implement FieldValue for null, boolean, and array in C++. (#637)

* implement FieldValue for null and boolean.

* refactoring to use union type instead of poly

* refactor using union design intead of poly

* refactoring to use anonymous union and fix styles

* small fix

* add field_value_test to the project

* fix warning of cmake and fix style

1 files changed, 22 insertions, 0 deletions

diff --git a/Firestore/core/src/firebase/firestore/model/CMakeLists.txt b/Firestore/core/src/firebase/firestore/model/CMakeLists.txt
new file mode 100644
index 0000000..1beb3bb
--- /dev/null
+++ b/Firestore/core/src/firebase/firestore/model/CMakeLists.txt
@@ -0,0 +1,22 @@
+# Copyright 2018 Google
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+add_library(
+  firebase_firestore_model
+  field_value.cc
+)
+target_link_libraries(
+  firebase_firestore_model
+  firebase_firestore_util
+)