C++ std::streambuf 小结

介绍

我们看到的 std:cin, std:cout 都是对 streambuf 的封装。你可以通过 std::cout.rdbuf () 来获取，也可以通过 std::cout.rdbuf (&newbuf) 来置换。

例子

重定向标准输出

让我们看看如何把 std::cout 重定向到一个文件。

ofstream：输出文件流，用于创建文件并向文件写入信息。

tests/streambuf.cpp

#include <ctime>
#include <fstream>
#include <iostream>

int
main ()
{
  std::ofstream ofs;
  ofs.open ("log.txt", std::ofstream::out | std::ofstream::app);
  std::cout.rdbuf(ofs.rdbuf());
  char const *message = "Hello, world!";
  std::cout << message;
  ofs.close ();

  return 0;
}

std::ofstream::app 表示追加模式。具体参考 ofstream::open - C++ Reference (cplusplus.com)

常用的函数

Input functions (get):

• in_avail

  Get number of characters available to read (public member function )

• snextc

  Advance to next position and get character (public member function )

• sbumpc

  Get current character and advance to next position (public member function )

• sgetc

  Get current character (public member function )

• sgetn

  Get sequence of characters (public member function )

• sputbackc

  Put character back (public member function )

• sungetc

  Decrease current position (public member function )

Output functions (put):

• sputc

  Store character at current put position and increase put pointer (public member function )

• sputn

  Put sequence of characters (public member function )

需要注意的是 uflow () ，如果它返回 EOF，则说明读到文件尾部。

printf

上面的例子虽然可以观察到 cout 输出到文件，但是如果使用 printf 则不会看到效果。要想重定向 printf，思路：

1. 使用 freopen
2. 使用自定义函数，覆盖 printf 函数
3. 使用 fprintf

参考

C++ 文件和流 | 菜鸟教程 (runoob.com)