How to convert errno to exception using <system_error> - c++

Question

I read a thoughtful series of blog posts about the new <system_error> header in C++11. It says that the header defines an error_code class that represents a specific error value returned by an operation (such as a system call). It says that the header defines a system_error class, which is an exception class (inherits from runtime_exception) and is used to wrap error_codess.
What I want to know is how to actually convert a system error from errno into a system_error so I can throw it. For example, the POSIX open function reports errors by returning -1 and setting errno, so if I want to throw an exception how should I complete the code below?
void x()
{
fd = open("foo", O_RDWR);
if (fd == -1)
{
throw /* need some code here to make a std::system_error from errno */;
}
}
I randomly tried:
errno = ENOENT;
throw std::system_error();
but the resulting exception returns no information when what() is called.
I know I could do throw errno; but I want to do it the right way, using the new <system_error> header.
There is a constructor for system_error that takes a single error_code as its argument, so if I can just convert errno to error_code then the rest should be obvious.
This seems like a really basic thing, so I don't know why I can't find a good tutorial on it.
I am using gcc 4.4.5 on an ARM processor, if that matters.

Answer 1

You are on the right track, just pass the error code and a std::generic_category object to the std::system_error constructor and it should work.
Example:
#include <assert.h>
#include <errno.h>
#include <iostream>
#include <system_error>
int main()
{
try
{
throw std::system_error(EFAULT, std::generic_category());
}
catch (std::system_error& error)
{
std::cout << "Error: " << error.code() << " - " << error.what() << '\n';
assert(error.code() == std::errc::bad_address);
}
}
Output from the above program on my system is
Error: generic:14 - Bad address

Answer 2

To add to the excellent accepted answer, you can enrich the error message with some contextual information in the 3rd argument, e.g. the failing file name:
std::string file_name = "bad_file_name.txt";
fd = open(file_name, O_RDWR);
if (fd < 0) {
throw std::system_error(errno, std::generic_category(), file_name);
}
Then when caught, e.what() will return, for example:
bad_file_name.txt: file not found