schemagen: fix nullable root types (#13)

The C++ code generator now handles schemas where the top-level type is
nullable ("type": ["object", "null"], ["string", "null"], or
["array", "null"]).

Changes to weaseljson_schemagen.py:
- Rename the inner object struct when the root is a nullable object, so
  the `using Root = std::optional<...>` alias no longer conflicts with
  `struct Root`.
- Treat nullable root objects and arrays as container roots, emplacing
  the inner value before pushing the root frame and pointing the frame at
  the contained value.
- For nullable root scalars/enums, engage() now returns a pointer to the
  value inside the optional rather than to the optional wrapper itself.
- cbNull() now safely accepts a top-level null when the root is nullable
  and rejects it otherwise.

Regression tests added:
- nullable_object.schema.json + test_nullable_root.cpp
- nullable_string.schema.json
- nullable_array.schema.json

Closes #13

contrib/schemagen/CMakeLists.txt

@@ -46,6 +46,49 @@ target_link_libraries(schemagen_big PRIVATE ${PROJECT_NAME})
 target_compile_options(schemagen_big PRIVATE -Wno-switch-enum)
 add_dependencies(schemagen_big schemagen_big_h)
 
+set(NULLABLE_OBJECT_SCHEMA
+    ${CMAKE_CURRENT_SOURCE_DIR}/nullable_object.schema.json)
+set(NULLABLE_STRING_SCHEMA
+    ${CMAKE_CURRENT_SOURCE_DIR}/nullable_string.schema.json)
+set(NULLABLE_ARRAY_SCHEMA
+    ${CMAKE_CURRENT_SOURCE_DIR}/nullable_array.schema.json)
+set(NULLABLE_OBJECT_H ${CMAKE_CURRENT_BINARY_DIR}/nullable_object.h)
+set(NULLABLE_STRING_H ${CMAKE_CURRENT_BINARY_DIR}/nullable_string.h)
+set(NULLABLE_ARRAY_H ${CMAKE_CURRENT_BINARY_DIR}/nullable_array.h)
+
+add_custom_command(
+  OUTPUT ${NULLABLE_OBJECT_H}
+  COMMAND ${Python3_EXECUTABLE} ${SCHEMAGEN_SCRIPT} ${NULLABLE_OBJECT_SCHEMA} -o
+          ${NULLABLE_OBJECT_H} --namespace nullable_object
+  DEPENDS ${SCHEMAGEN_SCRIPT} ${NULLABLE_OBJECT_SCHEMA}
+  COMMENT "Generating nullable_object.h")
+
+add_custom_command(
+  OUTPUT ${NULLABLE_STRING_H}
+  COMMAND ${Python3_EXECUTABLE} ${SCHEMAGEN_SCRIPT} ${NULLABLE_STRING_SCHEMA} -o
+          ${NULLABLE_STRING_H} --namespace nullable_string
+  DEPENDS ${SCHEMAGEN_SCRIPT} ${NULLABLE_STRING_SCHEMA}
+  COMMENT "Generating nullable_string.h")
+
+add_custom_command(
+  OUTPUT ${NULLABLE_ARRAY_H}
+  COMMAND ${Python3_EXECUTABLE} ${SCHEMAGEN_SCRIPT} ${NULLABLE_ARRAY_SCHEMA} -o
+          ${NULLABLE_ARRAY_H} --namespace nullable_array
+  DEPENDS ${SCHEMAGEN_SCRIPT} ${NULLABLE_ARRAY_SCHEMA}
+  COMMENT "Generating nullable_array.h")
+
+add_custom_target(
+  schemagen_nullable_h DEPENDS ${NULLABLE_OBJECT_H} ${NULLABLE_STRING_H}
+                               ${NULLABLE_ARRAY_H})
+
+add_executable(schemagen_nullable_root
+               ${CMAKE_CURRENT_SOURCE_DIR}/test_nullable_root.cpp)
+target_include_directories(schemagen_nullable_root
+                           PRIVATE include ${CMAKE_CURRENT_BINARY_DIR})
+target_link_libraries(schemagen_nullable_root PRIVATE ${PROJECT_NAME})
+target_compile_options(schemagen_nullable_root PRIVATE -Wno-switch-enum)
+add_dependencies(schemagen_nullable_root schemagen_nullable_h)
+
 add_test(
   NAME schemagen_example
   COMMAND schemagen_example
@@ -55,3 +98,8 @@ add_test(
   NAME schemagen_big
   COMMAND schemagen_big
   WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
+
+add_test(
+  NAME schemagen_nullable_root
+  COMMAND schemagen_nullable_root
+  WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})