file overwriting/truncating

[baniakjr]

i've got another problem hope that's the last one. I've been searching and trying diffrent think but i don't know what to do.

I've got some array of classes stored in file

void Newmap(char file[]) //Create new map file
{
  ofstream fout(file,ios::binary | ios::trunc);
  class Mapsqr Map[Map_Size][Map_Size];
  int coords[2];
  for (int i=0;i<Map_Size;i++)
  {
    for (int j=0;j<Map_Size;j++)
    {
      Map[i][j].SetParameters(i,j,Plains);
    }
  }
  fout.write((char*) &Map,sizeof Map);
  fout.close();
}

i can read it and everything is fine up to a point where i want to replace one Mapsqr class element in file.But i don't wont to read whole file and then rewrite it because it can be quit large. So i set a put pointer to corect place and wont to overwrite one element and end up with file with overwriten element end cutted off after it.

void EditOneTerrain (char file[],int coords[],TerrainType towhat)//edit terrain type
{
  class Mapsqr tmp;
  ofstream fout(file,ios::binary);
  tmp.SetParameters(coords[X],coords[Y],towhat);
  for (int i=0;i<coords[X];i++)
  {
    fout.seekp(Map_Size*sizeof(tmp), ios::cur);
  }

  for (int i=0;i<coords[Y];i++)
  {
    fout.seekp(sizeof(tmp), ios::cur);
  }
  fout.write((char*) &tmp,sizeof tmp);
  system("pause");
  fout.close();
}

When i read the file there are all the elements up to the changed one, but after it file ends. What's wrong? Does write automaticaly truncate file after it?

[Tonto]

>> Does write automaticaly truncate file after it?

No. But the openmode you specified does. It's totally wack yo. here's the deal with openmodes.

filebuf *open(const char *filename,
    ios_base::openmode mode);

The member function endeavors to open the file with filename filename, by calling fopen(filename, strmode). Here strmode is determined from mode & ~(ate | binary):

    * ios_base::in becomes "r" (open existing file for reading).
    * ios_base::out or ios_base::out | ios_base::trunc becomes "w" (truncate existing file or create for writing).
    * ios_base::out | ios_base::app becomes "a" (open existing file for appending all writes).
    * ios_base::in | ios_base::out becomes "r+" (open existing file for reading and writing).
    * ios_base::in | ios_base::out | ios_base::trunc becomes "w+" (truncate existing file or create for reading and writing).
    * ios_base::in | ios_base::out | ios_base::app becomes "a+" (open existing file for reading and for appending all writes).

If mode & ios_base::binary is nonzero, the function appends b to strmode to open a binary stream instead of a text stream. It then stores the value returned by fopen in the file pointer fp. If mode & ios_base::ate is nonzero and the file pointer is not a null pointer, the function calls fseek(fp, 0, SEEK_END) to position the stream at end-of-file. If that positioning operation fails, the function calls close(fp) and stores a null pointer in the file pointer.

If the file pointer is a null pointer, the function returns a null pointer. Otherwise, it returns this.

I think you can specify the openmode ios::binary | ios::out | ios::in and it will work.

[baniakjr]

Thanks.You're my hero.I was begining to warry that i'll have to change whol writing reading process. But now it works. Hmmm fun thing is that i was looking on openmodes but didn't manage to find anything. Thanks once again.