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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: (wyxplus)
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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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translated/tech/20210326 How to read and write files in C.md
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translated/tech/20210326 How to read and write files in C.md
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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: (wyxplus)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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如何用 C++ 读写文件
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======
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如果你知道如何在 C++ 中使用<ruby>输入输出<rt>I/O</rt></ruby>流，那么你便能够（原则上）处理任何类型的输入输出设备。
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![Computer screen with files or windows open][1]
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在 C++ 中，可以通过将输入输出流与流运算符 `>>` 和 `<<` 结合使用来进行文件读写。当读写文件的时候，这些运算符将应用于代表硬盘驱动器上文件类的实例上。这种基于流的方法有个巨大的优势：从 C++ 的角度，无论你要读取或写入的内容是文件、数据库、控制台，亦或是你通过网络连接的另外一台电脑，这都无关紧要。因此，知道如何使用流运算符来写入文件能够被利用到其他领域。
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### 输入输出流类
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C++ 标准库提供了 [ios_base][2] 类。该类充当所有 I/O 流的基类，例如 [basic_ofstream][3] 和 [basic_ifstream][4]。本例将使用特殊的类型来读写字符，`ifstream` 和 `ofstream`。
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- `ofstream`：输出文件流，并且其能通过插入运算符 `<<` 来实现。
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- `ifstream`：输入文件流，并且其能通过提取运算符 `>>` 来实现。
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该两种类型都是在头文件 `<fstream>` 中所定义。
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从 `ios_base` 继承的类在写入时可被视为数据接收器，在从其读取时可被视为数据源，与数据本身完全分离。这种面向对象的方法使 <ruby>[关注点分离][5]<rt>separation of concerns</rt></ruby> 和 <ruby>[依赖注入][6]<rt>dependency injection</rt></ruby> 等概念易于实现。
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### 一个简单的例子
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本例程是非常简单：实例化了一个 `ofstream` 来写入，和实例化一个 `ifstream` 来读取。
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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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该代码可以在 [GitHub][7] 上查看。当你编译并且执行它时，你应该能获得以下输出：
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![Console screenshot][8]
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(Stephan Avenwedde, [CC BY-SA 4.0][9])
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这是个简易、适合初学者的例子。如果你想去使用该代码在你自己的应用中，最好遵从以下建议：
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* 文件流在程序结束的时候自动关闭。如果你想继续执行，那么应该通过调用 `close()` 方法手动关闭。
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* 这些文件流类继承自 [basic_ios][10]（在多个级别上），并且重载了 `!` 运算符。这使你可以进行简单的检查，是否可以访问该流。在 [cppreference.com][11] 上，你可以找到该检查何时会（或不会）成功的概述，并且可以进一步实现错误处理。
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* 默认情况下，`ifstream` 停在空白处并跳过它。要逐行读取直到到达 [EOF][13] ，请使用 `getline(...)` 方法。
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* 为了读写二进制文件，请将 `std::ios::binary` 标志传递给构造函数：这样可以防止 [EOL][13] 字符附加到每一行。
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### 从系统角度进行写入
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写入文件时，数据将写入系统的内存写入缓冲区中。当系统收到系统调用 [sync][14] 时，此缓冲区的内容将被写入硬盘。这也是你在不告知系统的情况下，不要卸下 U 盘的原因。通常，守护进程会定期调用 _sync_。为了安全起见，也可以手动调用 _sync_：
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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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### 总结
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在 C++ 中读写文件并不那么复杂。更何况，如果你知道如何处理输入输出流，（原则上）那么你也知道如何处理任何类型的输入输出设备。对于各种输入输出设备的库能让你更容易地使用流运算符。这就是为什么知道输入输出流的流程会对你有所助益的原因。
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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：[wyxplus](https://github.com/wyxplus)
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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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