build the example as C++20

Author: PingLiuPing

example/CMakeLists.txt

@@ -20,13 +20,65 @@ cmake_minimum_required(VERSION 3.25)
 
 project(example)
 
-set(CMAKE_CXX_STANDARD 23)
+set(CMAKE_CXX_STANDARD 20)
 
 find_package(iceberg CONFIG REQUIRED COMPONENTS bundle rest)
 
+if(TARGET iceberg::iceberg_bundle_shared)
+  set(ICEBERG_BUNDLE_TARGET iceberg::iceberg_bundle_shared)
+else()
+  set(ICEBERG_BUNDLE_TARGET iceberg::iceberg_bundle_static)
+endif()
+
+if(TARGET iceberg::iceberg_rest_shared)
+  set(ICEBERG_REST_TARGET iceberg::iceberg_rest_shared)
+else()
+  set(ICEBERG_REST_TARGET iceberg::iceberg_rest_static)
+endif()
+
 add_executable(demo_example demo_example.cc)
 
-target_link_libraries(demo_example
-                      PRIVATE "$<IF:$<TARGET_EXISTS:iceberg::iceberg_bundle_shared>,iceberg::iceberg_bundle_shared,iceberg::iceberg_bundle_static>"
-                              "$<IF:$<TARGET_EXISTS:iceberg::iceberg_rest_shared>,iceberg::iceberg_rest_shared,iceberg::iceberg_rest_static>"
-)
+target_link_libraries(demo_example PRIVATE ${ICEBERG_BUNDLE_TARGET}
+                                           ${ICEBERG_REST_TARGET})
+
+# Compile every installed public header as a C++20 consumer. The installed include
+# tree is the public API contract: iceberg_install_all_headers excludes internal
+# headers before packaging them.
+get_target_property(ICEBERG_BUNDLE_INCLUDE_DIRS ${ICEBERG_BUNDLE_TARGET}
+                    INTERFACE_INCLUDE_DIRECTORIES)
+foreach(ICEBERG_INCLUDE_DIR IN LISTS ICEBERG_BUNDLE_INCLUDE_DIRS)
+  if(EXISTS "${ICEBERG_INCLUDE_DIR}/iceberg")
+    set(ICEBERG_PUBLIC_INCLUDE_DIR "${ICEBERG_INCLUDE_DIR}")
+    break()
+  endif()
+endforeach()
+
+if(NOT ICEBERG_PUBLIC_INCLUDE_DIR)
+  message(FATAL_ERROR "Could not locate iceberg's installed public headers")
+endif()
+
+file(GLOB_RECURSE ICEBERG_PUBLIC_HEADERS CONFIGURE_DEPENDS
+     "${ICEBERG_PUBLIC_INCLUDE_DIR}/iceberg/*.h"
+     "${ICEBERG_PUBLIC_INCLUDE_DIR}/iceberg/*.hpp")
+list(SORT ICEBERG_PUBLIC_HEADERS)
+
+set(ICEBERG_PUBLIC_HEADER_CHECK_SOURCE
+    "// Generated from iceberg's installed public headers.\n")
+foreach(ICEBERG_PUBLIC_HEADER IN LISTS ICEBERG_PUBLIC_HEADERS)
+  file(RELATIVE_PATH ICEBERG_PUBLIC_HEADER_RELATIVE_PATH
+       "${ICEBERG_PUBLIC_INCLUDE_DIR}" "${ICEBERG_PUBLIC_HEADER}")
+  string(APPEND ICEBERG_PUBLIC_HEADER_CHECK_SOURCE
+         "#include <${ICEBERG_PUBLIC_HEADER_RELATIVE_PATH}>\n")
+endforeach()
+string(APPEND ICEBERG_PUBLIC_HEADER_CHECK_SOURCE "\nint main() { return 0; }\n")
+
+file(GENERATE OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/public_headers_cxx20.cc"
+     CONTENT "${ICEBERG_PUBLIC_HEADER_CHECK_SOURCE}")
+
+add_executable(public_headers_cxx20
+               "${CMAKE_CURRENT_BINARY_DIR}/public_headers_cxx20.cc")
+set_target_properties(public_headers_cxx20 PROPERTIES CXX_STANDARD 20
+                                              CXX_STANDARD_REQUIRED ON
+                                              CXX_EXTENSIONS OFF)
+target_link_libraries(public_headers_cxx20
+                      PRIVATE ${ICEBERG_BUNDLE_TARGET} ${ICEBERG_REST_TARGET})