C++ Primer Plus 学习笔记第十七章 cin

太过爱你忘了你带给我的痛 2021-12-24 01:27 231阅读 0赞

cin支持的类型

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也支持 signed char\*, char\*, unsigned char\*

cin连续输入的话会跳过空格， 换行符， 制表符

会再不符合变量类型的输入流前面停止。并且将不符合的字符留在输入流中

#include <iostream>
    
    int main()
    {
      using namespace std;
      cout << "Enter numbers: ";
    
      int sum = 0;
      int input;
      while (cin >> input)
      {
        sum += input;
      }
    
      cout << "Last value entered = " << input <<endl;
      cout << "Sum = " << sum << endl;
      return 0;
    }

![20190626151434194.png][]

input最后返回的是0  嗯。。。为啥不是-123 难道是将false转换成int型么

流状态

iostate流状态数据成员的数据类型

3个ios\_base元素组成： eofbit, badbit, failbit.

都是1位

文件末尾 设置eofbit

未能读取到预期的字符 设置failbit

I/O失败 设置failbit

其他无法判断的失败 设置badbit

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设置状态

clear() 默认使用默认参数0 将3个状态位全部设置为0 或者传入特定的状态位排除掉（就是说这个参数不设置为0 其他都设为0）

clear(eofbit)

setstate()，必须制定参数 然后设置该参数位为0

![20190626152758815.png][]

exceptions（）

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#include <iostream>
    #include <exception>
    
    int main()
    {
      using namespace std;
      cin.exceptions(ios_base::failbit);
      cout << "Enter numbers: ";
      int sum = 0;
      int input;
      try {
        while (cin >> input)
          sum += input;
      }
      catch(ios_base::failure & bf)
      {
        cout << bf.what() << endl;
        cout << "O! the horror!\n";
      }
      cout << "Last value entered = " << input << endl;
      cout << "Sum = " << sum << endl;
      return 0;
    }

![201906261546162.png][]

异常抛的和书中不一样 嗯。。我不想去管它

其他istream类方法

get()读取下一个字符 转成整形 到文件尾 返回EOF

get(char & c) 转成字符型 跳过空格， 回车键停止

返回istream对象的引用

![20190626155953776.png][]

用哪个？

![20190626160055454.png][]

getline读取到换行符之后会抽出来并丢弃它

cin.ignore() 用来丢弃指定字符前的指定的字符或指定数量的字符

cin.ignore(255, '\\n') 丢弃255个字符或达到了第一个换行符前面的所有字符

程序示例

#include <iostream>
    const int Limit = 255;
    int main()
    {
      using std::cout;
      using std::cin;
      using std::endl;
    
      char input[Limit];
    
      cout << "Enter a string for getline() processing:\n";
      cin.getline(input, Limit, '#');
      cout << "Here is your input:\n";
      cout << input << "\nDone with phase 1\n";
    
      char ch;
      cin.get(ch);
      cout << "The next input character is " << ch << endl;
    
      if (ch != '\n')
        cin.ignore(Limit, '\n');
    
      cout << "Enter a string for get() processing:\n";
      cin.get(input, Limit, '#');
      cout << "Here is your input:\n";
      cout << input << "\nDone with phase 2\n";
    
      cin.get(ch);
      cout << "The next input character is " << ch << endl;
    
      return 0;
    }

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空行不会阻止getline()设置failbit

![20190626161523517.png][]

读满字符 设置failbit

![20190626161706187.png][]

其他istream方法

read()读取指定数量字符存到指定变量中 。不会转为字符串。也就是说结尾不会加上'\\0'

peek()读取下一个字符 但不抽取出来。字符依然在输入流中 用来做判断

gcount()返回最后一个不是通过>>读取的字符数 使用strlen()更快

putback() 将1个字符插到输入字符串中 该字符是吓一跳输入语句读取的第一个字符

#include <iostream>
    
    int main()
    {
      using std::cout;
      using std::cin;
      using std::endl;
    
      char ch;
    
      while(cin.get(ch))
      {
        if (ch != '#')
          cout << ch;
        else
        {
    // 将‘#’写到cin输入流开头
          cin.putback(ch);
          break;
        }
      }
    
      if (!cin.eof())
      {
        cin.get(ch);
        cout << endl << ch << " is next input character.\n";
      }
      else
      {
        cout << "End of file reached.\n";
        std::exit(0);
      }
    
      while(cin.peek() != '#')
      {
        cin.get(ch);
        cout << ch;
      }
      if (!cin.eof())
      {
        cin.get(ch);
        cout << endl << ch << " is next input character.\n";
      }
      else
        cout << "Enter of file reached.\n";
      
      return 0;
    }

![20190626163414867.png][]

peek()例子

#include <iostream>
    const int SLEN = 10;
    inline void eatline() {while (std::cin.get() != '\n') continue;}
    int main()
    {
      using std::cout;
      using std::cin;
      using std::endl;
    
      char name[SLEN];
      char title[SLEN];
      cout << "Enter your name: ";
      cin.get(name, SLEN);
    // 检查字符流中当前的字符
      if (cin.peek() != '\n')
        cout << "Sorry, we only have enough room for "
             << name << endl;
      eatline();
      cout << "Dear " << name << ", enter your title: \n";
      cin.get(title, SLEN);
      if (cin.peek() != '\n')
        cout << "We were forced to truncate your title.\n";
      eatline();
      cout <<" Name: " << name << "\nTitle: " << title << endl;
    
      return 0;
    }

![20190626164410937.png][]

cin内容完结 这节也好多内容 都是cin的

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[20190626151434194.png]: /images/20211223/bb28fda61bb740eea1c6c69b0728714b.png
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[201906261546162.png]: /images/20211223/74745c8e9af2446bb161bcea62146d94.png
[20190626155953776.png]: /images/20211223/c9eb52181eb741939c9419ad7b3971cc.png
[20190626160055454.png]: /images/20211223/665ba0d303ee4180ac60ab5498048205.png
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[20190626161706187.png]: /images/20211223/862b958967c146ed9e9f4ae3f078f11d.png
[20190626163414867.png]: /images/20211223/5816b0e860cf447b8d88be9320638d40.png
[20190626164410937.png]: /images/20211223/f6c8857e3ef448729810341321e3926d.png