Appending strings to text files in C++ - c++

Question

C++ beginner here,
I am trying to append some text to a pre-written .txt file where every line there is a word.
I have been using the method ofstream and ifstream as seen below, but everytime I try to write something, it erases the file. (I am not allowed to use ios:app or simillar)
int append_new_word() {
//First I read everything on the list and save it to a string called Words_in_List
ifstream data_wordlist_in("woerterliste"); //Opens the txt file
if (!data_wordlist_in) // checks if the file exists
{
cout << "File does not exist!" << endl;
return 1;
}
string Word;
int line = 0;
string Vorhandene_Woerter;
std::getline(data_wordlist_in, Wort);
do { //line counter, goes through all lines and save it to a string
line++;
std::getline(data_wordlist_in, Word);
Words_in_List = Words_in_List + "\n" + Word;
} while (!data_wordlist_in.eof());
cout << Words_in_List << endl;
data_wordlist_in.close();
//HEre it should save the string again in the list word per word with the neu appended word
ofstream data_wordlist_out("woerterliste"); //opens ofstream
if (!data_wordlist_out)
{
cout << "File does not exist!" << endl;
return 1;
}
string new_word_in_list;
cout << "\n Insert a Word to append: ";
cin >> new_word_in_list;
data_wordlist_out << Words_in_List << endl << new_word_in_list;
data_wordlist_out.close(); //closes ofstream
}
Everytime I try I open my program it erases the list.

Answer 1

Your code has some minor problems, but nothing that matches your description of it.
It does line based input, which is strange because nothing in the problem description indicates that reading a line at a time is necessary.
It counts lines, again for no obvious reason.
It skips the first line (maybe this is deliberate, but if so you didn't mention that).
The loop termination is incorrect (see link in the comments).
The function is declared as returning an int but no return is made.
Here some code that addresses these problems. It reads characters not lines (using get()) which makes reading the input simpler, but essentially it's the same technique as your code.
void append_new_word()
{
string existing_content;
ifstream in("file.txt");
char ch;
while (in.get(ch))
existing_content += ch;
in.close();
cout << "enter a new word ";
string new_word;
cin >> new_word;
ofstream out("file.txt");
out << existing_content << new_word << '\n';
}