Re: Why do Predef and Config exist?

From: John Maddock (jz.maddock_at_[hidden])
Date: 2022-12-23 18:33:45

>> Is someone willing to explain to me why we have Predef and Config?
> There are probably a couple of people who can explain. I might even be
> one of them. And this would be easier to explain if the Config
> documentation was better. Specifically in the area of stating its
> goal, what it's meant to do, its domain of operation, its reason for
> existence. Perhaps we are supposed to know everything just from its
> name? Do we dare ask for someone to volunteer such information be
> communicated in the documentation? Or do we all just know?

Perhaps we do (or not!)

I see the two libraries as orthogonal: Config answers the question "does 
the current compiler/platform/std lib have feature X?" without the user 
needing to know anything about what the current compiler/platform/stdlib 
actually is.  Where as predef provides a unified way to figure out what 
the current compiler/platform etc really is, and what version it is.  
This is obviously a brilliant fallback for situations when Config 
doesn't have a necessary feature test macro ;)

I should also state, that Config was never really intended to be a 
library for the end user (though I'm aware that some do use it), indeed 
we rather discouraged end-user usage.  Rather it was a bunch of shared 
configuration code, dating back to the heady days when there were only a 
handful of Boost libraries and everyone round here knew everything that 
was going on in each of them! Arguably it should be long dead by now, 
though I have to admit that I'm rather pleased it still mostly just 
plain works, and even the C++20 feature test macros are rather tedious 
to use in comparison (even though I was rather hoping they would 
eventually replace it).

So they could be merged, or not.  I'm not sure how much it matters 
either way ;)

John.