visibility=hidden by default

[huangqinjin]

b2 enables hidden visibility by default. https://github.com/boostorg/boost/blob/6d9f09bb168a735c69a291cf9601790e0e840b14/Jamroot#L191

boost-cmake should also enable by default:

set(CMAKE_VISIBILITY_INLINES_HIDDEN ON)
set(CMAKE_C_VISIBILITY_PRESET hidden)
set(CMAKE_CXX_VISIBILITY_PRESET hidden)

[Flamefire]

I think this makes sense. However they should really be the default only, i.e. guard them with if(DEFINED and maybe provide a BOOST_* variable to toggle inline and preset visibility with one switch. E.g. a bool BOOST_VISIBILITY_HIDDEN which when set will set the 2 variables (CMAKE_C_VISIBILITY_PRESET is not required for Boost) appropriately and if BOOST_VISIBILITY_HIDDEN is not set it will set the 2 variables if they are not set.

Hint for implementors: while option would be a good idea I remember an issue with some CMake versions and/or policies where a value set on the commandline via -D would be ignored (overwritten by option) unless the type is also given on the commandline which is very surprising behavior so that has to be considered/avoided. Also option would remove the possibility of the "tri-state" solution as it will be always set.

[pdimov]

OK, let's give it a go.

[Flamefire]

As for https://github.com/boostorg/cmake/commit/0e34b571cb1a8b310c5e2e9727cdcb03d4f5ff2d and my previous comment regarding option I made a little test:

cmake_minimum_required(VERSION 3.5...3.16)
project(Boost VERSION 1.81.0 LANGUAGES CXX)

set(FOO OFF)

option(FOO "DD" ON)
message(FATAL_ERROR "FOO ${FOO}")

I.e. assuming that some project sets it as a non-cache variable.

• CMake 3.12: First run ON, 2nd run OFF --> Worst case IMO
• CMake 3.13: All runs OFF

With -DFOO=ON: Ignored on CMake 3.13+

• CMake 3.12: All runs ON
• CMake 3.13: All runs OFF

With -DFOO:BOOL=ON: Always ignored

• CMake 3.12: All runs OFF
• CMake 3.13: All runs OFF

With -DFOO=OFF or -DFOO:BOOL=OFF it is always OFF (expected)

When setting the non-cache variable to ON:

cmake_minimum_required(VERSION 3.5...3.16)
project(Boost VERSION 1.81.0 LANGUAGES CXX)

set(FOO ON)

option(FOO "DD" ON)
message(FATAL_ERROR "FOO ${FOO}")

With no option or -DFOO=ON or -DFOO:BOOL=ON it is ON

-DFOO=OFF: Ignored on CMake 3.13+

• CMake 3.12: All runs OFF
• CMake 3.13: All runs ON

-DFOO:BOOL=OFF: Always ignored

• CMake 3.12: All runs ON
• CMake 3.13: All runs ON

So it only affects Boost-as-a-subdirectory use cases and only for CMake 3.12 and below but (I think) it will be ignoring the user-projects policy setting so if someone was setting CMAKE_VISIBILITY_INLINES_HIDDEN as a normal variable with a policy version of 3.12 (e.g. cmake_minimum_required(VERSION 3.8)) he might get a surprise.

The worst case is the different behavior between subsequent runs without changes. Maybe we should wrap all our option usages into a function the uses the current value as the default when the variable is defined to make it consistent.