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Subject: Re: [boost] [Config] Announce: New Build time configuration feature
From: Edward Diener (eldiener_at_[hidden])
Date: 2014-06-05 16:44:48

On 6/5/2014 4:09 PM, Edward Diener wrote:
> On 6/5/2014 2:01 PM, John Maddock wrote:
>> In develop only is a new feature to make configuring targets at build
>> time easier, from the docs:
>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> There are times when you want to control whether a build target gets
>> built or not, based on what features the compiler supports. For example,
>> suppose you have a test file "test_constexpr_128.cpp" which requires
>> three key features in order to build:
>> * The constexpr keyword as detected by BOOST_NO_CXX11_CONSTEXPR.
>> * User defined literals, as detected by
>> * The __int128 data type, as detected by BOOST_HAS_INT128.
>> Clearly we know that if these features are not supported by the
>> compiler, then there's simply no point in even trying to build the test
>> program. The main advantages being:
>> * Faster compile times - build configuration uses lightweight tests
>> the results of which are also cached.
>> * Less noise in build output - there's no reason to be faced with
>> pages of template instantiation backtrace if we know the file can never
>> compile anyway.
>> * Less noise in the online test results - the test will show up as
>> blank, rather than as a fail in the online test matrix.
>> * A better experience for end users building all of Boost, if those
>> libraries which can not be built for the current target compiler are
>> simply skipped, rather than generating pages of error output.
>> Returning to our example, the test case is probably executed in it's
>> Jamfile via the "run" rule:
>> run test_constexpr_128.cpp ;
>> We now need to make this target conditional on the necessary features.
>> We can do that by first importing the necessary rule at the start of the
>> Jamfile:
>> import path-to-config-lib/checks/config : requires ;
>> Assuming that the test case is in the usual directory:
>> libs/yourlib/test
>> then the import rule will actually be:
>> import ../../config/checks/config : requires ;
>> Then add a "requires" rule invocation to the requirements section of the
>> target:
>> run test_constexpr_128.cpp
>> : : : #requirements:
>> [ requires cxx11_constexpr cxx11_user_defined_literals int128 ] ;
>> Notice that multiple arguments can be added to the requires rule, and
>> that these are always the same as the Boost.Config macro name, but in
>> lower case and with the boost_no_ or boost_has_ prefix removed.
>> When building the above example, you will see at the start of the build
>> process the results of the configuration, for example GCC in C++11 mode
>> gives:
>> - Boost.Config Feature Check: int128 : yes
>> - Boost.Config Feature Check: cxx11_constexpr : yes
>> - Boost.Config Feature Check: cxx11_user_defined_literals : yes
>> That's all there is to this handy feature, should at any time you be
>> unsure of the feature-test names you can pass to the "requires" rule,
>> then search for the Boost.Config macro of interest in
>> libs/config/checks/Jamfiles.v2, and the name of the feature check will
>> follow it.
>> And finally, this feature is built around the Boost.Build built in rule
>> check-target-builds which can be used to perform more generalized
>> build-time feature testing. The checks in this library are provided as a
>> convenient shorthand without the need for you to write the test cases
>> yourself.
> <rant>
> As an example why Boost Build is so difficult for me, where is the Boost
> Build documentation for the "check-target-builds" feature, much less how
> to use it in a jamfile ?
> BTW I did search the Internet via Google since the Boost Build docs for
> the latest release. 1.55, still has no index. But even Google finds
> nothing of value in the way of documentation.
> <end-of-rant>
> Now I am peacable again. <g>

Apologies. I do see documentation for "check-target-builds" in the
latest doc in the 'develop' branch of Boost Build.

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