istream extraction operator: how to detect parse failure?

问题

How can I detect whether the istream extraction failed like this?

string s("x");
stringstream ss(s);
int i;
ss >> std::ios::hex >> i;

EDIT -- Though the question title covers this, I forgot to mention in the body: I really want to detect whether the failure is due to bad formatting, i.e. parsing, or due to any other IO-related issue, in order to provide proper feedback (an malformed_exception("x") or whatever).

回答1:

if(! (ss >> std::ios::hex >> i) ) 
{
  std::cerr << "stream extraction failed!" << std::endl;
}

It's just that easy.

ETA: Here's an example of how this test interacts with the end of a stream.

int i;
std::stringstream sstr("1 2 3 4");
while(sstr >> i)
{
    std::cout << i << std::endl;
    if(sstr.eof())
    {
        std::cout << "eof" << std::endl;
    }
}

will print
1
2
3
4
eof

If you were to check sstr.eof() or sstr.good() in the while loop condition, 4 would not be printed.

回答2:

Failure to extract the value will set the stream's "fail" bit, which can be detected by if (ss.fail()), or just if (!ss). Equivalently, you can test the result of the >> operation, since that returns a reference to the stream.

These will also detect other errors, which set the "bad" bit; you can distinguish these with ss.bad().

If you want to continue reading from the stream, you'll need to clear the state flags (ss.clear()).

回答3:

Errors during extraction are signaled by the internal state flags. You can check them by the good() member function. See also here: http://www.cplusplus.com/reference/iostream/stringstream

Or just using the if()-construction as suggested above. This works due to the bool cast operator of stream classes

回答4:

First off: thanks for the useful answers. However, after some investigation (cfr. cppreference) and verification, it seems that the one way to check for parse-failure only is by checking for the ios::failbit flag, as in

const bool parsing_failed = (ss >> ios::hex >> i).rdstate() & ios::failbit ;

While both the suggested istream::operator! and istream::operator bool mingle failbit and badbit (cfr here and there on cplusplusreference).

来源：https://stackoverflow.com/questions/8488937/istream-extraction-operator-how-to-detect-parse-failure

标签

c++

parsing

istream