Implement m-variables

The m-variables provide the same functionality as their
template metaprogramming-based version by using macros. This
new implementation ensures considerably faster compile times.

CMakeLists.txt

@@ -80,7 +80,7 @@ endif()
 list(APPEND UNGAR_INCLUDE_DIRECTORIES
      $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
      $<INSTALL_INTERFACE:include>)
-list(APPEND UNGAR_LINK_LIBRARIES Eigen3::Eigen hana)
+list(APPEND UNGAR_LINK_LIBRARIES Eigen3::Eigen hana Boost::preprocessor)
 
 # Optional modules.
 if(UNGAR_ENABLE_LOGGING)

external/CMakeLists.txt

@@ -48,6 +48,18 @@ elseif(NOT UNGAR_USE_SYSTEM_LIBRARIES)
   set(HANA_ROOT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/hana/install)
 endif()
 
+# Preprocessor.
+if(DEFINED Preprocessor_ROOT)
+  set(PREPROCESSOR_ROOT_DIRECTORY ${Preprocessor_ROOT})
+elseif(NOT UNGAR_USE_SYSTEM_LIBRARIES)
+  set(USE_BUNDLED_PREPROCESSOR ON)
+  set(BUNDLED_PREPROCESSOR_FILENAME
+      ${CMAKE_CURRENT_LIST_DIR}/config/preprocessor/preprocessor-1.84.0-ungar.zip
+  )
+  set(PREPROCESSOR_ROOT_DIRECTORY
+      ${CMAKE_CURRENT_BINARY_DIR}/preprocessor/install)
+endif()
+
 # GoogleTest.
 if(DEFINED GTest_ROOT)
   set(GTEST_ROOT_DIRECTORY ${GTest_ROOT})
@@ -161,6 +173,28 @@ else()
   message(STATUS "Hana found.")
 endif()
 
+# ##############################################################################
+# Add Preprocessor.
+# ##############################################################################
+message(STATUS "--------------------------------------------------")
+if(USE_BUNDLED_PREPROCESSOR)
+  message(STATUS "Using bundled Preprocessor...")
+  download_external_project(config/preprocessor/CMakeLists.txt.in preprocessor)
+  find_package(
+    Preprocessor REQUIRED PATHS
+    ${PREPROCESSOR_ROOT_DIRECTORY}/lib/cmake/preprocessor NO_DEFAULT_PATH)
+else()
+  message(STATUS "Using system-wide Preprocessor...")
+  find_package(Preprocessor REQUIRED)
+endif()
+set_target_properties(Boost::preprocessor PROPERTIES IMPORTED_GLOBAL TRUE)
+
+if(DEFINED PREPROCESSOR_ROOT_DIRECTORY)
+  message(STATUS "Preprocessor found at '${PREPROCESSOR_ROOT_DIRECTORY}'.")
+else()
+  message(STATUS "Preprocessor found.")
+endif()
+
 # ===========================================================================
 # 2) TESTING
 if(UNGAR_BUILD_TESTS)

external/config/preprocessor/CMakeLists.txt.in

@@ -0,0 +1,24 @@
+cmake_minimum_required(VERSION 3.20)
+
+project(ExternalPreprocessor NONE)
+
+include(ExternalProject)
+
+if(NOT @Preprocessor_CMAKE_FLAGS@ STREQUAL "")
+  message(
+    STATUS "Preprocessor additional CMake flags: @Preprocessor_CMAKE_FLAGS@")
+endif()
+
+ExternalProject_Add(
+  preprocessor-1.84.0-ungar
+  URL @BUNDLED_PREPROCESSOR_FILENAME@
+  URL_HASH
+    SHA256=6c5a4bf474c1ee5355d426a0a890d96186361eb66b73666bb09dc4062270435f
+  CMAKE_CACHE_ARGS
+    -DCMAKE_CXX_STANDARD:STRING=20
+    -DCMAKE_INSTALL_PREFIX:STRING=@CMAKE_CURRENT_BINARY_DIR@/preprocessor/install
+    -DBUILD_TESTING:BOOL=OFF
+    -DBOOST_PREPROCESSOR_INSTALL:BOOL=ON
+    @Preprocessor_CMAKE_FLAGS@
+  UPDATE_DISCONNECTED 1
+  DOWNLOAD_EXTRACT_TIMESTAMP 1)