Saving randomly generated passwords to a text file in order to display them later - c++-cli

Question

I'm currently in a traineeship and I currently have to softwares I'm working on. The most important was requested yesterday and I'm stucked on the failure of its main feature: saving passwords.
The application is developped in C++\CLR using Visual Studio 2013 (Couldn't install MFC libraries somehow, installation kept failing and crashing even after multiple reboots.) and aims to generate a password from a seed provided by the user. The generated password will be save onto a .txt file. If the seed has already been used then the previously generated password will show up.
Unfortunately I can't save the password and seed to the file, though I can write the seed if I don't get to the end of the document. I went for the "if line is empty then write this to the document" but it doesn't work and I can't find out why. However I can read the passwords without any problem.
Here's the interresting part of the source:
int seed;
char genRandom() {
static const char letters[] =
"0123456789"
"ABCDEFGHIJKLMNOPQRSTUVWXYZ"
"abcdefghijklmnopqrstuvwxyz";
int stringLength = sizeof(letters) - 1;
return letters[rand() % stringLength];
}
System::Void OK_Click(System::Object^ sender, System::EventArgs^ e) {
fstream passwords;
if (!(passwords.is_open())) {
passwords.open("passwords.txt", ios::in | ios::out);
}
string gen = msclr::interop::marshal_as<std::string>(GENERATOR->Text), line, genf = gen;
bool empty_line_found = false;
while (empty_line_found == false) {
getline(passwords, line);
if (gen == line) {
getline(passwords, line);
PASSWORD->Text = msclr::interop::marshal_as<System::String^>(line);
break;
}
if (line.empty()) {
for (unsigned int i = 0; i < gen.length(); i++) {
seed += gen[i];
}
srand(seed);
string pass;
for (int i = 0; i < 10; ++i) {
pass += genRandom();
}
passwords << pass << endl << gen << "";
PASSWORD->Text = msclr::interop::marshal_as<System::String^>(pass);
empty_line_found = true;
}
}
}
I've also tried replacing ios::in by ios::app and it doesn't work. And yes I have included fstream, iostream, etc.
Thanks in advance!
[EDIT]
Just solved this problem. Thanks Rook for putting me on the right way. It feels like a silly way to do it, but I've closed the file and re-openned it using ios::app to write at the end of it. I also solved a stupid mistake resulting in writing the password before the seed and not inserting a final line so the main loop can still work. Here's the code in case someone ends up with the same problem:
int seed;
char genRandom() {
static const char letters[] =
"0123456789"
"ABCDEFGHIJKLMNOPQRSTUVWXYZ"
"abcdefghijklmnopqrstuvwxyz";
int stringLength = sizeof(letters) - 1;
return letters[rand() % stringLength];
}
System::Void OK_Click(System::Object^ sender, System::EventArgs^ e) {
fstream passwords;
if (!(passwords.is_open())) {
passwords.open("passwords.txt", ios::in | ios::out);
}
string gen = msclr::interop::marshal_as<std::string>(GENERATOR->Text), line, genf = gen;
bool empty_line_found = false;
while (empty_line_found == false) {
getline(passwords, line);
if (gen == line) {
getline(passwords, line);
PASSWORD->Text = msclr::interop::marshal_as<System::String^>(line);
break;
}
if (line.empty()) {
passwords.close();
passwords.open("passwords.txt", ios::app);
for (unsigned int i = 0; i < gen.length(); i++) {
seed += gen[i];
}
srand(seed);
string pass;
for (int i = 0; i < 10; ++i) {
pass += genRandom();
}
passwords << gen << endl << pass << endl << "";
PASSWORD->Text = msclr::interop::marshal_as<System::String^>(pass);
empty_line_found = true;
}
}
passwords.close();
}

Answer 1

So, here's an interesting thing:
passwords << pass << endl << gen << "";
You're not ending that with a newline. This means the very end of your file could be missing a newline too. This has an interesting effect when you do this on the final line:
getline(passwords, line);
getline will read until it sees a line ending, or an EOF. If there's no newline, it'll hit that EOF and then set the EOF bit on the stream. That means the next time you try to do this:
passwords << pass << endl << gen << "";
the stream will refuse to write anything, because it is in an eof state. There are various things you can do here, but the simplest would be to do passwords.clear() to remove any error flags like eof. I'd be very cautious about accidentally clearing genuine error flags though; read the docs for fstream carefully.
I also reiterate my comment about C++/CLR being a glue language, and not a great language for general purpose development, which would be best done using C++ or a .net language, such as C#. If you're absolutely wedded to C++/CLR for some reason, you may as well make use of the extensive .net library so you don't have to pointlessly martial managed types back and forth. See System::IO::FileStream for example.