Question

I’m using the C++11 system_error error code library to create a custom error class for a library I’m making. I’ve done this before with boost::error_code, but I can’t quite it get it working with std::error_code. I’m using GCC 4.6.

Basically, I’ve laid out all the boilerplate code to create an error class, an error_category, and the conversion routines in the STD namespace to convert my custom enums into an std::error_code object:

namespace mylib
{
    namespace errc {

        enum my_error
        {
            failed = 0
        };

        inline const char* error_message(int c)
        {
            static const char* err_msg[] = 
            {
                "Failed",
            };

            assert(c < sizeof(err_msg) / sizeof(err_msg[0]));
            return err_msg[c];
        }

        class my_error_category : public std::error_category
        {
            public:

            my_error_category()
            { }

            std::string message(int c) const
            { 
                return error_message(c); 
            }

            const char* name() const { return "My Error Category"; }

            const static error_category& get()
            {
                const static my_error_category category_const;
                return category_const;
            }
        };

    } // end namespace errc
} // end namespace mylib

namespace std {

inline error_code make_error_code(mylib::errc::my_error e)
{
    return error_code(static_cast<int>(e), mylib::errc::my_error_category::get());
}

template<>
struct is_error_code_enum<mylib::errc::my_error>
    : std::true_type
{ }; 

The problem is, implicit conversion between my error code enums and std::error_code objects doesn’t seem to be working, so I can’t for example try and compare an instance of std::error_code with enum literals:

int main()
{
    std::error_code ec1 = std::make_error_code(mylib::errc::failed); // works
    std::error_code ec2 = mylib::errc::failed; // doesn't compile
    bool result = (ec2 == mylib::errc::failed); // doesn't compile
}

The expression ec2 == mylib::errc::failed won’t compile – I have to say ec2 == std::make_error_code(mylib::errc::failed).

The error the compiler emits is:

In file included from test6.cc:3:0:
/usr/include/c++/4.6/system_error: In constructor ‘std::error_code::error_code(_ErrorCodeEnum, typename std::enable_if<std::is_error_code_enum<_ErrorCodeEnum>::value>::type*) [with _ErrorCodeEnum = mylib::errc::my_error, typename std::enable_if<std::is_error_code_enum<_ErrorCodeEnum>::value>::type = void]’:
test6.cc:70:37:   instantiated from here
/usr/include/c++/4.6/system_error:127:9: error: cannot convert ‘mylib::errc::my_error’ to ‘std::errc’ for argument ‘1’ to ‘std::error_code std::make_error_code(std::errc)’

And here’s an Ideone link.

So, why isn’t this working? Do I need additional boilerplate code to enable mylib::errc::my_error enums to be implicitly convertible to std::error_code? I thought that the specialization of std::make_error_code takes care of that?

Answer 1

You have to move error_code make_error_code(mylib::errc::my_error e) function from std to your error namespace (mylib::errc). Please check http://ideone.com/eSfee.