error code vs error condition

Question

I don't quite get why do we need to make a distinction between error code (std::error_code) and an error condition(std::error_condition), aren't they the same thing? What are the advantages of an error condition vs error code?

Answer 1

From http://en.cppreference.com/w/cpp/error/error_condition

std::error_condition is a platform-independent error code. Like std::error_code, it is uniquely identified by an integer value and a std::error_category, but unlike std::error_code, the value is not platform-dependent.

So, the advantage is your error code isn't specific to the platform you're working on when using std::error_condition.

With an std::error_code

Each std::error_code object holds a pair of error code originating from the operating system, or some low-level interface

So, the error_code will reference something specific to your platform, a piece of hardware etc etc.

It may be advantageous to use both. The error_condition is the "portable abstraction" so would be the generic error message to give to the user and the error_code would be the platform dependent information that would be useful for specific debug.

A typical implementation [of error_condition] holds one integer data member (the value) and a pointer to an std::error_category.

Answer 2

The simplest answer to this question I found here: http://blog.think-async.com/2010/04/system-error-support-in-c0x-part-5.html.

• class std::error_code - represents a specific error value returned by an operation (such as a system call).
• class std::error_condition - something that you want to test for and, potentially, react to in your code.

I think it is applicable for C++11 too.

Answer 3

Good naming has a huge impact on understanding. With the zoo of very similarly named errorish things introduced in C++11...

std::errc            - an error code
std::error_code      - a different error code?
std::error_condition - yet another kind of error code 
std::error_category  - a utility helper interface for error codes 

...I had to make a mental map just to sort them all out:

std::errc              -> std::posix_error_code
std::error_code        -> std::platform_dependent_error
std::error_condition   -> std::platform_independent_error
std::error_category    -> std::error_domain/utility/mapper
std::system_category   -> std::platform_dependent_error_domain
std::generic_category  -> std::platform_independent_error_domain
std::future_errc       -> std::future_error_code
std::future_category   -> std::future_error_domain
std::io_errc           -> std::iostream_error_code
std::iostream_category -> std::iostream_error_domain

Both platform-dependent errors and platform-independent errors hold a code and a pointer to a utility helper class to map that domain-specific error codes to a message. Despite having nearly identical interfaces {assign, clear, value, message...}, they don't inherit from a common base class, meaning you can't generically call one or the other's methods. ‍♂️

Observe that std::error_code is not an error code, but rather a combo of error code + plus domain/category, whereas std::errc is an actual error code (a numeric value). Good ideas - awful naming .