c++ - 为什么 parse_config_file 在流上设置 failbit？

Question

这个最小的程序使用 boost::program_options 来解析 stringstream。奇怪的是，在解析之后，流不再处于“良好”状态，并且设置了 failbit 和 eofbit。

#include <iostream>
#include <sstream>
#include <boost/program_options/options_description.hpp>
#include <boost/program_options/parsers.hpp>
#include <boost/program_options/variables_map.hpp>

void test_stream(std::stringstream& s);

int main()
{
  using namespace std;
  namespace po = boost::program_options;

  stringstream s;
  s << "seed=3" << '\n';
  test_stream(s);

  po::options_description desc("");
  desc.add_options()
    ("seed", po::value<int>());
  po::variables_map vm;
  po::store(po::parse_config_file(s, desc, true), vm);
  po::notify(vm);

  test_stream(s);

  return 0;
}

void test_stream(std::stringstream& s)
{
  using namespace std;

  if (s.good())
    {
      cout << "stream is good" << endl;
    }
  else
    {
      cout << "stream is not good" << endl;
      if (s.rdstate() & ios_base::badbit)
    cout << "badbit is set" << endl;
      if (s.rdstate() & ios_base::failbit)
    cout << "failbit is set" << endl;
      if (s.rdstate() & ios_base::eofbit)
    cout << "eofbit is set" << endl;
    }
}

输出:

stream is good
stream is not good
failbit is set
eofbit is set

虽然 eof 条件在某种程度上是预期的，但由于推测解析器已读取流直到 EOF，为什么还设置了 failbit？

Answer 1

根据 ios::eof 的文档标记在某些情况下可能会发生这种情况:

Operations that attempt to read at the End-of-File fail, and thus both the eofbit and the failbit end up set. This function can be used to check whether the failure is due to reaching the End-of-File or to some other reason.

Boost 的解析器使用 std::copy() 和流上的迭代器来提取选项。正如 Jerry Coffin's answer 中指出的那样对于另一个问题，迭代器从序列中读取项目。读取最后一个后，序列的 eof 位被设置。当迭代器再次递增以获得流结束迭代器时，这是在 copy() 中离开循环的条件，它会尝试再次读取，因此也是流的 设置失败位。