问以可移植的方式读取C++中的文件并处理可能的错误

EN

  // Try to read first line from file at path, returning no string if file does not exist,
  // throwing exception for any other error
  optional<std::string> read_first_line(filesystem::path path)
  {
    using namespace AFIO_V2_NAMESPACE;
    // The result<T> is from WG21 P0762, it looks quite like an `expected<T, std::error_code>` object
    // See Outcome v2 at https://ned14.github.io/outcome/ and https://lists.boost.org/boost-announce/2017/06/0510.php

    // Open for reading the file at path using a null handle as the base
    result<file_handle> _fh = file({}, path);
    // If fh represents failure ...
    if(!_fh)
    {
      // Fetch the error code
      std::error_code ec = _fh.error();
      // Did we fail due to file not found?
      // It is *very* important to note that ec contains the *original* error code which could
      // be POSIX, or Win32 or NT kernel error code domains. However we can always compare,
      // via 100% C++ 11 STL, any error code to a generic error *condition* for equivalence
      // So this comparison will work as expected irrespective of original error code.
      if(ec == std::errc::no_such_file_or_directory)
      {
        // Return empty optional
        return {};
      }
      std::cerr << "Opening file " << path << " failed with " << ec.message() << std::endl;
    }
    // If errored, result<T>.value() throws an error code failure as if `throw std::system_error(fh.error());`
    // Otherwise unpack the value containing the valid file_handle
    file_handle fh(std::move(_fh.value()));
    // Configure the scatter buffers for the read, ideally aligned to a page boundary for DMA
    alignas(4096) char buffer[4096];
    // There is actually a faster to type shortcut for this, but I thought best to spell it out
    file_handle::buffer_type reqs[] = {{buffer, sizeof(buffer)}};
    // Do a blocking read from offset 0 possibly filling the scatter buffers passed in
    file_handle::io_result<file_handle::buffers_type> _buffers_read = read(fh, {reqs, 0});
    if(!_buffers_read)
    {
      std::error_code ec = _fh.error();
      std::cerr << "Reading the file " << path << " failed with " << ec.message() << std::endl;
    }
    // Same as before, either throw any error or unpack the value returned
    file_handle::buffers_type buffers_read(_buffers_read.value());
    // Note that buffers returned by AFIO read() may be completely different to buffers submitted
    // This lets us skip unnecessary memory copying

    // Make a string view of the first buffer returned
    string_view v(buffers_read[0].data, buffers_read[0].len);
    // Sub view that view with the first line
    string_view line(v.substr(0, v.find_first_of('\n')));
    // Return a string copying the first line from the file, or all 4096 bytes read if no newline found.
    return std::string(line);
  }

using namespace boost::beast;
using namespace boost::system;

file f;
error_code ec;
f.open("/path/to/file", file_mode::read, ec);
if(ec == errc::no_such_file_or_directory) {
    // ...
} else {
    // ...
}

#include <iostream>
#include <fstream>
#include <string>
#include <system_error>

using namespace std;

int
main()
{
    ifstream f("testfile.txt");
    if (!f.good()) {
        error_code e(errno, system_category());
        cerr << e.message();
        //...
    }
    // ...
}