C++11 introduced the <system_error> header containing a generic system to handle error codes. An std::error_code is a tuple containing an int, the error code, and a reference to an std::error_category, which defines the error domain and handling of the error code. The standard library comes with four categories: std::generic_category, std::system_category, std::future_category, and std::iostream_category.

There are conflicts on which category to use, both here on SO and on C++ reference sites, when creating std::error_codes/throwing std::system_errors with errno and WinAPI error codes:

• errno with std::generic_category: SO answer, llvm-commits, cplusplus.com


• errno with std::system_category: SO answer, cppreference.com


• GetLastError() with std::generic_category: SO answer


• GetLastError() with std::system_category: SO answer, SO comment

However, errno and GetLastError() can't use the same category, otherwise some error codes would be ambiguous. Error code 33 is one example, as it is both EDOM and ERROR_LOCK_VIOLATION.

There are even some places advocating a user-made category for the WinAPI, but I can't find any references to that at the moment. This alternative would be specially painful.

Which category should be used with errno, and which should be used with GetLastError() so that

• std::error_code::default_error_condition()


• std::error_code::message()

are unambinguous and appropriate to the underlying error code?