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, 44 insertions, 5 deletions

diff --git a/CMake/AbseilHelpers.cmake b/CMake/AbseilHelpers.cmake
index 6a64a2c7..17c4f449 100644
--- a/CMake/AbseilHelpers.cmake
+++ b/CMake/AbseilHelpers.cmake
@@ -254,9 +254,23 @@ Cflags: -I\${includedir}${PC_CFLAGS}\n")
       set_property(TARGET ${_NAME} PROPERTY FOLDER ${ABSL_IDE_FOLDER}/internal)
     endif()
 
-    # INTERFACE libraries can't have the CXX_STANDARD property set
-    set_property(TARGET ${_NAME} PROPERTY CXX_STANDARD ${ABSL_CXX_STANDARD})
-    set_property(TARGET ${_NAME} PROPERTY CXX_STANDARD_REQUIRED ON)
+    if(ABSL_PROPAGATE_CXX_STD)
+      # Abseil libraries require C++11 as the current minimum standard.
+      # Top-level application CMake projects should ensure a consistent C++
+      # standard for all compiled sources by setting CMAKE_CXX_STANDARD.
+      target_compile_features(${_NAME} PUBLIC cxx_std_11)
+    else()
+      # Note: This is legacy (before CMake 3.8) behavior. Setting the
+      # target-level CXX_STANDARD property to ABSL_CXX_STANDARD (which is
+      # initialized by CMAKE_CXX_STANDARD) should have no real effect, since
+      # that is the default value anyway.
+      #
+      # CXX_STANDARD_REQUIRED does guard against the top-level CMake project
+      # not having enabled CMAKE_CXX_STANDARD_REQUIRED (which prevents
+      # "decaying" to an older standard if the requested one isn't available).
+      set_property(TARGET ${_NAME} PROPERTY CXX_STANDARD ${ABSL_CXX_STANDARD})
+      set_property(TARGET ${_NAME} PROPERTY CXX_STANDARD_REQUIRED ON)
+    endif()
 
     # When being installed, we lose the absl_ prefix.  We want to put it back
     # to have properly named lib files.  This is a no-op when we are not being
@@ -287,6 +301,16 @@ Cflags: -I\${includedir}${PC_CFLAGS}\n")
         ${ABSL_DEFAULT_LINKOPTS}
     )
     target_compile_definitions(${_NAME} INTERFACE ${ABSL_CC_LIB_DEFINES})
+
+    if(ABSL_PROPAGATE_CXX_STD)
+      # Abseil libraries require C++11 as the current minimum standard.
+      # Top-level application CMake projects should ensure a consistent C++
+      # standard for all compiled sources by setting CMAKE_CXX_STANDARD.
+      target_compile_features(${_NAME} INTERFACE cxx_std_11)
+
+      # (INTERFACE libraries can't have the CXX_STANDARD property set, so there
+      # is no legacy behavior else case).
+    endif()
   endif()
 
   # TODO currently we don't install googletest alongside abseil sources, so
@@ -390,8 +414,23 @@ function(absl_cc_test)
   # Add all Abseil targets to a folder in the IDE for organization.
   set_property(TARGET ${_NAME} PROPERTY FOLDER ${ABSL_IDE_FOLDER}/test)
 
-  set_property(TARGET ${_NAME} PROPERTY CXX_STANDARD ${ABSL_CXX_STANDARD})
-  set_property(TARGET ${_NAME} PROPERTY CXX_STANDARD_REQUIRED ON)
+  if(ABSL_PROPAGATE_CXX_STD)
+    # Abseil libraries require C++11 as the current minimum standard.
+    # Top-level application CMake projects should ensure a consistent C++
+    # standard for all compiled sources by setting CMAKE_CXX_STANDARD.
+    target_compile_features(${_NAME} PUBLIC cxx_std_11)
+  else()
+    # Note: This is legacy (before CMake 3.8) behavior. Setting the
+    # target-level CXX_STANDARD property to ABSL_CXX_STANDARD (which is
+    # initialized by CMAKE_CXX_STANDARD) should have no real effect, since
+    # that is the default value anyway.
+    #
+    # CXX_STANDARD_REQUIRED does guard against the top-level CMake project
+    # not having enabled CMAKE_CXX_STANDARD_REQUIRED (which prevents
+    # "decaying" to an older standard if the requested one isn't available).
+    set_property(TARGET ${_NAME} PROPERTY CXX_STANDARD ${ABSL_CXX_STANDARD})
+    set_property(TARGET ${_NAME} PROPERTY CXX_STANDARD_REQUIRED ON)
+  endif()
 
   add_test(NAME ${_NAME} COMMAND ${_NAME})
 endfunction()