Way to rewrite only first 30 chars using std::ofstream?

Question

is there a way to rewrite only the first 30 characters of a text file using std::ofstream without clearing all the other contents of a text file? I looked in the web, but that didn't help. One guy offered to rewrite the entire file, but that's very unefficient (there are +900*30 other characters). Anyone? Oh, and here's my code:

void Unlock(int Level)
{
    ifstream CheckIfExists("levels.txt");
    if (!CheckIfExists.good())
    {
        SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR, "ERROR",
                                 "Unable to open the levels file. Next level won't be unlocked!", window);
        CheckIfExists.close();
        return;
    }
    CheckIfExists.close();
    if(Level >= 0 && Level <= 29)
    {
        ofstream New_Locks("levels.txt");
        Locked[Level] = UNLOCKED;
        for(int i = 0; i < 30; ++i) New_Locks << Locked[i];
        New_Locks.close();
        return;
    }
    else SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_INFORMATION, "Congratulations!",
                                  "You won! Thank you for playing! Maybe once more?", window);
    return;
}

Answer 1

use fstream instead of ofstream

ifstream is for reading only, ofstream for writing only, fstream does both.

Also note: it is not necessary to explicitly close the files. The fstream destructor will close them properly.