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Subject: [boost] Cmake
From: Peter Dimov (lists_at_[hidden])
Date: 2017-06-24 08:59:12

There is considerable interest in Boost supporting CMake, but it seems that
everyone has different ideas as to what this support will entail. After
deliberation and discussions, I have identified the following (separate)

1. The user installs a Boost release as usual with `b2 install`, which makes
the installation visible to CMake and usable via

2. The user brings several Boost libraries as git submodules into his
CMake-based project and uses add_subdirectory in his CMakeLists.txt to link
to them.

3. The user uses CMake to install an individual Boost library, which is then
available for find_package.

4. The user uses CTest to run the tests of an individual Boost library.

5. CMake is supported as a way to build and install the entire Boost, in
place of b2.

6. CTest is supported as a way to run the tests for the entire Boost, in
place of b2.

At the moment, I think that we should concentrate on (1) and (2) as
providing most bang for the buck. 5-6 in particular require that all
Jamfiles are ported at once, which is a serious undertaking for a
questionable benefit as what we have already works.

I've done a proof of concept for (2), which can be seen here:

This repository uses git submodules in ext/ to bring in Boost.System (our
guinea pig) and its dependencies, then uses them via add_subdirectory in

The required CMake infrastructure is on branch feature/cmake in those six
libraries (assert, config, core, predef, system, winapi.) It consists of
CMakeLists.txt in the root:

and supporting .cmake files in cmake/ :

The goal here is for Boost library developers to be able to remain ignorant
of CMake if they so choose; so of those files, only sources.cmake requires
their input (it's empty for header-only, so those require nothing at all.)

BoostVersion.cmake and default.cmake are common for all libraries and are
copied from the superproject. dependencies.cmake is automatically generated
with boostdep. There is a Jamfile in cmake/ that updates these three files:

and the intent is for this to be done automatically in a superproject script
that invokes "b2 cmake" for the superproject:

which in turn invokes "b2 cmake" in each library having "cmake/Jamfile".

Copying BoostVersion.cmake and default.cmake, instead of referring to them,
allows libraries to be used without a superproject, to support scenarios (2)
and (3) above. (Scenario (3) is also supported by default.cmake.)

What remains to be done is (a) to extend this to the rest of the Boost
libraries, if we decide to do so, which would require figuring out a way to
cope with the numeric/ irregular libraries, and (b) support for scenario (1)

Scenario (1), that is, `b2 install` making the installed Boost available for
CMake find_package use, is an entirely separate undertaking, completely
independent of what I've done - except for possibly reusing the
dependencies.cmake files. It would entail generating
boost_libname-config.cmake and boost_libname-config-version.cmake files for
each library, and boost_libname-config-<suffix>.cmake subconfiguration files
for each build variant (toolset, variant=debug/release, link=static/shared,
runtime-link=static/shared, runtime-debugging=on/off,
threading=single/multi, and possibly python-debugging=on/off.)

This would probably require us to define CMake variables controlling these
BOOST_RUNTIME_LINK, and so on, with them having appropriate default values
inferred from the CMake environment.

Then, boost_system-config-vc141-mt-gd-1_65.cmake will check these values and
declare an imported target pointing to
boost_system-vc141-mt-gd-1_65.lib/dll, otherwise it would do nothing. (Not
sure how relevant is threading=single/multi today, but in principle, it
should also check BOOST_THREADING == MULTI.)

Generating these -config.cmake files would ideally be done as part of the
`b2 install` procedure and require no changes to the individual libraries.
The changes to the `b2 install` procedure significantly exceed my bjam-fu
however, so if we decide to proceed with this - and I don't see how we could
claim CMake support in a meaningful way without it - the interested parties
would need to figure out a way to make that happen. At this point, I know
what needs to be done, but not how to bring it about.

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