C++ alternative to perror()

Question

I know we can use

perror()

in C to print errors. I was just wondering if there is a C++ alternative to this, or whether I have to include this (and therefore stdio.h) in my program. I am trying to avoid as many C functions as possible.

Solution 1

You could do something like:

std::cerr << strerror(errno) << std::endl;

That still ends up calling strerror, so you're really just substituting one C function for another. OTOH, it does let you write via streams, instead of mixing C and C++ output, which is generally a good thing. At least AFAIK, C++ doesn't add anything to the library to act as a substitute for strerror (other than generating an std::string, I'm not sure what it would change from strerror anyway).

Solution 2

You could use the boost::system_error::error_code class.

#include <boost/system/system_error.hpp>

#include <cerrno>
#include <iostream>

void
PrintError(
        const std::string& message,
        int error
        )
{
    std::cerr << message << ": " <<
            boost::system::error_code(
                error,
                boost::system::get_system_category()
                ).message()
            << std::endl;
}

int
main()
{
    PrintError( "something went wrong!", EINVAL );
    return 0;
}

it's a tad verbose, and somewhat overkill if you aren't already using the boost_system library.

Solution 3

With C++11, we have the <system_error> header, so you should be able to use:

std::error_code{errno, std::generic_category()}.message();

Example program:

#include <system_error>
#include <iostream>

int main() {
    std::cout << std::error_code{errno, std::generic_category()}.message() << '\n';
}

This prints Success.

See also:

• How to convert errno to exception using <system_error>
• <system_error> categories and standard/system error codes
  (regarding whether to use generic_category or system_category)

Comments

[Tyler McHenry]

A C++ version of strerror which would return a std::string would presumably also be thread-safe, which would be a nice improvement.

[Jerry Coffin]

@Tyler: Well, that's certainly possible, and would be a handy improvement. OTOH, getting thread safety out of anything that uses errno almost unavoidably uses thread local storage anyway (i.e., about the same as it takes to make strerror thread safe).

[Tyler McHenry]

@Jerry I'm not sure about other threading frameworks, but POSIX threads guarantee that errno is thread-local automatically. The problem with strerror is that it returns a pointer to a static buffer that is not thread-local.

[Jerry Coffin]

@Tyler: My point was that to make errno work, you need to have and use TLS anyway, and once you're using it, you might as well use it for strerror's buffer too.

[Simon]

@Tyler McHenry: Having an error-message-creation-function that creates a std::string would be very inappropriate. How would that function work if it would try to report "out of memory"? I would say that an error-function that takes a char* buffer with a given size would be the most appropriate. It's also "thread-safe".

[Simon]

Wouldn't that be pretty bad if you happen to do PrintError("We ran out of memory!", EINVAL); when you discover that you failed to do a memory-allocation?

[Tyler McHenry]

@Simon And there exists such a thing (perror_r), although you do have a point about out-of-memory errors.

[Sagar]

Can't actually use the boost library. This has to be as 'plain' (for lack of a better term) as possible. Trying not to include any 3rd party stuff. Thanks though!

[Sagar]

Thanks all. Really helpful! The fact that I can stream it makes me like it more. I don't like mixing C & C++ if I can help it. The sterror issue (non-local static buffers) is something I'll have to check, although we should be using POSIX threads, so...