Re: [Boost-build] switching on toolset

Subject: Re: [Boost-build] switching on toolset
From: Robert Ramey (ramey_at_[hidden])
Date: 2015-06-22 11:25:35

On 6/22/15 4:39 AM, Vladimir Prus wrote:

> you've made the same point earlier.

[snip]

I know, but it wasn't helpful.

> So, if you're interesting in improving things, could you act on earlier
> responses, please?

Stephan said he needed more specific suggestion.  Here's a specific one.

On 6/16/15 3:54 PM, Gennadiy Rozental wrote:
> Hi,
>
> I'd like specify different requirement (cxxflags specifically) depending on
> which toolset I am running against.
>
> I know I specify multiple entries in requirements for each toolset, but I'd
> rather have some if statement which explicit state what i mean. This is
> especially important to handle various  gcc versions. I'd rather not spell
> out each version, but have some conditions on version number.
>
> Any help appreciated.
>
> Gennadiy
>
> _______________________________________________
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>

So I read this as proposal to include logic inside he jamfile which will 
select features/attributes of the build environment without the user 
having to know what is going on.

I understand the motivation - it's so that the build "just works"

I also believe that doing such a thing is a bad idea.  The basic 
reasoning is:

a) it will sometimes fail.  Then the user will have to delve into the 
bjam code which he doesn't understand to try and figure out the problem.

b) Any failures are likely unforeseeable.  Hence when it fails it will 
not be obvious where the source of the failure is.  This will cause the 
user, even more difficulty.

c) Its totally unclear what happens if there is a conflict between the 
switches used on the command line and the switches automatically added 
in by the jamfile.  Which would take precedence? would this internal 
logic drop switches I've specifically requested?.  Would it add switches 
I don't know about?  What would it do?  How would I find this out?

d) what happens when I invoke b2 for the whole of boost with some set of 
switches. Now would some libraries get built with one set of switches 
and other libraries get built with a different set?  How would I know 
what's getting built with what?  Suppose one library uses another 
library, how do we know that there won't be any incompatibilities?  If 
there are any, and they show up later in the process when I try to use 
the libraries, How will we track down the source?  And how will we fix 
the problem?

What I suggest is that if there is some sort of requirements related to 
the selection of toolset options/features/attributes which cannot be 
supported, the the bjam code just detect them and bail with an error 
message which tells the user what he did wrong and maybe how to fix it. 
This avoids the problems above.

Robert Ramey