Adding a minimal implementation of std::optional. (#783)

We're using C++11, which doesn't have std::optional. We need a few
features of std::optional for improved GPS support. These are
implemented here.

cartographer/common/optional.h

@@ -0,0 +1,55 @@
+/*
+ * Copyright 2017 The Cartographer Authors
+ *
+ * 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 CARTOGRAPHER_COMMON_OPTIONAL_H_
+#define CARTOGRAPHER_COMMON_OPTIONAL_H_
+
+#include <memory>
+
+#include "cartographer/common/make_unique.h"
+#include "glog/logging.h"
+
+namespace cartographer {
+namespace common {
+
+template <class T>
+class optional {
+ public:
+  optional() {}
+
+  optional(const optional& other) {
+    if (other.has_value()) {
+      value_ = common::make_unique<T>(other.value());
+    }
+  }
+
+  explicit optional(const T& value) { value_ = common::make_unique<T>(value); }
+
+  bool has_value() const { return value_ != nullptr; }
+
+  const T& value() const {
+    CHECK(value_ != nullptr);
+    return *value_;
+  }
+
+ private:
+  std::unique_ptr<T> value_;
+};
+
+}  // namespace common
+}  // namespace cartographer
+
+#endif  // CARTOGRAPHER_COMMON_OPTIONAL_H_

cartographer/common/optional_test.cc

@@ -0,0 +1,49 @@
+/*
+ * Copyright 2017 The Cartographer Authors
+ *
+ * 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.
+ */
+
+#include "cartographer/common/optional.h"
+
+#include "gtest/gtest.h"
+
+namespace cartographer {
+namespace common {
+namespace {
+
+TEST(OptionalTest, CreateDisengagedObject) {
+  const optional<int> o;
+  EXPECT_FALSE(o.has_value());
+  const optional<float> x;
+  EXPECT_FALSE(x.has_value());
+}
+
+TEST(OptionalTest, CreateWithValue) {
+  const optional<int> a(5);
+  EXPECT_TRUE(a.has_value());
+  EXPECT_EQ(5, a.value());
+}
+
+TEST(OptionalTest, CreateFromOtherOptional) {
+  const optional<int> a(5);
+  const optional<int> b = a;
+  EXPECT_TRUE(a.has_value());
+  EXPECT_TRUE(b.has_value());
+  EXPECT_EQ(5, a.value());
+  EXPECT_EQ(5, b.value());
+}
+
+}  // namespace
+}  // namespace common
+}  // namespace cartographer