逐字节读取二进制 istream
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 方法来一次读取二进制文件的整个块，没有任何问题。但我当前的任务适合逐字节进行，并依靠 io 系统中的缓冲来提高效率。问题是我似乎比应有的时间早了几个字节到达文件末尾。所以我编写了以下测试程序：
#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;
}
这会转储 test.dat 的内容，每行一个字节，显示前后的文件位置。
果然，如果我的文件碰巧有两字节序列 0x0D-0x0A（对应于回车符和换行符），那么这些字节将被跳过。
- 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？
- 提取运算符是否始终使用文本模式？
- 从二进制 istream 中逐字节读取的正确方法是什么？
Windows 上的 MSVC++ 2008。
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:
#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.
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.
- 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?
MSVC++ 2008 on Windows.
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>>提取器用于格式化输入；他们跳过空白（通过
默认）。对于单个字符的无格式输入，您可以使用
istream::get()（返回int，如果读取失败则为 EOF，或者[0,UCHAR_MAX]) 或 istream::get(char&) 范围内的值（将
在参数中读取的字符，返回转换为的内容
bool，如果读取成功则为 true，如果失败则为 false。The >> extractors are for formatted input; they skip white space (by
default). For single character unformatted input, you can use
istream::get()(returns anint, either EOF if the read fails, ora value in the range [0,UCHAR_MAX]) or
istream::get(char&)(puts thecharacter read in the argument, returns something which converts to
bool, true if the read succeeds, and false if it fails.有一个 read() 成员函数，您可以在其中指定字节数。
there is a read() member function in which you can specify the number of bytes.
为什么使用格式化提取，而不是
.read()？Why are you using formatted extraction, rather than
.read()?会给你一个字节。它是无格式输入函数。
运算符>>是格式化输入函数，可能意味着跳过空白字符。
will give you a single byte. It is unformatted input function.
operator>> is formatted input function that may imply skipping whitespace characters.
正如其他人提到的，您应该使用
istream::read()。但是，如果必须使用格式化提取，请考虑std::noskipws。As others mentioned, you should use
istream::read(). But, if you must use formatted extraction, considerstd::noskipws.