use C++17 (#703)

* create branch

* fix

CMakeLists.txt

@@ -1,7 +1,12 @@
 cmake_minimum_required (VERSION 3.18)
 
-set(CMAKE_CXX_STANDARD 14)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
+
+if(NOT DEFINED CMAKE_CXX_STANDARD)
+  set(CMAKE_CXX_STANDARD 17)
+endif()
+
+
 set(CMAKE_POSITION_INDEPENDENT_CODE ON)
 set(CMAKE_INSTALL_RPATH_USE_LINK_PATH TRUE)
 set(CMAKE_CUDA_SEPARABLE_COMPILATION ON)
@@ -128,7 +133,7 @@ list(APPEND CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/cmake)
 
 target_compile_features(hiop_options
   INTERFACE
-  cxx_std_14
+  cxx_std_17
   cxx_alignas
   cxx_alignof
   cxx_attributes
@@ -238,7 +243,7 @@ if(HIOP_USE_GPU)
     check_language(CUDA)
 
     if(NOT DEFINED CMAKE_CUDA_STANDARD)
-      set(CMAKE_CUDA_STANDARD 14)
+      set(CMAKE_CUDA_STANDARD 17)
       set(CMAKE_CUDA_STANDARD_REQUIRED ON)
     endif()