If you know how to use I/O streams in C++, you can (in principle) handle
any kind of I/O device.
![Computer screen with files or windows open][1]
In C++, reading and writing to files can be done by using I/O streams in conjunction with the stream operators `>>` and `<<`. When reading or writing to files, those operators are applied to an instance of a class representing a file on the hard drive. This stream-based approach has a huge advantage: From a C ++ perspective, it doesn't matter what you are reading or writing to, whether it's a file, a database, the console, or another PC you are connected to over the network. Therefore, knowing how to write files using stream operators can be transferred to other areas.
### I/O stream classes
The C++ standard library provides the class [ios_base][2]. This class acts as the base class for all I/O stream-compatible classes, such as [basic_ofstream][3] and [basic_ifstream][4]. This example will use the specialized types for reading/writing characters, `ifstream` and `ofstream`.
*`ofstream` means _output file stream_, and it can be accessed with the insertion operator, `<<`.
*`ifstream` means _input file stream_, and it can be accessed with the extraction operator, `>>`.
Both types are defined inside the header `<fstream>`.
A class that inherits from `ios_base` can be thought of as a data sink when writing to it or as a data source when reading from it, completely detached from the data itself. This object-oriented approach makes concepts such as [separation of concerns][5] and [dependency injection][6] easy to implement.
### A simple example
This example program is quite simple: It creates an `ofstream`, writes to it, creates an `ifstream`, and reads from it:
std::ofstream fileSink(sFilename); // Creates an output file stream
if (!fileSink) {
std::cerr << "Canot open " << sFilename << std::endl;
exit(-1);
}
/* std::endl will automatically append the correct EOL */
fileSink << "Hello Open Source World!" << std::endl;
/******************************************
**
*READING *
**
******************************************/
std::ifstream fileSource(sFilename); // Creates an input file stream
if (!fileSource) {
std::cerr << "Canot open " << sFilename << std::endl;
exit(-1);
}
else {
// Intermediate buffer
std::string buffer;
// By default, the >> operator reads word by workd (till whitespace)
while (fileSource >> buffer)
{
std::cout << buffer << std::endl;
}
}
exit(0);
}
```
This code is available on [GitHub][7]. When you compile and execute it, you should get the following output:
![Console screenshot][8]
(Stephan Avenwedde, [CC BY-SA 4.0][9])
This is a simplified, beginner-friendly example. If you want to use this code in your own application, please note the following:
* The file streams are automatically closed at the end of the program. If you want to proceed with the execution, you should close them manually by calling the `close()` method.
* These file stream classes inherit (over several levels) from [basic_ios][10], which overloads the `!` operator. This lets you implement a simple check if you can access the stream. On [cppreference.com][11], you can find an overview of when this check will (and won't) succeed, and you can implement further error handling.
* By default, `ifstream` stops at white space and skips it. To read line by line until you reach [EOF][12], use the `getline(...)`-method.
* For reading and writing binary files, pass the `std::ios::binary` flag to the constructor: This prevents [EOL][13] characters from being appended to each line.
### Writing from the systems perspective
When writing files, the data is written to the system's in-memory write buffer. When the system receives the system call [sync][14], this buffer's contents are written to the hard drive. This mechanism is also the reason you shouldn't remove a USB stick without telling the system. Usually, _sync_ is called on a regular basis by a daemon. If you really want to be on the safe side, you can also call _sync_ manually:
```
#include <unistd.h> // needs to be included
sync();
```
### Summary
Reading and writing to files in C++ is not that complicated. Moreover, if you know how to deal with I/O streams, you also know (in principle) how to deal with any kind of I/O device. Libraries for various kinds of I/O devices let you use stream operators for easy access. This is why it is beneficial to know how I/O steams work.
[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/browser_screen_windows_files.png?itok=kLTeQUbY (Computer screen with files or windows open)
