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 12:58 423阅读 0赞

std :: istream

  1. typedef basic_istream<char> istream;

Input stream

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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实例 具有以下模板参数：



















模板参数 定义 评论
charT char 成员char_type的别名 
traits char_traits<char> 成员traits_type的别名 

std :: istream::istream
















initialization (1) 
  1. explicit istream (streambuf* sb);
copy (2) 
  1. istream& (const istream&) = delete;
move (3) 
  1. protected: istream& (istream&& x);

作用：构造对象

  1. // istream constructor
  2. #include <iostream>     // std::ios, std::istream, std::cout
  3. #include <fstream>      // std::filebuf
  5. int main () {
  6.   std::filebuf fb;
  7.   if (fb.open ("test.txt",std::ios::in))
  8.   {
  9.     std::istream is(&fb);
  10.     while (is)
  11.       std::cout << char(is.get());
  12.     fb.close();
  13.   }
  14.   return 0;
  15. }

" class="reference-link">20210303110634308.png

std :: istream::operator>>
















arithmetic types (1) 
  1. istream& operator>> (bool& val);
    istream& operator>> (short& val);
    istream& operator>> (unsigned short& val);
    istream& operator>> (int& val);
    istream& operator>> (unsigned int& val);
    istream& operator>> (long& val);
    istream& operator>> (unsigned long& val);
    istream& operator>> (long long& val);
    istream& operator>> (unsigned long long& val);
    istream& operator>> (float& val);
    istream& operator>> (double& val);
    istream& operator>> (long double& val);
    istream& operator>> (void& val);
stream buffers (2) 
  1. istream& operator>> (streambuf sb );
manipulators (3) 
  1. istream& operator>> (istream& (pf)(istream&));
    istream& operator>> (ios& (    pf)(ios&));
    istream& operator>> (ios_base& (*pf)(ios_base&));

作用：提取格式化的输入

  1. #include <iostream>     // std::cin, std::cout, std::he
  3. int main () {
  4.     int n;
  6.     std::cout << "Enter a number: ";
  7.     std::cin >> n;
  8.     std::cout << "You have entered: " << n << '\n';
  10.     std::cout << "Enter a hexadecimal number: ";
  11.     std::cin >> std::hex >> n;
  12.     std::cout << "Its decimal equivalent is: " << n << '\n';
  13.     return 0;
  14. }

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std :: istream::gcount

  1. streamsize gcount（）const;

作用：获取字符数

  1. #include <iostream>  // std::cin, std::cout
  3. int main () {
  4.     char str[20];
  6.     std::cout << "Please, enter a word: ";
  7.     std::cin.getline(str, 20);
  8.     std::cout << std::cin.gcount() << " characters read: " << str << "\n";
  9.     return 0;
  10. }

20210303103834521.png

std :: istream::get
















single character (1) 
  1. int get();
    istream& get (char& c);
c-string (2) 
  1. istream& get (char s, streamsize n);
    istream& get (char     s, streamsize n, char delim);
stream buffer (3) 
  1. istream& get (streambuf& sb);
    istream& get (streambuf& sb, char delim);

作用：从流中提取字符，作为未格式化的输入

  1. #include <iostream>     // std::cin, std::cout
  2. #include <fstream>      // std::ifstream
  4. int main () {
  5.     char str[256];
  7.     std::cout << "输入现有文本文件的名称: ";
  8.     std::cin.get (str,256);    // get c-string
  10.     std::ifstream  is(str);
  12.     char c;
  13.     while (is.get(c)){
  14.         std::cout << c;
  15.     }
  17.     is.close();
  19.     return 0;
  20. }

20210303104253117.png

std :: istream::getline

  1. istream＆getline（char * s，streamsize n）;
  2. 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

  1. #include <iostream>     // std::cin, std::cout
  2. #include <fstream>      // std::ifstream
  4. int main () {
  5.     char name[256], title[256];
  7.     std::cout << "Please, enter your name: ";
  8.     std::cin.getline(name, 256);
  10.     std::cout << "Please, enter your favourite movie: ";
  11.     std::cin.getline(title, 256);
  13.     std::cout << name << "'s favourite movie is " << title;
  14.     return 0;
  15. }

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std :: istream::ignore

  1. 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。

  1. // istream::ignore example
  2. #include <iostream>     // std::cin, std::cout
  4. int main () {
  5.     char first, last;
  7.     std::cout << "Please, enter your first name followed by your surname: ";
  9.     first = std::cin.get();     // get one character
  10.     std::cin.ignore(256,' ');   // ignore until space
  12.     last = std::cin.get();      // get one character
  14.     std::cout << "Your initials are " << first << last << '\n';
  16.     return 0;
  17. }

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std :: istream::peek

  1. 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。

  1. // istream::peek example
  2. #include <iostream>     // std::cin, std::cout
  3. #include <string>       // std::string
  4. #include <cctype>       // std::isdigit
  6. int main () {
  8.   std::cout << "Please, enter a number or a word: ";
  9.   std::cout.flush();    // ensure output is written
  11.   std::cin >> std::ws;  // eat up any leading white spaces
  12.   int c = std::cin.peek();  // peek character
  14.   if ( c == EOF ) return 1;
  15.   if ( std::isdigit(c) )
  16.   {
  17.     int n;
  18.     std::cin >> n;
  19.     std::cout << "You entered the number: " << n << '\n';
  20.   }
  21.   else
  22.   {
  23.     std::string str;
  24.     std::cin >> str;
  25.     std::cout << "You entered the word: " << str << '\n';
  26.   }
  28.   return 0;
  29. }

20210303105310419.png

std :: istream::read、std :: istream :: tellg、std :: istream :: seekg

  1. 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。

  1. streampos tellg();

作用：返回当前字符在输入流中的位置。

  1. istream& seekg (streampos pos);
  2. 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

  1. // read a file into memory
  2. #include <iostream>     // std::cout
  3. #include <fstream>      // std::ifstream
  5. int main () {
  7.   std::ifstream is ("test.txt", std::ifstream::binary);
  8.   if (is) {
  9.     // get length of file:
  10.     is.seekg (0, is.end);
  11.     int length = is.tellg();
  12.     is.seekg (0, is.beg);
  14.     char * buffer = new char [length];
  16.     std::cout << "Reading " << length << " characters... ";
  17.     // read data as a block:
  18.     is.read (buffer,length);
  20.     if (is)
  21.       std::cout << "all characters read successfully.";
  22.     else
  23.       std::cout << "error: only " << is.gcount() << " could be read";
  24.     is.close();
  26.     // ...buffer contains the entire file...
  28.     delete[] buffer;
  29.   }
  30.   return 0;
  31. }

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std :: istream::putback、std::istream::unget

  1. istream& putback (char c);
  2. 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

  1. // istream::putback example
  2. #include <iostream>     // std::cin, std::cout
  3. #include <string>       // std::string
  5. int main () {
  6.     std::cout << "Please, enter a number or a word: ";
  7.     char c = std::cin.get();
  9.     if ( (c >= '0') && (c <= '9') )
  10.     {
  11.         int n;
  12.         std::cin.putback (c);
  13.       // std::cin.unget();
  14.         std::cin >> n;
  15.         std::cout << "You entered a number: " << n << '\n';
  16.     }
  17.     else
  18.     {
  19.         std::string str;
  20.         std::cin.putback (c);
  21.        //std::cin.unget();
  22.         getline (std::cin,str);
  23.         std::cout << "You entered a word: " << str << '\n';
  24.     }
  25.     return 0;
  26. }

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std :: istream :: sync

  1. int sync();

作用：同步输入缓冲区

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