[optinal] use std::optional on newer compilers

cmake/modules/CheckCXXFeatures.cmake

@@ -22,6 +22,9 @@
 #    :code:`DUNE_HAVE_CXX_CLASS_TEMPLATE_ARGUMENT_DEDUCTION`
 #       True if C++17's class template argument deduction is supported
 #
+#    :code:`DUNE_HAVE_CXX_OPTIONAL`
+#       True if C++17's optional implementation is supported
+#
 # .. cmake_variable:: DISABLE_CXX_VERSION_CHECK
 #
 #    You may set this variable to TRUE to disable checking for
@@ -350,6 +353,21 @@ check_cxx_source_compiles("
 " DUNE_HAVE_CXX_CLASS_TEMPLATE_ARGUMENT_DEDUCTION
   )
 
+
+# support for C++17's optional implementation
+check_cxx_source_compiles("
+  #include <optional>
+  #include <string>
+
+  int main()
+  {
+    std::optional< std::string > a;
+    std::string b = a.value_or( \"empty\" );
+  }
+" DUNE_HAVE_CXX_OPTIONAL
+  )
+
+
 # find the threading library
 # Use a copy FindThreads from CMake 3.1 due to its support of pthread
 if(NOT DEFINED THREADS_PREFER_PTHREAD_FLAG)

config.h.cmake

@@ -29,6 +29,9 @@
 /* does the compiler support C++17's class template argument deduction? */
 #cmakedefine DUNE_HAVE_CXX_CLASS_TEMPLATE_ARGUMENT_DEDUCTION 1
 
+/* does the compiler support C++17's optional? */
+#cmakedefine DUNE_HAVE_CXX_OPTIONAL 1
+
 /* does the compiler support conditionally throwing exceptions in constexpr context? */
 #cmakedefine DUNE_SUPPORTS_CXX_THROW_IN_CONSTEXPR 1
 

dune/common/std/optional.hh

@@ -7,12 +7,24 @@
 #include <type_traits>
 #include <utility>
 
+#ifdef DUNE_HAVE_CXX_OPTIONAL
+#include <optional>
+#endif // #ifdef DUNE_HAVE_CXX_OPTIONAL
+
+
 namespace Dune
 {
 
   namespace Std
   {
 
+#ifdef DUNE_HAVE_CXX_OPTIONAL
+    // In case of C++ standard >= 17 we forward optionals into our namespace
+    template< class T >
+    using optional = std::optional< T >;
+#else
+    // In case of C++ standard < 17 we take the fallback implementation
+
     // nullopt
     // -------
 
@@ -414,11 +426,14 @@ namespace Dune
       return optional< typename std::decay< T >::type >( std::forward< T >( value ) );
     }
 
+#endif //#ifdef DUNE_HAVE_CXX_OPTIONAL
+
   } // namespace Std
 
 } // namespace Dune
 
 
+#ifndef DUNE_HAVE_CXX_OPTIONAL
 namespace std
 {
 
@@ -447,4 +462,6 @@ namespace std
 
 } // namespace std
 
+#endif //#ifndef DUNE_HAVE_CXX_OPTIONAL
+
 #endif // #ifndef DUNE_COMMON_STD_OPTIONAL_HH