9/3/2023 0 Comments Istream open![]() In this case, the data in the buffer is not written to disk, and is lost forever. In these cases, the destructors for the file stream classes are not executed, which means the files are never closed, which means the buffers are never flushed. The main culprit in this case is when there is data in the buffer, and then program terminates immediately (either by crashing, or by calling exit()). One way to cause the buffer to be flushed is to close the file - the contents of the buffer will be flushed to disk, and then the file will be closed.īuffering is usually not a problem, but in certain circumstance it can cause complications for the unwary. When a buffer is written to disk, this is called flushing the buffer. This is done primarily for performance reasons. Instead, several output operations may be batched and handled together. This means that anything that is output to a file stream may not be written to disk immediately. When inf goes out of scope, the ifstream ![]() read stuff from the file into a string and print it While there's still stuff left to read Std::cerr << "Uh oh, Sample.txt could not be opened for reading!\n" If we couldn't open the input file stream for reading If we couldn't open the output file stream for reading We'll read from a file called Sample.txt ![]() We’ll use this fact to determine how much to read. Note that ifstream returns a 0 if we’ve reached the end of the file (EOF). Now, we’ll take the file we wrote in the last example and read it back in from disk. Note that it is also possible to use the put() function to write a single character to the file. If you open it with a text editor, you will see that it indeed contains two lines we wrote to the file. If you look in your project directory, you should see a file called Sample.txt. When outf goes out of scope, the ofstream Std::cerr << "Uh oh, Sample.txt could not be opened for writing!\n" If we couldn't open the output file stream for writing This is extremely straighforward: #include To do file output in the following example, we’re going to use the ofstream class. Once you are done, there are several ways to close a file: explicitly call the close() function, or just let the file I/O variable go out of scope (the file I/O class destructor will close the file for you). Then use the insertion (>) operator to write to or read data from the file. However, this is extremely simple: to open a file for reading and/or writing, simply instantiate an object of the appropriate file I/O class, with the name of the file as a parameter. Unlike the cout, cin, cerr, and clog streams, which are already ready for use, file streams have to be explicitly set up by the programmer. To use the file I/O classes, you will need to include the fstream header. These classes do file input, output, and input/output respectively. There are 3 basic file I/O classes in C++: ifstream (derived from istream), ofstream (derived from ostream), and fstream (derived from iostream). In C++17, file I/O is better done using std::filesystem.įile I/O in C++ works very similarly to normal I/O (with a few minor added complexities). This lesson and the next were written pre-C++17.
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