#include <iostream>	// include iostream so we can do std::cin, std::cout, and std::cerr
#include <fstream>	// include fstream so we can do file input and output.

int main(int argc, char* argv[]){
	if(argc!=3){
		std::cout << "Usage: ./a.out input.txt output.txt" << std::endl;
		exit(1);
	}

	// store the first argument in inputFileName, as a C++ string
	std::string inputFileName = std::string(argv[1]);

	// but we can't just use that string to open a file, we have to create an std::ifstream object.
        std::ifstream inputFile(inputFileName);

	// this is how we actually open the input file.
        if (!inputFile.is_open()) {
		// if the file can't be opened, we print an error message.
                std::cerr << "Failed to open the input file." << std::endl;
                exit(1);
        }

	// store the second argument in outputFileName, as a C++ string
	std::string outputFileName = std::string(argv[2]);

	// but we can't just use that string to open a file, we have to create an std::ofstream object.
        std::ofstream outputFile(outputFileName);

	// this is how we actually open the output file.
        if (!outputFile.is_open()) {
		// if the file can't be opened, we print an error message.
                std::cerr << "Failed to open the output file." << std::endl;
                exit(1);
        }

	std::string line;

	// read the input file one line each time, and store the content of that one line into this string variable line.
	// The getline function returns the input stream (inputFile in this case), 
	// and the loop condition while(getline(inputFile, line)) checks whether the stream is in a good state.
	// If getline successfully reads a line from inputFile, 
	// it returns the stream (inputFile) which evaluates to true in a boolean context.
	// If getline encounters the end-of-file (EOF) while reading, it sets the end-of-file flag on the stream, 
	// and the next attempt to read from the stream will fail. 
	// in this case, getline returns the stream in a boolean context, which evaluates to false.
	while(getline(inputFile, line)){
		// print that one line to the console
		std::cout << line << std::endl;

		// print again, but this time print it to a file.
		outputFile << line << std::endl;
	}

	// after using the files, close them before exit the program.
	inputFile.close();
	outputFile.close();
	return 0;
}