seekp and bring it back to the beginning before I do a write?

Yes, you will need to seek to the position where size is stored in the file before trying to (over)write it.