Implement assignment operator for common::optional. (#800)

Implement assignment operator for common::optional.

cartographer/common/optional.h

@@ -45,6 +45,20 @@ class optional {
     return *value_;
   }
 
+  optional<T>& operator=(const T& other_value) {
+    this->value_ = common::make_unique<T>(other_value);
+    return *this;
+  }
+
+  optional<T>& operator=(const optional<T>& other) {
+    if (!other.has_value()) {
+      this->value_ = nullptr;
+    } else {
+      this->value_ = common::make_unique<T>(other.value());
+    }
+    return *this;
+  }
+
  private:
   std::unique_ptr<T> value_;
 };

cartographer/common/optional_test.cc

@@ -44,6 +44,20 @@ TEST(OptionalTest, CreateFromOtherOptional) {
   EXPECT_EQ(5, b.value());
 }
 
+TEST(OptionalTest, AssignmentOperator) {
+  optional<int> a(5);
+  optional<int> b(4);
+  optional<int> c;
+  a = b;
+  EXPECT_TRUE(a.has_value());
+  EXPECT_EQ(4, a.value());
+  a = c;
+  EXPECT_FALSE(a.has_value());
+  a = 3;
+  EXPECT_TRUE(a.has_value());
+  EXPECT_EQ(3, a.value());
+}
+
 }  // namespace
 }  // namespace common
 }  // namespace cartographer