如何在C ++中使用std :: getline（）？

叁歲伎倆 2023-02-28 08:38 3阅读 0赞

In this article, we’ll take a look at using the function std::getline() in C++. This is a very handy function if you want to read characters from an input stream.

在本文中，我们将介绍在C ++中使用函数std :: getline（）的情况。 如果您想从输入流中读取字符，这是一个非常方便的功能。

Let’s find out how we can use this properly, using some illustrative examples.

让我们使用一些说明性示例来找出如何正确使用它。

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## C ++中std :: getline（）的基本语法 **(****Basic Syntax of std::getline() in C++****)** ##

This function reads characters from an input stream and puts them onto a **string**.

此函数从输入流中读取字符，并将其放入**字符串中** 。

We need to import the header file `<string>`, since `getline()` is a part of this file.

我们需要导入头文件`<string>` ，因为`getline()`是此文件的一部分。

While this takes template arguments, we’ll focus on string inputs (characters) , since the output is written to a string.

尽管这需要模板参数，但我们将重点放在字符串输入（字符）上，因为输出已写入字符串。

istream& getline(istream& input_stream, string& output, char delim);

What this says is that `getline()` takes an input stream, and writes it to `output`. Delimiters can be optionally specified using `delim`.

这说明`getline()`接受输入流，并将其写入`output` 。 可以使用`delim`指定分隔符。

This also returns a reference to the same input stream, but for most cases, we don’t need this handle.

这也返回对相同输入流的引用，但是在大多数情况下，我们不需要此句柄。

## 使用std :: getline（）从输入流中读取 **(****Using std::getline() to read from input streams****)** ##

Now that we know the basic syntax, let’s get input from `std::cin` (standard input stream) to a string.

现在我们已经了解了基本语法，现在让我们从`std::cin` （标准输入流）向字符串获取输入。

#include <iostream>
    #include <string>
    
    int main() {
        // Define a name (String)
        std::string name;
    
        std::cout << "Enter the name: ";
    
        // Get the input from std::cin and store into name
        std::getline(std::cin, name);
    
        std::cout << "Hello " << name << "!\n";
    
        return 0;
    }

**Output**

**输出量**

Enter the name: JournalDev
    Hello JournalDev!

Indeed, we were able to get the input from `std::cin` without any problems!

确实，我们能够毫无问题地从`std::cin`获得输入！

Let’s now take another example, where we have a file `input.txt` containing the following content:

现在再来看一个示例，其中有一个包含以下内容的文件`input.txt` ：

$ cat input.txt
    Hello from JournalDev
    Second Line of file
    Last line

Let’s now read the file line by line and store them into a [vector ][vector]of strings!

现在让我们逐行读取文件并将其存储到字符串[向量][vector]中！

The core logic will be to keep reading using `std::getline(file)` until the input stream reaches **EOF**.

核心逻辑是继续使用`std::getline(file)`读取，直到输入流到达**EOF**为止。

We can easily write this using this format:

我们可以使用以下格式轻松编写此代码：

std::ifstream infile("input.txt");
    
    // Temporary buffer
    std::string temp;
    
    // Get the input from the input file until EOF
    while (std::getline(infile, temp)) {
      // Add to the list of output strings
      outputs.push_back(temp);
    }

The complete code is shown below:

完整的代码如下所示：

#include <iostream>
    #include <string>
    #include <vector> // For std::vector
    #include <fstream> // For std::ifstream and std::ofstream
    
    int main() {
        // Store the contents into a vector of strings
        std::vector<std::string> outputs;
    
        std::cout << "Reading from input.txt....\n";
    
        // Create the file object (input)
        std::ifstream infile("input.txt");
    
        // Temporary buffer
        std::string temp;
    
        // Get the input from the input file until EOF
        while (std::getline(infile, temp)) {
            // Add to the list of output strings
            outputs.push_back(temp);
        }
    
        // Use a range-based for loop to iterate through the output vector
        for (const auto& i : outputs)
            std::cout << i << std::endl;
    
        return 0;
    }

**Output**

**输出量**

Reading from input.txt....
    Hello from JournalDev
    Second Line of file
    Last line

## 在C ++中使用std :: getline（）使用定界符分割输入 **(****Using std::getline() in C++ to split the input using delimiters****)** ##

We can also use the `delim` argument to make the **getline** function split the input in terms of a delimiter character.

我们还可以使用`delim`参数使**getline**函数根据分隔符对输入进行分割。

By default, the delimiter is `\n` (newline). We can change this to make `getline()` split the input based on other characters too!

默认情况下，分隔符为`\n` （换行符）。 我们可以更改它，使`getline()`根据其他字符拆分输入！

Let’s set the delim character to a space ‘ ‘ character to the above example and see what happens!

让我们在上面的示例中将delim字符设置为空格''字符，看看会发生什么！

#include <iostream>
    #include <string>
    #include <vector> // For std::vector
    #include <fstream> // For std::ifstream and std::ofstream
    
    int main() {
        // Store the contents into a vector of strings
        std::vector<std::string> outputs;
    
        std::cout << "Reading from input.txt....\n";
    
        // Create the file object (input)
        std::ifstream infile("input.txt");
    
        // Temporary buffer
        std::string temp;
    
        // Get the input from the input file until EOF
        while (std::getline(infile, temp, ' ')) {
            // Add to the list of output strings
            outputs.push_back(temp);
        }
    
        // Use a range-based for loop to iterate through the output vector
        for (const auto& i : outputs)
            std::cout << i << std::endl;
    
        return 0;
    }

**Output**

**输出量**

Reading from input.txt....
    Hello
    from
    JournalDev
    Second
    Line
    of
    file
    Last
    line

Indeed, we have our space separated string now!

确实，我们现在有了用空格分隔的字符串！

--------------------

## 使用std :: getline（）的潜在问题 **(****Potential Issues with using std::getline()****)** ##

While `std::getline()` is a very useful function, there could be some problems that you may face when using it along with some input streams such as `std::cin`.

尽管`std::getline()`是一个非常有用的函数，但在将其与诸如`std::cin`类的一些输入流一起使用时，可能会遇到一些问题。

*  `std::getline()` does **not** ignore any leading white-space / newline characters.
    
    `std::getline()` **不**忽略任何前导空格/换行符。

Because of this, if you call `std::cin >> var;` just before `getline()`, there will be a newline still remaining in the input stream, after reading the input variable.

因此，如果您调用`std::cin >> var;` 在读取输入变量之后，就在`getline()`之前，在输入流中仍然会有换行符。

So, if you call `getline()` immediately after `cin`, you will get a newline instead, since it is the first character in the input stream!

因此，如果您在`cin`之后立即调用`getline()` ，则会得到一个换行符，因为它是输入流中的第一个字符！

To avoid this, simply add a dummy `std::getline()` to consume this new-line character!

为避免这种情况，只需添加一个虚拟`std::getline()`即可使用此换行符！

The below program shows an issue with using `cin` just before `getline()`.

以下程序显示了在`getline()`之前使用`cin`的问题。

#include <iostream>
    #include <string>
    
    int main() {
        // Define a name (String)
        std::string name;
    
        int id;
    
        std::cout << "Enter the id: ";
    
        std::cin >> id;
    
        std::cout << "Enter the Name: ";
    
        // Notice std::cin was the last input call!
        std::getline(std::cin, name);
    
        std::cout << "Id: " << id << std::endl;
        std::cout << "Name: " << name << "\n";
    
        return 0;
    }

**Output**

**输出量**

Enter the id: 10
    Enter the Name: Id: 10
    Name:

Notice that I wasn’t able to enter the name at all! Since a trailing newline was there in the input stream, it simply took that, and since it is a delimiter, it stopped reading!

请注意，我根本无法输入名称！ 由于输入流中存在尾随换行符，因此它就简单地接受了它，并且由于它是一个定界符，因此它停止了读取！

Now let’s add a dummy `std::getline()` call just before our actual `std::getline()`.

现在让我们在实际的`std::getline()`之前添加一个虚拟的`std::getline()`调用。

#include <iostream>
    #include <string>
    
    int main() {
        // Define a name (String)
        std::string name;
    
        int id;
    
        std::cout << "Enter the id: ";
    
        std::cin >> id;
    
        std::cout << "Enter the Name: ";
    
        // Add a dummy getline() call
        std::getline(std::cin, name);
        // Notice std::cin was the last input call!
        std::getline(std::cin, name);
    
        std::cout << "Id: " << id << std::endl;
        std::cout << "Name: " << name << "\n";
    
        return 0;
    }

**Output**

**输出量**

Enter the id: 10
    Enter the Name: JournalDev
    Id: 10
    Name: JournalDev

We’ve finally fixed our bug! This hopefully makes you think a bit more before blindly using `std::getline()`.

我们终于修复了我们的错误！ 希望这会使您在盲目使用`std::getline()`之前多加考虑。

Unfortunately, there are no elegant methods to get input in C++, so we must make do with what we have!

不幸的是，没有优雅的方法来获取C ++中的输入，因此我们必须利用已有的东西！

--------------------

## 结论 **(****Conclusion****)** ##

In this article, we learned about using std::getline() in C++. We also look at some examples which illustrate the power, and pitfalls of this function.

在本文中，我们学习了在C ++中使用std :: getline（）的知识。 我们还将查看一些示例，这些示例说明了此功能的功能和缺陷。

## 参考资料 **(****References****)** ##

*  [cppreference.com page][] std::getline()
    
    [cppreference.com页面][cppreference.com page] std :: getline（）
 *  [StackOverflow Question][] on using std::getline()
    
    关于使用std :: getline（）的[StackOverflow问题][StackOverflow Question]

--------------------

> 翻译自: [https://www.journaldev.com/39743/getline-in-c-plus-plus][https_www.journaldev.com_39743_getline-in-c-plus-plus]

[vector]: https://www.journaldev.com/35402/vectors-in-c-plus-plus
[cppreference.com page]: https://en.cppreference.com/w/cpp/string/basic_string/getline
[StackOverflow Question]: https://stackoverflow.com/questions/6649852/getline-not-working-properly-what-could-be-the-reasons
[https_www.journaldev.com_39743_getline-in-c-plus-plus]: https://www.journaldev.com/39743/getline-in-c-plus-plus