CMake: option to use cxx_std_11 (minimum) that propagates. (#986)

See https://github.com/abseil/abseil-cpp/issues/259 for context.

This change introduces the ABSL_PROPAGATE_CXX_STD option (default to OFF for now, though it prints a warning in CMake 2.8+ builds that a future Abseil release will default to ON). When enabled, all Abseil CMake targets will set the cxx_std_11 target meta feature (which will then propagate to targets that depend upon Abseil) rather than setting the target-level CXX_STANDARD property to CMAKE_CXX_STANDARD (which is the default value anyway), which doesn't propagate.

Updates README documentation to clarify behavior and with a different example recommendation for library projects (which should generally leave CMAKE_CXX_STANDARD to the root application project).

See https://crascit.com/2015/03/28/enabling-cxx11-in-cmake/ for a useful overview of these different CMake features surrounding C++ standard version configuration.

1 files changed, 4 insertions, 1 deletions

diff --git a/FAQ.md b/FAQ.md
index 78028fc0..fbd92ce9 100644
--- a/FAQ.md
+++ b/FAQ.md
@@ -27,7 +27,10 @@ compiler, there several ways to do this:
   file](https://docs.bazel.build/versions/master/guide.html#bazelrc)
 
 If you are using CMake as the build system, you'll need to add a line like
-`set(CMAKE_CXX_STANDARD 17)` to your top level `CMakeLists.txt` file. See the
+`set(CMAKE_CXX_STANDARD 17)` to your top level `CMakeLists.txt` file. If you
+are developing a library designed to be used by other clients, you should
+instead leave `CMAKE_CXX_STANDARD` unset and configure the minimum C++ standard
+required by each of your library targets via `target_compile_features`. See the
 [CMake build
 instructions](https://github.com/abseil/abseil-cpp/blob/master/CMake/README.md)
 for more information.