C++ ofstream writing variables into a text file - c++

Question

I wasn't able to find any obvious answers to this, and I'm pretty stumped.
Do I need to do something different when I want to write a variable string into a text document compared to what I need to do when I want to write just a set string into said document?
This code would properly write "set string" on 8 individual lines on newfile.txt (and create it).
string output_file = "newfile.txt"
ofstream file_object_2;
file_object_2.open(output_file, std::ofstream::out | std::ofstream::app);
string nextline;
for (int i = 0; i <= row_number; ++i)
{
file_object_2 << "set string" << "\n";
}
file_object_2.close();
return 0;
But this would leave the file completely empty, even if line_vector[i] itself has strings within it (and cout can print them)
string output_file = "newfile.txt"
ofstream file_object_2;
file_object_2.open(output_file, std::ofstream::out | std::ofstream::app);
string nextline;
for (int i = 0; i <= row_number; ++i)
{
nextline = line_vector[i];
file_object_2 << nextline << "\n";
}
file_object_2.close();
return 0;
I tried to look at the documentation and do it the same way as they had done, but I'm having no success here. Obviously because of my own failure, but I just can't figure out where I'm in the wrong here.
The only difference with those two codes are in the lines where I'm trying to write into the document
file_object_2 << nextline << "\n";
vs
file_object_2 << "set string" << "\n";
main() that I tried to trim down so it has less dynamic functionality (no manual inputs) but still does not work:
The text file "a.txt" has just a couple of lines of random strings
[a.txt]
("Yogi has a best friend too
Boo Boo, Boo Boo
Yogi has a best friend too
Boo Boo, Boo Boo Bear
Boo Boo, Boo Boo Bear
Boo Boo, Boo Boo Bear
Yogi has a best friend too
Boo Boo, Boo Boo Bear")
And the function itself
#include <iostream>
#include <fstream>
#include <string>
#include <vector>
using namespace std;
int main() {
string input_file = "a.txt";
vector<string> line_vector;
string output_file = "output.txt";
// reads from the original text file. Included in the example because I might
// be constructing the vector in a stupid way and this is where it happens
int row_number = 0;
string line;
while (getline(file_object, line) )
{
cout << line << endl;
line_vector.push_back(line);
++row_number;
}
file_object.close();
// writing onto a new file starts, this is where I'd assume the problem is
ofstream file_object_2;
file_object_2.open(output_file, std::ofstream::out | std::ofstream::app);
string nextline;
for (int i = 0; i <= row_number; ++i)
{
nextline = i + " " + line_vector[i];
file_object_2 << nextline << "\n";
}
file_object_2.close();
return 0;
}

Answer 1

This is broken:
i + " " + line_vector[i]
You meant to_string(i) + " " + line_vector[i], but you used + directly on i and "", which the compiler determines is operator+(int, const char*) which is pointer arithmetic, not a conversion of i to a string. Since the string in question only has one character (a NUL), adding 1 number results in a past-the-end pointer which can't be dereferenced, adding larger numbers is already undefined behavior.
The simplest fix would be to keep i separate from nextline, and write it to the file directly. Your loop body becomes:
{
nextline = line_vector[i];
file_object_2 << i << " " << nextline << "\n";
}
But it should also work to convert i to a string using std::to_string().
As a side note, the first snippet of code you claimed is broken (where nextline = line_vector[i];) actually is just fine. If you had bothered to test that you might have found the problem yourself.