[boost] CMake, modular Boost, and other stories

Subject: [boost] CMake, modular Boost, and other stories
From: Peter Dimov (pdimov_at_[hidden])
Date: 2019-04-23 16:40:24

It's been quiet lately, so it must be time for another CMake discussion.

As you know, Boost 1.70 installs CMake configuration files on `b2 install`, 
and we've started to receive feedback, some of which more interesting than 
what doesn't work.

At present, `b2 install --with-filesystem` for example installs a 
configuration file boost_filesystem-config.cmake, which enables CMake users 
to issue the command `find_package(boost_filesystem)` and then use 
Boost.Filesystem. This mimics what an ordinary, independent library that 
happened to be called "boost_filesystem" would do.

It also installs an umbrella BoostConfig.cmake file, which enables the 
alternative use of `find_package(Boost COMPONENTS filesystem)`, which treats 
Filesystem as a component of the package Boost, rather than as a standalone 
library. This is compatible with the existing, pre-1.70 use of 
FindBoost.cmake to locate Boost from CMake.

Brad King (from Kitware, who should know) argues that installing separate 
configuration files for all the libraries is wrong. He says that the 
preferred practice is to only support the second use, with Boost as the 
package, and the libraries as components, not accessible as packages on 
their own. (The discussion is here: 
https://gitlab.kitware.com/cmake/cmake/issues/19186)

The reason I went with installing separate configuration files was that I 
expected that (a) this would be our preferred future approach when, or if, 
we switch to building with CMake and (b) even if not, this seemed like a 
step in the right, "modular", direction where libraries are treated as 
independent rather than parts of the monolithic package Boost.

I'm having second thoughts though. It looks to me that Brad is right, and 
that the various library configuration files should be implementation 
details of a single Boost package (in the CMake sense) and not be 
user-visible. That is, only support

    find_package(Boost COMPONENTS filesystem)

and drop

    find_package(boost_filesystem)

Thoughts?