adding get line example

lectures/01_introduction/getline.cpp

@@ -0,0 +1,54 @@
+#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;
+	}
+	return 0;
+}