C++ iostream >> operator behaves differently than get() unsigned char

问题

I was working on a piece of code to do some compression, and I wrote a bitstream class.

My bitstream class kept track of the current bit we are reading and the current byte (unsigned char).

I noticed that reading the next unsigned character from the file was done differently if I used the >> operator vs get() method in the istream class.

I was just curious why I was getting different results?

ex:

this->m_inputFileStream.open(inputFile, std::ifstream::binary);   
unsigned char currentByte;
this->m_inputFileStream >> currentByte;

vs.

this->m_inputFileStream.open(inputFile, std::ifstream::binary);
unsigned char currentByte;
this->m_inputFileStream.get((char&)currentByte);

Additional Info:

To be specific the byte I was reading was 0x0A however when using >> it would read it as 0x6F

I'm not sure how they're even related ? (they're not the 2s complement of each other?)

The >> operator is also defined to work for unsigned char as well however (see c++ istream class reference

回答1:

If you aren't parsing text, don't use operator>> or operator<<. You'll get weird bugs that are hard to track down. They are also resilient to unit tests, unless you know what to look for. Reading a uint8 for instance will fail on 9 for instance.

edit:

#include <iostream>
#include <sstream>
#include <cstdint>

void test(char r) {
        std::cout << "testing " << r << std::endl;
        char t = '!';
        std::ostringstream os(std::ios::binary);
        os << r;
        if (!os.good()) std::cout << "os not good" << std::endl;
        std::istringstream is(os.str(), std::ios::binary);
        is >> t;
        if (!is.good()) std::cout << "is not good" << std::endl;
        std::cout << std::hex << (uint16_t)r 
             << " vs " << std::hex << (uint16_t)t << std::endl;
}

int main(int argc, char ** argv) {
        test('z');
        test('\n');
        return 0;
}

produces:

testing z
7a vs 7a
testing 

is not good
a vs 21

I suppose that would never have been evident a priori.

回答2:

operator>> is for formatted input. It'll read "23" as an integer if you stream it into an int, and it'll eat whitespace between tokens. get() on the other hand is for unformatted, byte-wise input.

回答3:

C++'s formatted input (operator >>) treats char and unsigned char as a character, rather than an integer. This is a little annoying, but understandable.

You have to use get, which returns the next byte, instead.

However, if you open a file with the binary flag, you should not be using formatted I/O. You should be using read, write and related functions. Formatted I/O won't behave correctly, as it's intended to operate on text formats, not binary formats.

来源：https://stackoverflow.com/questions/6792462/c-iostream-operator-behaves-differently-than-get-unsigned-char

标签

c++

iostream

unsigned-char