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diff --git a/Firestore/core/src/firebase/firestore/model/base_path.h b/Firestore/core/src/firebase/firestore/model/base_path.h
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+/*
+ * 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_BASE_PATH_H_
+#define FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_MODEL_BASE_PATH_H_
+
+#include <algorithm>
+#include <cctype>
+#include <initializer_list>
+#include <string>
+#include <utility>
+#include <vector>
+
+#include "Firestore/core/src/firebase/firestore/util/firebase_assert.h"
+
+namespace firebase {
+namespace firestore {
+namespace model {
+namespace impl {
+
+/**
+ * BasePath represents a path sequence in the Firestore database. It is composed
+ * of an ordered sequence of string segments.
+ *
+ * BasePath is reassignable and movable. Apart from those, all other mutating
+ * operations return new independent instances.
+ *
+ * ## Subclassing Notes
+ *
+ * BasePath is strictly meant as a base class for concrete implementations. It
+ * doesn't contain a single virtual method, can't be instantiated, and should
+ * never be used in any polymorphic way. BasePath is templated to allow static
+ * factory methods to return objects of the derived class (the expected
+ * inheritance would use CRTP: struct Derived : BasePath<Derived>).
+ */
+template <typename T>
+class BasePath {
+ protected:
+  using SegmentsT = std::vector<std::string>;
+
+ public:
+  using const_iterator = SegmentsT::const_iterator;
+
+  /** Returns i-th segment of the path. */
+  const std::string& operator[](const size_t i) const {
+    FIREBASE_ASSERT_MESSAGE(i < segments_.size(), "index %u out of range", i);
+    return segments_[i];
+  }
+
+  /** Returns the first segment of the path. */
+  const std::string& first_segment() const {
+    FIREBASE_ASSERT_MESSAGE(!empty(),
+                            "Cannot call first_segment on empty path");
+    return segments_[0];
+  }
+  /** Returns the last segment of the path. */
+  const std::string& last_segment() const {
+    FIREBASE_ASSERT_MESSAGE(!empty(), "Cannot call last_segment on empty path");
+    return segments_[size() - 1];
+  }
+
+  size_t size() const {
+    return segments_.size();
+  }
+  bool empty() const {
+    return segments_.empty();
+  }
+
+  const_iterator begin() const {
+    return segments_.begin();
+  }
+  const_iterator end() const {
+    return segments_.end();
+  }
+
+  /**
+   * Returns a new path which is the result of concatenating this path with an
+   * additional segment.
+   */
+  T Append(const std::string& segment) const {
+    auto appended = segments_;
+    appended.push_back(segment);
+    return T{std::move(appended)};
+  }
+  T Append(std::string&& segment) const {
+    auto appended = segments_;
+    appended.push_back(std::move(segment));
+    return T{std::move(appended)};
+  }
+
+  /**
+   * Returns a new path which is the result of concatenating this path with an
+   * another path.
+   */
+  T Append(const T& path) const {
+    auto appended = segments_;
+    appended.insert(appended.end(), path.begin(), path.end());
+    return T{std::move(appended)};
+  }
+
+  /**
+   * Returns a new path which is the result of omitting the first n segments of
+   * this path.
+   */
+  T PopFirst(const size_t n = 1) const {
+    FIREBASE_ASSERT_MESSAGE(n <= size(),
+                            "Cannot call PopFirst(%u) on path of length %u", n,
+                            size());
+    return T{begin() + n, end()};
+  }
+
+  /**
+   * Returns a new path which is the result of omitting the last segment of
+   * this path.
+   */
+  T PopLast() const {
+    FIREBASE_ASSERT_MESSAGE(!empty(), "Cannot call PopLast() on empty path");
+    return T{begin(), end() - 1};
+  }
+
+  /**
+   * Returns true if this path is a prefix of the given path.
+   *
+   * Empty path is a prefix of any path. Any path is a prefix of itself.
+   */
+  bool IsPrefixOf(const T& rhs) const {
+    return size() <= rhs.size() && std::equal(begin(), end(), rhs.begin());
+  }
+
+  bool operator==(const BasePath& rhs) const {
+    return segments_ == rhs.segments_;
+  }
+  bool operator!=(const BasePath& rhs) const {
+    return segments_ != rhs.segments_;
+  }
+  bool operator<(const BasePath& rhs) const {
+    return segments_ < rhs.segments_;
+  }
+  bool operator>(const BasePath& rhs) const {
+    return segments_ > rhs.segments_;
+  }
+  bool operator<=(const BasePath& rhs) const {
+    return segments_ <= rhs.segments_;
+  }
+  bool operator>=(const BasePath& rhs) const {
+    return segments_ >= rhs.segments_;
+  }
+
+ protected:
+  BasePath() = default;
+  template <typename IterT>
+  BasePath(const IterT begin, const IterT end) : segments_{begin, end} {
+  }
+  BasePath(std::initializer_list<std::string> list) : segments_{list} {
+  }
+  BasePath(SegmentsT&& segments) : segments_{std::move(segments)} {
+  }
+
+ private:
+  SegmentsT segments_;
+};
+
+}  // namespace impl
+}  // namespace model
+}  // namespace firestore
+}  // namespace firebase
+
+#endif  // FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_MODEL_BASE_PATH_H_