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Subject: Re: [boost] [cmake] Minimum viable cmakeification for Boost
From: Thomas Heller (thom.heller_at_[hidden])
Date: 2017-06-20 13:48:59

Am 20.06.2017 2:17 nachm. schrieb "Niall Douglas via Boost" <

On 20/06/2017 07:27, P F wrote:
>> On Jun 19, 2017, at 6:42 PM, Niall Douglas via Boost
>> <boost_at_[hidden]> wrote:
>> I've finished the mockup which can be studied at:
>> Just Boost.System was cmakeified. Extensive comments are throughout
>> to explain every single line of cmake and why it's there. A small
>> sample program is in the root of the repo to show how this cmake
>> would be used by end user programs.
> Here some feedback, I have:
> - Having things like `::hl` or `::sl` is quite strange. In cmake, I
> set `BUILD_SHARED_LIBS=On` when I want a shared library or I set it
> to `BUILD_SHARED_LIBS=Off` when I want a static library. There should
> only be one target `boost::config` or `boost::system` that I use.

BUILD_SHARED_LIBS is a cmake2-ism and should not be present in modern
cmake. Moreover, cmake now supports header only libraries, and
BUILD_SHARED_LIBS causes problems with eventual C++ Modules support. The
::hl suffix means "link against the header only library edition", the
::sl suffix means "link against the static library edition" and the ::dl
suffix means "link against the dynamic library edition".

Err, I tend to highly disagree here. As a library author, I can not know if
my users need a static or shared library. For a header only library, this
hardly matters. Having those different conventions greatly increases the
complexity for consuming your library (which do I need again in which
circumstances?) so we need to depend on variables to link against the right
target (so much for cmake2isms).

Some bikeshedding over those names occurred off list. They don't hugely
matter, they could be boost::system::header, boost::system::static and
boost::system::shared if you wanted.

Some bikeshedding also occurred over what boost::system with no suffix
should mean. I recommend "whatever library edition the maintainer
recommends as the best one" which is NOT necessarily the header only
edition. For example, in the future with C++ Modules support the
additional suffixs ::hlm, ::slm and ::dlm will be needed. Or longer
named, boost::system::header_module, boost::system::static_module and

So if C++ Modules were available, the maintainer might recommend the
::dlm variety, but recommend the ::hl variety if they weren't available.
I think that will hugely vary per library as C++ Modules won't
necessarily be always a win. Also, end users may wish to assemble all of
Boost into a single C++ Module. We don't want to get in the way.

> - Each project do things like
> `include("${CMAKE_CURRENT_SOURCE_DIR}/../../cmake/BoostVersion.cmake”)`
> which is broken when built on its own.

That was intentional. Some bikeshedding occurred over that too with
respect to which Boost library guaranteed to be a dependency for all
other libraries ought to host the version setting script. Boost.Config
is most likely. So the above is a placeholder.

> - Doing `if(NOT TARGET boost::assert::hl)` is wrong. A target does
> not have to exists to be able to link against it. Plus, it should
> call `find_package(boost_assert)` to bring in the target for when its
> not built in the superproject.

That part is there solely to enable modular build. If you don't want to
bother with modular build as this is a bare minimum viable
cmakeification, you can eliminate it.

> - There is no installation of the targets

Not necessary right now, so out of scope.

> - Doing `target_include_directories(boost_core-hl INTERFACE
> "include”)` is wrong as this will only add the include directories
> for the build. The include directories should be added for the build
> and for the installation.

As I've repeatedly explained now, installation logic is best implemented
by a rootmost CMakeLists. Not in the per-library CMakeLists. We do tell
cmake what targets are used by installation via install(TARGETS ...).
That's the bare minimum needed for other cmake to implement the
remaining installation logic.

(On wider picture stuff, install paths need to be absolute I've found,
whereas build paths can usually be relative. Relative paths are much
nicer during build. In a rootmost CMakeLists which implements some
installation logic, you'd iterate the targets for the Boost libraries
and configure each programmatically for the appropriate install for that
library and that type of library. All the metadata you need to make
decisions is available)


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