C/C++编程：字符流输入流std :: istream

╰+哭是因爲堅強的太久メ 2022-05-19 12:58 361阅读 0赞

> # std :: istream #
> 
>     typedef basic_istream<char> istream;
> 
> Input stream
> 
> ![20210303110025392.png][]
> 
> 输入流对象可以读取和解释字符序列的输入
> 
> 这是一个[basic\_istream][basic_istream]实例 具有以下模板参数：
> 
> <table> 
>  <tbody> 
>   <tr> 
>    <th>模板参数</th> 
>    <th>定义</th> 
>    <th>评论</th> 
>   </tr> 
>   <tr> 
>    <td style="border-color:#000000;">charT</td> 
>    <td style="border-color:#000000;"><code>char</code></td> 
>    <td style="border-color:#000000;">成员char_type的别名&nbsp;</td> 
>   </tr> 
>   <tr> 
>    <td style="border-color:#000000;">traits</td> 
>    <td style="border-color:#000000;"><code><a href="http://www.cplusplus.com/char_traits" rel="nofollow">char_traits</a>&lt;char&gt;</code></td> 
>    <td style="border-color:#000000;">成员traits_type的别名&nbsp;</td> 
>   </tr> 
>  </tbody> 
> </table>

## std :: istream::istream ##

> <table> 
>  <tbody> 
>   <tr> 
>    <th>initialization (1)</th> 
>    <td> <pre><code>explicit istream (streambuf* sb);
> </code></pre> </td> 
>   </tr> 
>   <tr> 
>    <th>copy (2)</th> 
>    <td> <pre><code>istream&amp; (const istream&amp;) = delete;
> </code></pre> </td> 
>   </tr> 
>   <tr> 
>    <th>move (3)</th> 
>    <td> <pre><code>protected: istream&amp; (istream&amp;&amp; x);</code></pre> </td> 
>   </tr> 
>  </tbody> 
> </table>
> 
> 作用：构造对象

// istream constructor
    #include <iostream>     // std::ios, std::istream, std::cout
    #include <fstream>      // std::filebuf
    
    int main () {
      std::filebuf fb;
      if (fb.open ("test.txt",std::ios::in))
      {
        std::istream is(&fb);
        while (is)
          std::cout << char(is.get());
        fb.close();
      }
      return 0;
    }

## ![20210303110634308.png][] ##

## std :: istream::operator>> ##

> <table> 
>  <tbody> 
>   <tr> 
>    <th>arithmetic types (1)</th> 
>    <td> <pre><code>istream&amp; operator&gt;&gt; (bool&amp; val);
> istream&amp; operator&gt;&gt; (short&amp; val);
> istream&amp; operator&gt;&gt; (unsigned short&amp; val);
> istream&amp; operator&gt;&gt; (int&amp; val);
> istream&amp; operator&gt;&gt; (unsigned int&amp; val);
> istream&amp; operator&gt;&gt; (long&amp; val);
> istream&amp; operator&gt;&gt; (unsigned long&amp; val);
> istream&amp; operator&gt;&gt; (long long&amp; val);
> istream&amp; operator&gt;&gt; (unsigned long long&amp; val);
> istream&amp; operator&gt;&gt; (float&amp; val);
> istream&amp; operator&gt;&gt; (double&amp; val);
> istream&amp; operator&gt;&gt; (long double&amp; val);
> istream&amp; operator&gt;&gt; (void*&amp; val);
> </code></pre> </td> 
>   </tr> 
>   <tr> 
>    <th>stream buffers (2)</th> 
>    <td> <pre><code>istream&amp; operator&gt;&gt; (streambuf* sb );
> </code></pre> </td> 
>   </tr> 
>   <tr> 
>    <th>manipulators (3)</th> 
>    <td> <pre><code>istream&amp; operator&gt;&gt; (istream&amp; (*pf)(istream&amp;));
> istream&amp; operator&gt;&gt; (ios&amp; (*pf)(ios&amp;));
> istream&amp; operator&gt;&gt; (ios_base&amp; (*pf)(ios_base&amp;));</code></pre> </td> 
>   </tr> 
>  </tbody> 
> </table>
> 
> 作用：提取格式化的输入

#include <iostream>     // std::cin, std::cout, std::he
    
    int main () {
        int n;
    
        std::cout << "Enter a number: ";
        std::cin >> n;
        std::cout << "You have entered: " << n << '\n';
    
        std::cout << "Enter a hexadecimal number: ";
        std::cin >> std::hex >> n;
        std::cout << "Its decimal equivalent is: " << n << '\n';
        return 0;
    }

![20210303103402801.png][]

## std :: istream::gcount ##

>     streamsize gcount（）const;
> 
> 作用：获取字符数

#include <iostream>  // std::cin, std::cout
    
    int main () {
        char str[20];
    
        std::cout << "Please, enter a word: ";
        std::cin.getline(str, 20);
        std::cout << std::cin.gcount() << " characters read: " << str << "\n";
        return 0;
    }

![20210303103834521.png][]

## std :: istream::get ##

> <table> 
>  <tbody> 
>   <tr> 
>    <th>single character (1)</th> 
>    <td> <pre><code>int get();
> istream&amp; get (char&amp; c);
> </code></pre> </td> 
>   </tr> 
>   <tr> 
>    <th>c-string (2)</th> 
>    <td> <pre><code>istream&amp; get (char* s, streamsize n);
> istream&amp; get (char* s, streamsize n, char delim);
> </code></pre> </td> 
>   </tr> 
>   <tr> 
>    <th>stream buffer (3)</th> 
>    <td> <pre><code>istream&amp; get (streambuf&amp; sb);
> istream&amp; get (streambuf&amp; sb, char delim);</code></pre> </td> 
>   </tr> 
>  </tbody> 
> </table>
> 
> 作用：从流中提取字符，作为*未格式化的输入*

#include <iostream>     // std::cin, std::cout
    #include <fstream>      // std::ifstream
    
    int main () {
        char str[256];
    
        std::cout << "输入现有文本文件的名称: ";
        std::cin.get (str,256);    // get c-string
    
        std::ifstream  is(str);
    
        char c;
        while (is.get(c)){
            std::cout << c;
        }
    
        is.close();
    
        return 0;
    }

![20210303104253117.png][]

## std :: istream::getline ##

>     istream＆getline（char * s，streamsize n）;
>     istream＆getline（char * s，streamsize n，char delim）;
> 
> 作用：从标准输入流中获取行
> 
> 从流中提取字符作为*未格式化的输入*，并将它们作为c字符串存储到s中，直到提取的字符是定界字符，或者将n个字符写入s（包括终止的空字符）为止

#include <iostream>     // std::cin, std::cout
    #include <fstream>      // std::ifstream
    
    int main () {
        char name[256], title[256];
    
        std::cout << "Please, enter your name: ";
        std::cin.getline(name, 256);
    
        std::cout << "Please, enter your favourite movie: ";
        std::cin.getline(title, 256);
    
        std::cout << name << "'s favourite movie is " << title;
        return 0;
    }

![20210303104535987.png][]

## std :: istream::ignore ##

>     istream& ignore (streamsize n = 1, int delim = EOF);
> 
> 作用：提取并丢弃字符
> 
> 从输入序列中提取字符并丢弃它们，直到提取出n个字符或一个等于delim的比较为止。

// istream::ignore example
    #include <iostream>     // std::cin, std::cout
    
    int main () {
        char first, last;
    
        std::cout << "Please, enter your first name followed by your surname: ";
    
        first = std::cin.get();     // get one character
        std::cin.ignore(256,' ');   // ignore until space
    
        last = std::cin.get();      // get one character
    
        std::cout << "Your initials are " << first << last << '\n';
    
        return 0;
    }

![20210303104858339.png][]

## std :: istream::peek ##

>     int peek（）;
> 
> 作用：偷看下一个字符
> 
> 返回输入序列中的下一个字符，而不提取该字符：保留该字符作为要从流中提取的下一个字符。

// istream::peek example
    #include <iostream>     // std::cin, std::cout
    #include <string>       // std::string
    #include <cctype>       // std::isdigit
    
    int main () {
    
      std::cout << "Please, enter a number or a word: ";
      std::cout.flush();    // ensure output is written
    
      std::cin >> std::ws;  // eat up any leading white spaces
      int c = std::cin.peek();  // peek character
    
      if ( c == EOF ) return 1;
      if ( std::isdigit(c) )
      {
        int n;
        std::cin >> n;
        std::cout << "You entered the number: " << n << '\n';
      }
      else
      {
        std::string str;
        std::cin >> str;
        std::cout << "You entered the word: " << str << '\n';
      }
    
      return 0;
    }

![20210303105310419.png][]

## std :: istream::read、std :: [istream][] :: tellg、std :: [istream][] :: seekg ##

>     istream＆read（char * s，streamsize n）;
> 
> 作用：读取数据块
> 
> 从流中 提取n个字符并将其存储在s指向的数组中。
> 
>     streampos tellg();
> 
> 作用：返回当前字符在输入流中的位置。
> 
>     istream& seekg (streampos pos);
>     istream& seekg (streamoff off, ios_base::seekdir way);
> 
> 作用：设置要从输入流中提取的下一个字符的位置

// read a file into memory
    #include <iostream>     // std::cout
    #include <fstream>      // std::ifstream
    
    int main () {
    
      std::ifstream is ("test.txt", std::ifstream::binary);
      if (is) {
        // get length of file:
        is.seekg (0, is.end);
        int length = is.tellg();
        is.seekg (0, is.beg);
    
        char * buffer = new char [length];
    
        std::cout << "Reading " << length << " characters... ";
        // read data as a block:
        is.read (buffer,length);
    
        if (is)
          std::cout << "all characters read successfully.";
        else
          std::cout << "error: only " << is.gcount() << " could be read";
        is.close();
    
        // ...buffer contains the entire file...
    
        delete[] buffer;
      }
      return 0;
    }

![20210303111114410.png][]

## std :: istream::putback、std::[istream][]::unget ##

>     istream& putback (char c);
>     istream& unget();
> 
> 作用：放回字符
> 
> 尝试将流中当前位置减少一个字符，使从流中提取的最后一个字符再次可用于输入操作

// istream::putback example
    #include <iostream>     // std::cin, std::cout
    #include <string>       // std::string
    
    int main () {
        std::cout << "Please, enter a number or a word: ";
        char c = std::cin.get();
    
        if ( (c >= '0') && (c <= '9') )
        {
            int n;
            std::cin.putback (c);
          // std::cin.unget();
            std::cin >> n;
            std::cout << "You entered a number: " << n << '\n';
        }
        else
        {
            std::string str;
            std::cin.putback (c);
           //std::cin.unget();
            getline (std::cin,str);
            std::cout << "You entered a word: " << str << '\n';
        }
        return 0;
    }

![2021030311153249.png][]

## std :: [istream][] :: sync ##

>     int sync();
> 
> 作用：同步输入缓冲区

[20210303110025392.png]: /images/20220519/4d1dd31221784bf9a0c6168097a79578.png
[basic_istream]: http://www.cplusplus.com/basic_istream
[20210303110634308.png]: /images/20220519/b208a7284a594efc84ed6d3342eeaea7.png
[20210303103402801.png]: /images/20220519/d4101323ca644fb598cb461c49a17adb.png
[20210303103834521.png]: /images/20220519/f348c792a055443c84cc59f40378fe3c.png
[20210303104253117.png]: /images/20220519/3de08fd05d334a3f89ff1865120635f3.png
[20210303104535987.png]: /images/20220519/14ddb77cf22d4d5a860f46ea8d494565.png
[20210303104858339.png]: /images/20220519/21cf346580af4e6ab55aa00c61f29549.png
[20210303105310419.png]: /images/20220519/c99be8d6653f4b6f8a492ce36c07a4fa.png
[istream]: http://www.cplusplus.com/reference/istream/istream/
[20210303111114410.png]: /images/20220519/f089f0c72ebd4e978fe2e480f2da9334.png
[2021030311153249.png]: /images/20220519/33fe57909c71440096f62c62916c4cdc.png