vector<string> doesnt work when using seperate classes - c++

Question

I have a txt file which has a rogue-like level
I load it like so:
void File::LoadLevel()
{
ifstream input_file;
input_file.open("Level_1.txt");
if (input_file.fail())
{
perror("Level_1.txt");
}
while (input_file >> _level)
{
_level_instance.push_back(_level);
}
}
variables:
string _level;
vector<string> _level_instance;
I print it out like this:
for (int i = 0; i < _level_instance.size(); i++)
{
cout << _level_instance[i] << endl;
}
which works fine.
however I have a vector in another class as well and i use a getter like this:
vector<string>GetlevelData(){ return _level_data; }
and I change the LoadLevel() from this:
_level_instance.push_back(_level);
to this:
Level Lvl;
Lvl.GetLevelData().pushback(_level);
I make an method in 'Level' class which prints it out to the screen same as before
it compiles but it doesnt print out anything why?
By the way in the int main() neccesary methods are executed which is OpenLevel() from File class and Print() from Level class
EDIT:
passing it &by reference didnt work
here are both methods in Level.h:
void SetLevelData(const std::string &string) {
_level_data.push_back(string);
}
//Getters
vector<string>& GetlevelData(){ return _level_data; }
in File.cpp:
void File::LoadLevel()
{
ifstream input_file;
Level lvl;
input_file.open("Level_1.txt");
if (input_file.fail())
{
perror("Level_1.txt");
}
while (input_file >> _level)
{
lvl.GetlevelData().push_back(_level);
}
}
it doesnt work it prints nothing, even though i added '&'
the result is same when i try it with 'push_back' method in Level.h
Here is int main() just in case:
int main()
{
File f;
Level lvl;
f.LoadLevel();
lvl.PrintLevel();
system("PAUSE");
return 0;
}
And PrintLevel() in Level.cpp:
void Level::PrintLevel()
{
for (int i = 0; i < _level_data.size(); i++)
{
cout << _level_data[i] << endl;
}
}

Answer 1

The GetlevelData function returns its vector by value which means a whole new copy of it is created. Anything you push into that vector will be lost when the copy goes out of scope (which happens when the expression Lvl.GetLevelData().pushback(_level) is done).
You should return it by reference instead:
vector<string>& GetlevelData(){ return _level_data; }
// ^
// |
// Return by reference

Answer 2

vector<string>GetlevelData() returns a copy of the vector and not a reference to it.
So your Lvl.GetLevelData().push_back(_level); returns a copy adds data to the vector and then deletes that copy again. So you add it to a vector that will be immediately be deleted again.
You would need to return it either by reference:
vector<string>& GetlevelData(){ return _level_data; }
Or write a method to push back the data:
void push_back( const std::string &string) {
_level_data.push_back(string);
}