Compilers: Add common macros to be used by various compilers

With numerous compilers now supporting multiple language standards in CMake,
some common design patterns have begun to emerge.  This is intended to
be a collection of macros implementing many of those common design
patterns that can be factored out and leveraged by many compilers.
Currently this only contains the code to set the default language
standard of not preset.  More functionality will be added in the future
as appropriate.

1 files changed, 62 insertions, 0 deletions

diff --git a/Modules/Compiler/CMakeCommonCompilerMacros.cmake b/Modules/Compiler/CMakeCommonCompilerMacros.cmake
new file mode 100644
index 0000000..cb365d6
--- /dev/null
+++ b/Modules/Compiler/CMakeCommonCompilerMacros.cmake
@@ -0,0 +1,62 @@
+# Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+# file Copyright.txt or https://cmake.org/licensing for details.
+
+# This module is shared by multiple languages and compilers; use include guard
+if (__COMPILER_CMAKE_COMMON_COMPILER_MACROS)
+  return()
+endif ()
+set(__COMPILER_CMAKE_COMMON_COMPILER_MACROS 1)
+
+
+# Check that a compiler's language standard is properly detected
+# Parameters:
+#   lang   - Language to check
+#   stdver1 - Minimum version to set a given default for
+#   std1    - Default to use for compiler ver >= stdver1
+#   stdverN - Minimum version to set a given default for
+#   stdN    - Default to use for compiler ver >= stdverN
+#
+#   The order of stdverN stdN pairs passed as arguments is expected to be in
+#   monotonically increasing version order.
+#
+# Note:
+#   This macro can be called with multiple version / std pairs to convey that
+#   newer compiler versions may use a newer standard default.
+#
+# Example:
+#   To specify that compiler version 6.1 and newer defaults to C++11 while
+#   4.8 <= ver < 6.1 default to C++98, you would call:
+#
+# __compiler_check_default_language_standard(CXX 4.8 98 6.1 11)
+#
+macro(__compiler_check_default_language_standard lang stdver1 std1)
+  set(__std_ver_pairs "${stdver1};${std1};${ARGN}")
+  string(REGEX REPLACE " *; *" " " __std_ver_pairs "${__std_ver_pairs}")
+  string(REGEX MATCHALL "[^ ]+ [^ ]+" __std_ver_pairs "${__std_ver_pairs}")
+
+  # If the compiler version is below the threshold of even having CMake
+  # support for language standards, then don't bother.
+  if (CMAKE_${lang}_COMPILER_VERSION VERSION_GREATER_EQUAL "${stdver1}")
+    if (NOT CMAKE_${lang}_COMPILER_FORCED)
+      if (NOT CMAKE_${lang}_STANDARD_COMPUTED_DEFAULT)
+        message(FATAL_ERROR "CMAKE_${lang}_STANDARD_COMPUTED_DEFAULT should be set for ${CMAKE_${lang}_COMPILER_ID} (${CMAKE_${lang}_COMPILER}) version ${CMAKE_${lang}_COMPILER_VERSION}")
+      endif ()
+      set(CMAKE_${lang}_STANDARD_DEFAULT ${CMAKE_${lang}_STANDARD_COMPUTED_DEFAULT})
+    else ()
+      list(REVERSE __std_ver_pairs)
+      foreach (__std_ver_pair IN LISTS __std_ver_pairs)
+        string(REGEX MATCH "([^ ]+) (.+)" __std_ver_pair "${__std_ver_pair}")
+        set(__stdver ${CMAKE_MATCH_1})
+        set(__std ${CMAKE_MATCH_2})
+        if (CMAKE_${lang}_COMPILER_VERSION VERSION_GREATER_EQUAL __stdver AND
+          NOT DEFINED CMAKE_${lang}_STANDARD_DEFAULT)
+          # Compiler id was forced so just guess the default standard level.
+          set(CMAKE_${lang}_STANDARD_DEFAULT ${__std})
+        endif ()
+        unset(__std)
+        unset(__stdver)
+      endforeach ()
+    endif ()
+  endif ()
+  unset(__std_ver_pairs)
+endmacro()