[feat][cmake] Support c++20. default c++17.

CMakeLists.txt

@@ -51,12 +51,15 @@ option(BRPC_ENABLE_CPU_PROFILER "Enable brpc cpu profiler" OFF)
 option(XDPROCKS_PATH "Enable xdprocks raw engine")
 option(BUILD_PYTHON_SDK "Build python sdk" OFF)
 
+
 message(STATUS CMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE})
 message(STATUS THIRD_PARTY_BUILD_TYPE=${THIRD_PARTY_BUILD_TYPE})
 
 SET(CMAKE_C_FLAGS_DEBUG "$ENV{CFLAGS} -O0  -g3 -ggdb")
 SET(CMAKE_CXX_FLAGS_DEBUG "$ENV{CXXFLAGS} -O0  -g3 -ggdb")
 
+message(STATUS "CMAKE_CXX_STANDARD: ${CMAKE_CXX_STANDARD}")
+
 # xdprocks
 if(XDPROCKS_PATH)
     message(STATUS "Enable xdprocks, path: ${XDPROCKS_PATH}")
@@ -104,19 +107,33 @@ if(ENABLE_COVERAGE)
 endif()
 
 include(CheckCXXCompilerFlag)
-check_cxx_compiler_flag("-std=c++17" COMPILER_SUPPORTS_CXX17)
-if(COMPILER_SUPPORTS_CXX17)
-    set(CMAKE_CXX_STANDARD 17)
-    set(CMAKE_CXX_STANDARD_REQUIRED TRUE)
+if (CMAKE_CXX_STANDARD EQUAL 20)
+    check_cxx_compiler_flag("-std=c++20" COMPILER_SUPPORTS_CXX20)
+    if(COMPILER_SUPPORTS_CXX20)
+        set(CMAKE_CXX_STANDARD 20)
+        set(CMAKE_CXX_STANDARD_REQUIRED TRUE)
+    else()
+        message(FATAL_ERROR "The compiler ${CMAKE_CXX_COMPILER} has no C++20 support. Please use a different C++ compiler.")
+    endif()
 else()
-    message(FATAL_ERROR "The compiler ${CMAKE_CXX_COMPILER} has no C++17 support. Please use a different C++ compiler.")
+    check_cxx_compiler_flag("-std=c++17" COMPILER_SUPPORTS_CXX17)
+    if(COMPILER_SUPPORTS_CXX17)
+        set(CMAKE_CXX_STANDARD 17)
+        set(CMAKE_CXX_STANDARD_REQUIRED TRUE)
+    else()
+        message(FATAL_ERROR "The compiler ${CMAKE_CXX_COMPILER} has no C++17 support. Please use a different C++ compiler.")
+    endif()
 endif()
 
 if(THIRD_PARTY_BUILD_TYPE MATCHES "Debug")
     set(CMAKE_STATIC_LIBRARY_SUFFIX "d.a")
 endif()
 
-set(CMAKE_CXX_STANDARD 17)
+if (CMAKE_CXX_STANDARD EQUAL 20)
+    set(CMAKE_CXX_STANDARD 20)
+else()
+    set(CMAKE_CXX_STANDARD 17)
+endif()
 # set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 
 # Avoid warning about DOWNLOAD_EXTRACT_TIMESTAMP in CMake 3.24: