C++ 逐字节读取二进制 istream
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Reading binary istream byte by byte
提问by Adrian McCarthy
I was attempting to read a binary file byte by byte using an ifstream. I've used istream methods like get() before to read entire chunks of a binary file at once without a problem. But my current task lends itself to going byte by byte and relying on the buffering in the io-system to make it efficient. The problem is that I seemed to reach the end of the file several bytes sooner than I should. So I wrote the following test program:
我试图使用 ifstream 逐字节读取二进制文件。我之前使用过像 get() 这样的 istream 方法一次读取整个二进制文件块没有问题。但是我当前的任务适合逐字节处理并依靠 io 系统中的缓冲来使其高效。问题是我似乎比我应该提前几个字节到达文件末尾。于是我写了下面的测试程序：
#include <iostream>
#include <fstream>
int main() {
typedef unsigned char uint8;
std::ifstream source("test.dat", std::ios_base::binary);
while (source) {
std::ios::pos_type before = source.tellg();
uint8 x;
source >> x;
std::ios::pos_type after = source.tellg();
std::cout << before << ' ' << static_cast<int>(x) << ' '
<< after << std::endl;
}
return 0;
}
This dumps the contents of test.dat, one byte per line, showing the file position before and after.
这将转储 test.dat 的内容，每行一个字节，显示之前和之后的文件位置。
Sure enough, if my file happens to have the two-byte sequence 0x0D-0x0A (which corresponds to carriage return and line feed), those bytes are skipped.
果然，如果我的文件恰好有两个字节的序列 0x0D-0x0A（对应于回车和换行），这些字节将被跳过。
- I've opened the stream in binary mode. Shouldn't that prevent it from interpreting line separators?
- Do extraction operators always use text mode?
- What's the right way to read byte by byte from a binary istream?
- 我已经以二进制模式打开了流。这不应该阻止它解释行分隔符吗？
- 提取操作员总是使用文本模式吗？
- 从二进制 istream 逐字节读取的正确方法是什么？
MSVC++ 2008 on Windows.
Windows 上的 MSVC++ 2008。
采纳答案by James Kanze
The >> extractors are for formatted input; they skip white space (by
default). For single character unformatted input, you can use
istream::get()(returns an int, either EOF if the read fails, or
a value in the range [0,UCHAR_MAX]) or istream::get(char&)(puts the
character read in the argument, returns something which converts to
bool, true if the read succeeds, and false if it fails.
>> 提取器用于格式化输入；他们跳过空白（默认情况下）。对于单字符无格式输入，您可以使用
istream::get()(返回int，如果读取失败，则返回EOF，或范围 [0,UCHAR_MAX] 中的值) 或istream::get(char&)(将读取的字符放入参数中，返回转换为的内容bool，如果为
true读取成功，如果失败则返回 false。
回答by stefaanv
回答by Lightness Races in Orbit
Why are you using formatted extraction, rather than .read()?
为什么要使用格式化提取，而不是.read()?
回答by Serge Dundich
source.get()
will give you a single byte. It is unformatted input function. operator>> is formatted input function that may imply skipping whitespace characters.
会给你一个字节。它是无格式输入函数。operator>> 是格式化的输入函数，可能意味着跳过空白字符。
回答by Rob?
As others mentioned, you should use istream::read(). But, if you must use formatted extraction, consider std::noskipws.
正如其他人提到的，您应该使用istream::read(). 但是，如果您必须使用格式化提取，请考虑std::noskipws.
