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Subject: [boost] [cmake] Minimum viable cmakeification for Boost
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2017-06-19 23:42:28

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. follows for those who don't like clicking on links:

--- cut ---

This is a mock up of a **bare minimum viable** cmakeification of the
Boost C++ libraries for the community to study and evaluate. It only
cmakeifies Boost.System alone, its upstream dependencies are
stubbed/faked. A demo program which shows how one would link against
Boost.System is in this directory (`example_client_program`) and it
demos linking against boost::system as a header only library, as a
static library and as a shared library.

# Specific design goals of this mock up:

- Modern, **highly reusable by unknown third party cmake** cmake3 only

- Minimum possible cmake complexity, no cmake innovating, even at the
cost of extra boilerplate.

- No custom macros, functions, nor custom build logic. Only vanilla
cmake3. Keep the learning curve for the build system as gentle as is

- **Strict** separation of non-fixed configuration and build. Child
CMakeLists must only EVER define build and *fixed* configuration like "I
need C++ 11 minimum". *Variable* configuration like naming, directory
layout for outputs, optimisation flags etc is ALWAYS defined in the
**rootmost** CMakeLists. NEVER in child CMakeLists.

- All custom build logic should always be placed in rootlevel cmake
scripts (command line programs written in cmake, runnable using `cmake
-V`, these can take args etc) or rootlevel CMakeLists, or combinations

# Stuff in there we might yet remove:

- This cmake mockup explicitly documents dependencies between Boost
libraries in cmake, and so when I link against say Boost.System, cmake
will also link in the things Boost.System depends against. Moreover this
is minimal, so ONLY the stuff minimally necessary to fulfil the need for
Boost.System is built.

- We allow those Boost libraries support it to present header only,
static and dynamic linkage editions of themselves. This does complicate
the CMakeLists however.

# What we don't do:

- Test the boundaries of cmake or push any limits of what's possible.

- Bespoke libraries of common functions dragged in wherever.

- Complex custom build logic which needs to be documented and learned
off by library developers.

- Do anything which gets in the way of CMakeLists reuse by any arbitrary
third party cmake written by any Boost user.

--- cut ---

The code should be fairly self explanatory. Yes there are no tests
support. We are limiting the scope of discussion to build only for now.
Technical questions are welcome.


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