Possible compiler bug while reading file and outputting contents - c++

Question

While trying to help a friend with a problem with his code, I encountered a very weird bug when compiling the following code with GCC.
#include <fstream>
#include <iostream>
#include <string>
int main() {
std::ifstream classes("classes.txt");
std::string line;
std::string txt = ".txt";
while (std::getline(classes, line)) {
std::cout << "[-]: " << line << "," << txt << std::endl;
}
return 0;
}
classes.txt contains the following:
CSC1
CSC2
CSC46
CSC151
MTH121
When compiled with Clang or MSVC, the output is as follows:
[-]: CSC1,.txt
[-]: CSC2,.txt
[-]: CSC46,.txt
[-]: CSC151,.txt
[-]: MTH121,.txt
But, when compiled with GCC, this is what the code outputs:
,.txtCSC1
,.txtCSC2
,.txtCSC46
,.txtCSC151
[-]: MTH121,.txt
I cannot make sense of whats happening here. Can anyone explain this?
Image with GCC version and output:

Answer 1

No, this is not a compiler bug. You are running into line-ending differences between operating systems. My magic ball tells me that if you run dos2unix classes.txt, the problem will go away. Similarly, cat -v classes.txt should output something similar to the following:
CSC1^M
CSC2^M
CSC46^M
CSC151^M
MTH121^M
Here, the ^M denotes \r\n. This is known as a CRLF or "carriage return line feed". On Linux when the carriage return is encountered, it instructs the terminal to go back to the beginning of the line. This results in .txt overwriting whatever you had output previously.
N.B if you are running Clang on an Apple system, which I'm guessing you are, certain versions of Mac use \r, but not \r\n or \n..