Use https://github.com/martinmoene/optional-lite instead of std::optional

Since the only reason we used C++17 was to get std::optional, we downgrade
to C++11 in CMakeLists.

This commit imports version 3.2.0 of optional-lite.

Author: svedi

CMakeLists.txt

@@ -28,7 +28,7 @@ add_library (cryptolens STATIC ${SRC})
 target_link_libraries (cryptolens ${LIBS})
 target_include_directories (cryptolens PRIVATE "${cryptolens_SOURCE_DIR}/include/cryptolens")
 target_include_directories (cryptolens PUBLIC "${cryptolens_SOURCE_DIR}/include")
-set_property (TARGET cryptolens PROPERTY CXX_STANDARD 17)
+set_property (TARGET cryptolens PROPERTY CXX_STANDARD 11)
 set_property (TARGET cryptolens PROPERTY CXX_STANDARD_REQURED ON)
 
 if (${CRYPTOLENS_BUILD_TESTS})

examples/unix/cmake/CMakeLists.txt

@@ -12,25 +12,25 @@ add_subdirectory ("../../.." "cryptolens-cpp")
 
 add_executable(example_activate ../example_activate.cpp)
 target_link_libraries(example_activate cryptolens)
-set_property(TARGET example_activate PROPERTY CXX_STANDARD 17)
+set_property(TARGET example_activate PROPERTY CXX_STANDARD 11)
 set_property(TARGET example_activate PROPERTY CXX_STANDARD_REQURED ON)
 
 add_executable(example_create_trial_key ../example_create_trial_key.cpp)
 target_link_libraries(example_create_trial_key cryptolens)
-set_property(TARGET example_create_trial_key PROPERTY CXX_STANDARD 17)
+set_property(TARGET example_create_trial_key PROPERTY CXX_STANDARD 11)
 set_property(TARGET example_create_trial_key PROPERTY CXX_STANDARD_REQURED ON)
 
 add_executable(example_external ../example_external.cpp)
 target_link_libraries(example_external cryptolens)
-set_property(TARGET example_external PROPERTY CXX_STANDARD 17)
+set_property(TARGET example_external PROPERTY CXX_STANDARD 11)
 set_property(TARGET example_external PROPERTY CXX_STANDARD_REQURED ON)
 
 add_executable(example_floating ../example_floating.cpp)
 target_link_libraries(example_floating cryptolens)
-set_property(TARGET example_floating PROPERTY CXX_STANDARD 17)
+set_property(TARGET example_floating PROPERTY CXX_STANDARD 11)
 set_property(TARGET example_floating PROPERTY CXX_STANDARD_REQURED ON)
 
 add_executable(example_offline ../example_offline.cpp)
 target_link_libraries(example_offline cryptolens)
-set_property(TARGET example_offline PROPERTY CXX_STANDARD 17)
+set_property(TARGET example_offline PROPERTY CXX_STANDARD 11)
 set_property(TARGET example_offline PROPERTY CXX_STANDARD_REQURED ON)

include/cryptolens/imports/std/optional

@@ -1,5 +1,25 @@
 #pragma once
 
+#if __cplusplus < 201703L
+
+#include "../../../../third_party/optional-lite/optional.hpp"
+
+namespace cryptolens_io {
+
+namespace v20190401 {
+
+template<typename T>
+using optional = ::nonstd::optional<T>;
+
+using ::nonstd::make_optional;
+using ::nonstd::nullopt;
+
+} // namespace v20190401
+
+} // namespace cryptolens_io
+
+#else
+
 #include <optional>
 
 namespace cryptolens_io {
@@ -16,6 +36,8 @@ using std::nullopt;
 
 } // namespace cryptolens_io
 
+#endif
+
 namespace cryptolens_io {
 
 namespace latest {