void edit_member()
{
    int ID;
    int slot;
    streampos pos;
    cout << "n"
         << "Enter member ID#: ";
    cin.ignore();
    ID  = only_int();
    int mems = count_members();
    for (int a = 0; a < mems; a++)
    {
        if (ID == member[a].ID)
        {
            i_file.open("member_data.txt");
            for (int b = 0; b <= a; b++)
            {
                string void_data;
                getline(i_file, void_data, '#');
                pos = i_file.tellg();
                slot = a;
            }
            i_file.close();
            break;
        }
    }
    o_file.open("member_data.txt");
    cout << "nEnter new name for ID# " << ID << ": ";
    cin.ignore();
    getline(cin, member[slot].name, 'n');
    o_file.seekp(pos) << member[slot].name;
    o_file.close();
    load();
}

member[slot].name.length()

//when you open the file in write only mode, its content deleted.
//to keep the old file content, you need to open for both read and write:
fstream outfile;
outfile.open ("test.txt",ios::in|ios::out);