Subject: Re: [boost] [cmake] Pull request announcement
From: James E. King III (jking_at_[hidden])
Date: 2018-09-16 11:46:45
On Sat, Sep 15, 2018 at 11:33 AM mike via Boost <boost_at_[hidden]>
> > -----Original Message-----
> > From: Boost <boost-bounces_at_[hidden]> On Behalf Of Bjorn Reese
> via Boost
> > Sent: Saturday, September 15, 2018 2:11 PM
> > To: Mike Dev via Boost <boost_at_[hidden]>
> > Cc: Bjorn Reese <breese_at_[hidden]>
> > Subject: Re: [boost] [cmake] Pull request announcement
> > On 09/14/18 13:44, Mike Dev via Boost wrote:
> > > This will not address
> > > - Testing
> > [...]
> > > In short: all the difficult stuff is left out ;)
> > > The goal is not to present a solution that can replace b2 but to have
> > > a minimal starting point upon which individual library authors or the
> > > community as a whole can make iterative improvements. If and when BCM
> > > (or an alternative) is accepted into boost, individual libraries can
> > > then easily switch to this full-fledged solution and of course any
> > > author can create a more complete CMake solution for his library at
> > > any time (as some already have).
> > In my experience, we want to have both an integration and standalone
> > CMake file. This split is necessary to avoid problems with multiple
> > instances of enable_testing() when using one library from another.
> > * The integration file builds only what is necessary for integration
> > into other projects. This is similar to what you are proposing.
> > * The standalone file builds everything including test and
> > documentation. This reuses the integration file to build the library
> > itself.
> > If we place your proposed CMake file in the library root, and dependent
> > libraries start using that location, then it becomes difficult to later
> > have the standalone CMake file in the library root, which is the more
> > natural choice. So we should be cautious about where we place these
> > files.
> Could you elaborate, why this would require separate/conflicting
> cmake files as opposed to having multiple different targets in a
> single file?
> More to the point: Anything related to testing should reside in a cmake
> file inside the "tests" folder, which is called from the cmake
> file at the root, but that is mainly a matter of properly structuring the
> cmake code - just as you usually don't put all your code into a single
> header file, even if it might end up in a single translation unit in the
> The same is true for docs.
> In particular: Just because a cmake file is processed doesn't mean any
> of the targets it contains have to be build automatically.
> If you have genuine concerns however, I'm happy to further discuss
> them in detail. I certainly don't want to do anything that will prevent
> a future full-fledged cmake version.
I have used CMake for about 10 years now on pretty large projects, and I
never seen a need to have different CMakeLists files for different purposes.
If you want to provide a build option where tests are not built or
you would add a top level option (throwing in examples and docs too):
OPTION(BOOST_DOCS "Build documentation." OFF) # most people won't have
the required tooling
OPTION(BOOST_EXAMPLES "Build examples." ON)
OPTION(BOOST_TESTS "Build the tests." ON)
Unlike b2, tests do not typically run as part of a build when you use
There are three phases of a cmake based build (well, four really...)
1. Create an out-of-tree build directory and generate the build environment
2. Build using the generated build environment (which can be invoked with
"cmake --build .")
3. Run tests using ctest.
(4. cpack to build packages)
If you were to perform step 1 (generating the build) with "cmake
then step 3 would either do nothing or produce an error indicating tests
Someone asked what the naming convention for the libraries should be.
You can teach CMake how to name libraries; the naming convention can be
to how b2 does it. I don't see why it has to be any different than b2.
CMake isn't without it's oddities either. Selecting the build architecture
and type (debug, x64)
is done in step 1 on unix, but it is done on step 2 on windows. CMake does
not natively provide
windows package management. In all of the CMake build systems I have used
or created so
far I have always had to make an environmental harness to deal with third
party deps. If anyone
has tried to build libicu on windows (as an example), you know what a pain
this can be.
Therefore for it to be successful, tight integration with a package manager
that supports Windows
would be ideal - something that can provide libicu and the other boost
I believe for example conan has some cmake integration.
Finally, someone suggested the project maintain both build systems long
That would be a mistake.
I've seen what this can do in other projects (like Apache Thrift, which
uses autoconf for a
complete linux build, and cmake for windows which is only a subset of
Each check-in would need to prove it builds correctly in two very different
It is a lot to ask of folks submitting changes, but obviously the
"complete" build environment
has to work until the new one becomes "complete" and replaces it.
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