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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: Gavin Lambert (gavinl_at_[hidden])
Date: 2018-01-15 22:48:10

On 15/01/2018 13:01, Niall Douglas wrote:
> Is anybody actually generating a totally unique message per invocation
> for the same error code which couldn't be cached and reused? I've never
> seen it yet. Almost all use of message() is for printing to human
> readable output.

The most common dynamic message generation case is where a standard
message can't be found, so it returns a generic "error #nnn occurred"
message. While these *could* be cached, it seems a bit wasteful,
especially if nothing eventually tidies them away (but when is it safe
to do so?).

But there are other cases as well; Windows' FormatMessage allows
specifying additional parameters to fill in replacement values for parts
of the text, which might be contextual information about eg. which file
couldn't be loaded. And the standard error messages usually are defined
with these parameters in mind.

Granted in the context of an error_code alone being passed around it's
unlikely for there to be sufficient context to supply the parameters
except back at the original error site (which then isn't an error_code
any more) -- but given something like outcome which can pass the error
code *and* the necessary parameters to fill in the message when it does
eventually get converted to text, that becomes more useful.

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