Set minimum supported C++ standard to 17

.github/workflows/root-ci-config/buildconfig/ubuntu18.txt

@@ -1,2 +1,2 @@
-CMAKE_CXX_STANDARD=14
+CMAKE_CXX_STANDARD=17
 root7=OFF

cmake/modules/CheckCompiler.cmake

@@ -123,10 +123,15 @@ else()
                    OUTPUT_VARIABLE CXX_STANDARD_STRING
                    ERROR_QUIET
                    OUTPUT_STRIP_TRAILING_WHITESPACE)
-   # if the above command fails to set the variable for any reason, let's default to 2011 with a warning
+   # if the above command fails to set the variable for any reason, let's default to 2017 with a warning
    if (NOT CXX_STANDARD_STRING)
-      message(WARNING "Could not detect the default C++ standard in use by the detected compiler (${CMAKE_CXX_COMPILER}). Falling back to C++14 as a default, can be overridden by setting CMAKE_CXX_STANDARD.")
-      set(CXX_STANDARD_STRING 2014)
+      message(WARNING "Could not detect the default C++ standard in use by the detected compiler (${CMAKE_CXX_COMPILER}). Falling back to C++17 as a default, can be overridden by setting CMAKE_CXX_STANDARD.")
+      set(CXX_STANDARD_STRING 2017)
+   endif()
+   # If the native compiler defaults to a C++ standard lower than 17, overwrite the default value
+   if (NOT ${CXX_STANDARD_STRING} STRGREATER "201402L")
+      message(STATUS "The default C++ standard in use by the detected compiler (${CMAKE_CXX_COMPILER}) is lower than C++17. Setting C++17 as the minimum standard.")
+      set(CXX_STANDARD_STRING 2017)
    endif()
 endif()
 # Lexicographically compare the value of __cplusplus (e.g. "201703L" for C++17) to figure out
@@ -140,8 +145,8 @@ if (${CXX_STANDARD_STRING} STRGREATER "201703L")
 elseif(${CXX_STANDARD_STRING} STRGREATER "201402L")
    set(CXX_STANDARD_STRING 17 CACHE STRING "")
 else()
-   # We stick to C++14 as a minimum value
-   set(CXX_STANDARD_STRING 14 CACHE STRING "")
+   # We stick to C++17 as a minimum value
+   set(CXX_STANDARD_STRING 17 CACHE STRING "")
 endif()
 set(CMAKE_CXX_STANDARD ${CXX_STANDARD_STRING} CACHE STRING "")
 set(CMAKE_CXX_STANDARD_REQUIRED TRUE)
@@ -164,8 +169,8 @@ if(cxx11 OR cxx14 OR cxx17)
   unset(cxx11 CACHE)
 endif()
 
-if(NOT CMAKE_CXX_STANDARD MATCHES "14|17|20")
-  message(FATAL_ERROR "Unsupported C++ standard: ${CMAKE_CXX_STANDARD}")
+if(NOT CMAKE_CXX_STANDARD MATCHES "17|20")
+  message(FATAL_ERROR "Unsupported C++ standard: ${CMAKE_CXX_STANDARD}. Supported standards are: 17, 20.")
 endif()
 
 # needed by roottest, to be removed once roottest is fixed

cmake/modules/RootBuildOptions.cmake

@@ -369,11 +369,6 @@ foreach(opt ${root_build_options})
   endif()
 endforeach()
 
-#---ROOT 7 requires C++17 standard or higher---------------------------------------------------
-if(NOT CMAKE_CXX_STANDARD GREATER 14)
-  set(root7_defvalue OFF)
-endif()
-
 #---webgui by default always build together with root7-----------------------------------------
 set(webgui_defvalue ${root7_defvalue})
 
@@ -396,24 +391,6 @@ if (NOT builtin_cling)
   endif()
 endif(NOT builtin_cling)
 
-if(root7)
-  if(NOT CMAKE_CXX_STANDARD)
-    set(CMAKE_CXX_STANDARD 17 CACHE STRING "C++17 standard used with root7")
-    message(STATUS "Enabling C++17 for compilation of root7 components")
-  elseif(NOT CMAKE_CXX_STANDARD GREATER 14)
-    message(FATAL_ERROR ">>> At least C++17 standard required with root7, please enable it using CMake option: -DCMAKE_CXX_STANDARD=17")
-  endif()
-endif()
-
-#---check if webgui can be built-------------------------------
-if(webgui)
-  if(NOT CMAKE_CXX_STANDARD GREATER 11)
-    set(webgui OFF CACHE BOOL "(WebGUI requires at least C++14)" FORCE)
-  elseif(NOT http)
-    set(http ON CACHE BOOL "(Enabled since it's needed by webgui)" FORCE)
-  endif()
-endif()
-
 if(NOT webgui)
    set(qt5web OFF CACHE BOOL "Disabled because webgui not build" FORCE)
    set(qt6web OFF CACHE BOOL "Disabled because webgui not build" FORCE)

core/foundation/inc/ROOT/RConfig.hxx

@@ -44,10 +44,10 @@
 #  define R__NULLPTR
 # endif
 #else
-# if defined(__cplusplus) && (__cplusplus < 201402L)
-#  error "ROOT requires support for C++14 or higher."
+#if defined(__cplusplus) && (__cplusplus < 201703L)
+#error "ROOT requires support for C++17 or higher."
 #  if defined(__GNUC__) || defined(__clang__)
-#   error "Pass `-std=c++14` as compiler argument."
+#error "Pass `-std=c++17` as compiler argument."
 #  endif
 # endif
 #endif
@@ -380,7 +380,7 @@
 #      define R__OLDHPACC
 #      define R__TEMPLATE_OVERLOAD_BUG
 #      define R__GLOBALSTL       /* STL in global name space */
-#      error "ROOT requires proper support for C++11 or higher"
+#error "ROOT requires proper support for C++17 or higher"
 #   else
 #      define R__PLACEMENTDELETE /* supports overloading placement delete */
 #      define R__TMPLTSTREAM     /* std::iostream implemented with templates */

core/meta/test/CMakeLists.txt

@@ -6,9 +6,7 @@
 
 ROOT_ADD_GTEST(testStatusBitsChecker testStatusBitsChecker.cxx LIBRARIES Core)
 ROOT_ADD_GTEST(testHashRecursiveRemove testHashRecursiveRemove.cxx LIBRARIES Core)
-if(NOT MSVC OR win_broken_tests OR CMAKE_CXX_STANDARD GREATER 14)
-  ROOT_ADD_GTEST(testTClass testTClass.cxx LIBRARIES Core)
-endif()
+ROOT_ADD_GTEST(testTClass testTClass.cxx LIBRARIES Core)
 ROOT_ADD_GTEST(testTDataType testTDataType.cxx LIBRARIES Core)
 ROOT_ADD_GTEST(testTEnum testTEnum.cxx LIBRARIES Core)
 configure_file(stlDictCheck.h . COPYONLY)

core/metacling/src/CMakeLists.txt

@@ -137,12 +137,12 @@ if(MSVC)
         __std_find_trivial_8
     )
   endif()
-  if(CMAKE_CXX_STANDARD GREATER 14)
-    set(cling_exports ${cling_exports}
-      _Smtx_lock_shared
-      _Smtx_unlock_shared
-    )
-  endif()
+
+  set(cling_exports ${cling_exports}
+    _Smtx_lock_shared
+    _Smtx_unlock_shared
+  )
+
   if(CMAKE_SIZEOF_VOID_P EQUAL 8)
     set(cling_exports ${cling_exports}
       ??2@YAPEAX_K@Z

tutorials/CMakeLists.txt

@@ -106,12 +106,11 @@ elseif(MSVC AND NOT win_broken_tests)
       list(APPEND dataframe_veto dataframe/df016_vecOps.py)
       list(APPEND dataframe_veto dataframe/df017_vecOpsHEP.py)
     endif()
-    if(CMAKE_CXX_STANDARD GREATER 14)
-        list(APPEND dataframe_veto dataframe/df032_RDFFromNumpy.py)
-        list(APPEND dataframe_veto dataframe/df033_Describe.py)
-    else()
-        list(APPEND dataframe_veto dataframe/*.py)
-    endif()
+
+    # Why do we need to veto these tutorials on Windows with C++17?
+    list(APPEND dataframe_veto dataframe/df032_RDFFromNumpy.py)
+    list(APPEND dataframe_veto dataframe/df033_Describe.py)
+
 endif()
 
 if(NOT sqlite)
@@ -150,9 +149,6 @@ if(NOT ROOT_roofit_FOUND)
 else()
   if(MSVC AND NOT win_broken_tests)
     set(roofit_veto roofit/rf401_importttreethx.py roofit/rf708_bphysics.py roofit/rf501_simultaneouspdf.py)
-    if(CMAKE_CXX_STANDARD LESS 17)
-      list(APPEND roofit_veto roofit/rf408_RDataFrameToRooFit.py)
-    endif()
     if(CMAKE_SIZEOF_VOID_P EQUAL 8)
       list(APPEND roofit_veto roofit/rf409_NumPyPandasToRooFit.py)
     endif()

tutorials/v7/global_temperatures.cxx

@@ -1,10 +1,11 @@
 /// \file
 /// \ingroup tutorial_v7
 ///
-/// This ROOT 7 example demonstrates how to use RNTuple in combination with ROOT 6 features like RDataframe and visualizations.
-/// It ingests climate data and creates a model with fields like AverageTemperature. Then it uses RDataframe to process and filter the climate data for
-/// average temperature per city by season. Then it does the same for average temperature per city for the years between 1993-2002, and 2003-2013.
-/// Finally, the tutorial visualizes this processed data through histograms.
+/// This ROOT 7 example demonstrates how to use RNTuple in combination with ROOT 6 features like RDataframe and
+/// visualizations. It ingests climate data and creates a model with fields like AverageTemperature. Then it uses
+/// RDataframe to process and filter the climate data for average temperature per city by season. Then it does the same
+/// for average temperature per city for the years between 1993-2002, and 2003-2013. Finally, the tutorial visualizes
+/// this processed data through histograms.
 ///
 /// TODO(jblomer): re-enable once issues are fixed (\macro_image (rcanvas_js))
 /// \macro_code
@@ -17,7 +18,7 @@
 /// \warning The RNTuple classes are experimental at this point.
 /// Functionality, interface, and data format is still subject to changes.
 /// Do not use for real data! During ROOT setup, configure the following flags:
-/// `-DCMAKE_CXX_STANDARD=14 -Droot7=ON -Dwebgui=ON`
+/// `-DCMAKE_CXX_STANDARD=17 -Droot7=ON -Dwebgui=ON`
 ///
 /// \date 2021-02-26
 /// \author John Yoon

tutorials/webgui/qtweb/Readme.md

@@ -24,7 +24,7 @@ Compile ROOT with qt5web support in Release mode:
     cd C:\
     mkdir root
     cd C:\root
-    cmake -G"Visual Studio 16 2019" -A Win32 -Thost=x64 c:\git\root -Droot7=ON -DCMAKE_CXX_STANDARD=14 -Dwebgui=ON -Dqt5web=ON
+    cmake -G"Visual Studio 16 2019" -A Win32 -Thost=x64 c:\git\root -Droot7=ON -DCMAKE_CXX_STANDARD=17 -Dwebgui=ON -Dqt5web=ON
     cmake --build . --config Release -- /maxcpucount
 
 Configure ROOT, create build directory and build qt5web tutorial: