很多时候较大数据量的文件 IO 总是成为瓶颈，为了提高效率，有时想要先将文件大块大块的读入再行处理。下面分析两种惯常的处理手法。
1. 将文件一次性读入 string 中。
貌似 std::getline 、 istream::getline 或是 operator<< operator>> 等都不提供一次读到文件结尾的机制，只有 istreambuf_iterator 可以做到：
string 的构造函数前一个参数要多加一层 () 以免编译器误认为是函数声明 = = ...
这样读入 string 会随着内容动态增长，空间不足时会触发额外的 realloc 及 copy 操作，为提高效率有必要预分配足够的空间：
2. 将文件一次性读入 stringstream 中。
filebuf 和 stringbuf 无法直接通过 rdbuf() 重定向，因此从 filebuf 到 stringbuf 需要一次 copy 操作。最简单的方法是直接复制整个 streambuf ：
与 string 的情况相同，这里同样也有一个空间 realloc 及 copy 的问题。但 streambuf 的缓冲区不是那么方便操作的，解决方法是我们给他手动指定一个空间：
最后再顺便 BS 一下 VC 的 STL = =...
虽然 VC 的编译器效率没的说，但被 STL 拖后腿的话不就白搭了嘛。在文件 IO 方面 (fstream) 比起 MinGW (GCC 4.4.0) 带的要慢好几倍。GCC 的 fstream 格式化读写效率与 C 的比已经不分伯仲，以后应该还会有进一步的提升空间 (编译时格式控制 vs 执行时)
另外上面最后一段程序在 VS2008 (VC9.0) 下应该无法得到预想的结果，跟踪进去看了一下，VC 标准库里的 pubsetbuf 函数体居然是空的！内容如下(中间还有一层函数调用)：
看来是等着我们来继承了啊 = = 。而在 MinGW (GCC 4.4.0) 中可以得到预期的结果。
2022年8月16日 21:29
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2022年12月29日 19:54
In C++, the fstream file IO library provides various functions for reading from and writing to files. One of the most commonly used is the `getline` function, which reads a line of text from a file and stores it in a string Lab grown diamonds object. Other commonly used functions include `read`, `write`, and `seekg`, which allow you to read or write a certain number of bytes from a file, or seek to a specific position in a file, respectively.
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