Integrate with C++11 system_error (see PR #27)
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(Lodging an issue for this; I might be inclined to develop a pull request.)
C++11 introduces an error_code class that can accommodate both POSIX standard error codes and alternative collections of error codes such as NNG's. An error_code includes an integer value (0 for no error) and an error category which can be either the C++/POSIX one or some custom category. nngpp would benefit from implementing a custom error category.
https://en.cppreference.com/w/cpp/error/error_category
Implementations of std::error_category have a few responsibilities: Categorizing error codes, providing string representations, and mapping platform-dependent codes to platform-agnostic ones (including standard C++/POSIX error codes). These functions serve to make codebases more consistent and integrable; for example, libasio uses std::error_code extensively.
I recommend making nng::error convertible to std::error_code and/or adding a method for doing so to nng::exception.
The POSIX error codes supported by C++11 are listed here: https://en.cppreference.com/w/cpp/error/errc
The following codes have no direct mapping to a C++/POSIX equivalent.
| NNG CODE | possible C++ mapping |
|---|---|
| ECLOSED | |
| ESTATE | |
| EUNREACHABLE, EADDRINVAL | address_not_available |
| ENOFILES | |
| EREADONLY / EWRITEONLY | |
| ECRYPTO / EPEERAUTH | |
| ENOARG / EAMBIGUOUS / EBADTYPE | |
| ECONNSHUT | connection_aborted |
| EINTERNAL | |
| ESYSERR | |
| ETRANERR |
These codes would need to be either lossily mapped to C++ codes or implemented as an NNG-specific error_condition. It would be somewhat advantageous to assign them values that don't conflict with the standard C++ ones.
I'm working on a pull request which adds support for <system_error> to error.h without modifying or removing any current functionality.
These are my notes as I attempt to complete the mapping to generic_error_category based on the POSIX standard and a partial mapping in NNG's nn.c.
| NNG CODE | POSIX | C++ errc | Basis of mapping |
|---|---|---|---|
| ECLOSED | EBADF | bad_file_descriptor | nn.c |
| ESTATE | ENOTTY | inappropriate_io_control_operation | POSIX (nn.c uses EFSM) |
| EUNREACHABLE | EHOSTUNREACH | host_unreachable | nn.c |
| EADDRINVAL | EADDRNOTAVAIL | address_not_available | POSIX (nn.c uses EINVAL) |
| ENOFILES | EMFILE | too_many_files_open | nn.c |
| EREADONLY / EWRITEONLY | †EACCES | permission_denied | nn.c |
| ECRYPTO / EPEERAUTH | †EACCES | permission_denied | nn.c |
| ENOARG | †EINVAL | invalid_argument | "option requires argument" |
| EAMBIGUOUS | †EINVAL | invalid_argument | nn.c |
| EBADTYPE | †EINVAL | invalid_argument | nn.c |
| ECONNSHUT | †ECONNABORTED | connection_aborted | closest available |
| EINTERNAL | — | — | NNG-specific |
| ESYSERR (flag) | — | — | platform-specific |
| ETRANERR (flag) | — | — | transport-specific |
† "Lossy" mappings; multiple NNG error codes would map to one POSIX error in default_error_condition.