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145 lines
7.1 KiB
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[#]: subject: (How to read and write files in C++)
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[#]: via: (https://opensource.com/article/21/3/ccc-input-output)
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[#]: author: (Stephan Avenwedde https://opensource.com/users/hansic99)
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[#]: collector: (lujun9972)
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[#]: translator: ( )
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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How to read and write files in C++
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======
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If you know how to use I/O streams in C++, you can (in principle) handle
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any kind of I/O device.
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![Computer screen with files or windows open][1]
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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.
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### I/O stream classes
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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`.
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* `ofstream` means _output file stream_, and it can be accessed with the insertion operator, `<<`.
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* `ifstream` means _input file stream_, and it can be accessed with the extraction operator, `>>`.
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Both types are defined inside the header `<fstream>`.
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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.
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### A simple example
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This example program is quite simple: It creates an `ofstream`, writes to it, creates an `ifstream`, and reads from it:
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```
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#include <iostream> // cout, cin, cerr etc...
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#include <fstream> // ifstream, ofstream
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#include <string>
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int main()
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{
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std::string sFilename = "MyFile.txt";
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/******************************************
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* *
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* WRITING *
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* *
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******************************************/
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std::ofstream fileSink(sFilename); // Creates an output file stream
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if (!fileSink) {
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std::cerr << "Canot open " << sFilename << std::endl;
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exit(-1);
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}
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/* std::endl will automatically append the correct EOL */
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fileSink << "Hello Open Source World!" << std::endl;
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/******************************************
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* *
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* READING *
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* *
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******************************************/
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std::ifstream fileSource(sFilename); // Creates an input file stream
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if (!fileSource) {
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std::cerr << "Canot open " << sFilename << std::endl;
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exit(-1);
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}
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else {
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// Intermediate buffer
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std::string buffer;
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// By default, the >> operator reads word by workd (till whitespace)
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while (fileSource >> buffer)
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{
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std::cout << buffer << std::endl;
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}
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}
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exit(0);
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}
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```
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This code is available on [GitHub][7]. When you compile and execute it, you should get the following output:
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![Console screenshot][8]
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(Stephan Avenwedde, [CC BY-SA 4.0][9])
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This is a simplified, beginner-friendly example. If you want to use this code in your own application, please note the following:
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* 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.
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* 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.
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* By default, `ifstream` stops at white space and skips it. To read line by line until you reach [EOF][12], use the `getline(...)`-method.
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* 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.
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### Writing from the systems perspective
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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:
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```
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#include <unistd.h> // needs to be included
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sync();
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```
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### Summary
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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.
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/21/3/ccc-input-output
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作者：[Stephan Avenwedde][a]
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选题：[lujun9972][b]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://opensource.com/users/hansic99
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[b]: https://github.com/lujun9972
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[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)
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[2]: https://en.cppreference.com/w/cpp/io/ios_base
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[3]: https://en.cppreference.com/w/cpp/io/basic_ofstream
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[4]: https://en.cppreference.com/w/cpp/io/basic_ifstream
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[5]: https://en.wikipedia.org/wiki/Separation_of_concerns
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[6]: https://en.wikipedia.org/wiki/Dependency_injection
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[7]: https://github.com/hANSIc99/cpp_input_output
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[8]: https://opensource.com/sites/default/files/uploads/c_console_screenshot.png (Console screenshot)
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[9]: https://creativecommons.org/licenses/by-sa/4.0/
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[10]: https://en.cppreference.com/w/cpp/io/basic_ios
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[11]: https://en.cppreference.com/w/cpp/io/basic_ios/operator!
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[12]: https://en.wikipedia.org/wiki/End-of-file
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[13]: https://en.wikipedia.org/wiki/Newline
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[14]: https://en.wikipedia.org/wiki/Sync_%28Unix%29
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