Need for noskipws on stream with T=unsigned char i

问题:

I came across Reading the binary file into the vector of unsigned chars and tested the code discussed in the question.

The code of interest is below:

typedef unsigned char BYTE;

std::ifstream file(filename, std::ios::binary);
file.unsetf(std::ios::skipws);

std::vector<BYTE> vec;
vec.insert(vec.begin(),
           std::istream_iterator<BYTE>(file),
           std::istream_iterator<BYTE>());

According to Benjamin Lindley's answer at Why std::istream_iterator ignores newline characters?, istream::operator>>(char) skips white spaces. But the type above is unsigned char, and the file was opened with std::binary.

Why did the code require an explicit call to file.unsetf(std::ios::skipws) (or alternately, file >> std::noskipws)?

回答1:

The basic algorithm for >> of a string is:

1) skip whitespace
2) read and extract until next whitespace

If you use noskipws, then the first step is skipped.

After the first read, you are positionned on a whitespace, so the next (and all following) reads will stop immediatly, extracting nothing.