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, 130 insertions, 0 deletions

diff --git a/Firestore/core/src/firebase/firestore/model/field_value.h b/Firestore/core/src/firebase/firestore/model/field_value.h
new file mode 100644
index 0000000..781e34f
--- /dev/null
+++ b/Firestore/core/src/firebase/firestore/model/field_value.h
@@ -0,0 +1,130 @@
+/*
+ * 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.
+ */
+
+#ifndef FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_MODEL_FIELD_VALUE_H_
+#define FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_MODEL_FIELD_VALUE_H_
+
+#include <memory>
+#include <vector>
+
+namespace firebase {
+namespace firestore {
+namespace model {
+
+/**
+ * tagged-union class representing an immutable data value as stored in
+ * Firestore. FieldValue represents all the different kinds of values
+ * that can be stored in fields in a document.
+ */
+class FieldValue {
+ public:
+  /**
+   * All the different kinds of values that can be stored in fields in
+   * a document. The types of the same comparison order should be defined
+   * together as a group. The order of each group is defined by the Firestore
+   * backend and is available at:
+   *     https://firebase.google.com/docs/firestore/manage-data/data-types
+   */
+  enum class Type {
+    Null,     // Null
+    Boolean,  // Boolean
+    Long,     // Number type starts here
+    Double,
+    Timestamp,  // Timestamp type starts here
+    ServerTimestamp,
+    String,     // String
+    Blob,       // Blob
+    Reference,  // Reference
+    GeoPoint,   // GeoPoint
+    Array,      // Array
+    Object,     // Object
+    // New enum should not always been added at the tail. Add it to the correct
+    // position instead, see the doc comment above.
+  };
+
+  FieldValue() : tag_(Type::Null) {
+  }
+
+  // Do not inline these ctor/dtor below, which contain call to non-trivial
+  // operator=.
+  FieldValue(const FieldValue& value);
+  FieldValue(FieldValue&& value);
+
+  ~FieldValue();
+
+  FieldValue& operator=(const FieldValue& value);
+  FieldValue& operator=(FieldValue&& value);
+
+  /** Returns the true type for this value. */
+  Type type() const {
+    return tag_;
+  }
+
+  /** factory methods. */
+  static const FieldValue& NullValue();
+  static const FieldValue& TrueValue();
+  static const FieldValue& FalseValue();
+  static const FieldValue& BooleanValue(bool value);
+  static FieldValue ArrayValue(const std::vector<const FieldValue>& value);
+  static FieldValue ArrayValue(std::vector<const FieldValue>&& value);
+
+  friend bool operator<(const FieldValue& lhs, const FieldValue& rhs);
+
+ private:
+  explicit FieldValue(bool value) : tag_(Type::Boolean), boolean_value_(value) {
+  }
+
+  /**
+   * Switch to the specified type, if different from the current type.
+   */
+  void SwitchTo(const Type type);
+
+  Type tag_;
+  union {
+    // There is no null type as tag_ alone is enough for Null FieldValue.
+    bool boolean_value_;
+    std::vector<const FieldValue> array_value_;
+  };
+};
+
+/** Compares against another FieldValue. */
+bool operator<(const FieldValue& lhs, const FieldValue& rhs);
+
+inline bool operator>(const FieldValue& lhs, const FieldValue& rhs) {
+  return rhs < lhs;
+}
+
+inline bool operator>=(const FieldValue& lhs, const FieldValue& rhs) {
+  return !(lhs < rhs);
+}
+
+inline bool operator<=(const FieldValue& lhs, const FieldValue& rhs) {
+  return !(lhs > rhs);
+}
+
+inline bool operator!=(const FieldValue& lhs, const FieldValue& rhs) {
+  return lhs < rhs || lhs > rhs;
+}
+
+inline bool operator==(const FieldValue& lhs, const FieldValue& rhs) {
+  return !(lhs != rhs);
+}
+
+}  // namespace model
+}  // namespace firestore
+}  // namespace firebase
+
+#endif  // FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_MODEL_FIELD_VALUE_H_