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Subject: Re: [boost] [system] Would it be possible to trial a breaking change to Boost.System and see what happens?
From: Vinnie Falco (vinnie.falco_at_[hidden])
Date: 2018-01-19 17:16:04

On Sat, Jan 13, 2018 at 8:05 PM, Christopher Kohlhoff via Boost
<boost_at_[hidden]> wrote:
> The error_code class itself deliberately does *not* imbue zero values with
> the meaning 'success' and non-zero values with the meaning 'failure'. An
> error_code simply represents an integer and a category, where the category
> identifies the source of a particular integer value. The specification of the
> error_code class carefully avoids making any judgement as to whether a
> particular value represents success or failure. The construct:
> if (ec) ...
> does not, in and of itself, mean 'if an error ...'. Instead, operator bool is
> specified to behave as the ints do, and the above construct should simply
> be read as 'if non-zero ...'.

That might have been the original design intention, but that ship sailed a long
time ago. It is well established that users are habituated to think of the bool
conversion as meaning success or failure.

> When defining your own API you are free to define your own notion of success
> or failure. One way would be to define your own error_condition for this
> (an intended use case for error_condition), but you may also use some other
> mechanism entirely (indicate failure via exception, return value, etc.). You
> might like to consider this approach for your own API that wraps NT kernel calls.

Nobody wants to write

    if(ec == my_condition::success)

When then can instead write

    if(! ec)

Even in your own code you write `if(! ec)`:


Whether it was intended or not, established practice is to treat the bool
conversion of error_code as false==success and true==failure.

I have long felt that the documentation for <error_code> and <error_condition>
is terribly inadequate which probably accounts for the pervasive misconceptions.
The standard is completely unhelpful in offering guidance on its use, and all of
the usual websites ( or the boost docs for example) are
similarly unhelpful. Chris' blog posts on error_code were instructive but
incomplete. Andrezj's blog posts did the best job of providing tutorial-like
guidance but of course I read them too late.

People are still trying to figure out how to use error_code and having trouble
because it isn't well explained. Ideas like "if(! ec)" should not be used to
check for success do nothing to improve this situation.


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