binarystore: Cleanup optional config with default values.

Change-Id: If76d85f7aafbec30756dd01d871a78389d6e87a8
Signed-off-by: Willy Tu <wltu@google.com>
diff --git a/include/parse_config.hpp b/include/parse_config.hpp
index 5bef8ad..766830c 100644
--- a/include/parse_config.hpp
+++ b/include/parse_config.hpp
@@ -31,16 +31,12 @@
     j.at("sysFilePath").get_to(config.sysFilePath);
     if (j.contains("offsetBytes"))
     {
-        uint32_t val;
-        j.at("offsetBytes").get_to(val);
-        config.offsetBytes = val;
+        j.at("offsetBytes").get_to(config.offsetBytes.emplace());
     }
 
     if (j.contains("maxSizeBytes"))
     {
-        uint32_t val;
-        j.at("maxSizeBytes").get_to(val);
-        config.maxSizeBytes = val;
+        j.at("maxSizeBytes").get_to(config.maxSizeBytes.emplace());
     }
 }
 
diff --git a/src/main.cpp b/src/main.cpp
index bb45f55..8e3d244 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -65,9 +65,8 @@
             "Loading from config with",
             entry("BASE_ID=%s", config.blobBaseId.c_str()),
             entry("FILE=%s", config.sysFilePath.c_str()),
-            entry("MAX_SIZE=%llx",
-                  static_cast<unsigned long long>(
-                      config.maxSizeBytes ? *config.maxSizeBytes : 0)));
+            entry("MAX_SIZE=%llx", static_cast<unsigned long long>(
+                                       config.maxSizeBytes.value_or(0))));
 
         auto file = std::make_unique<binstore::SysFileImpl>(config.sysFilePath,
                                                             config.offsetBytes);
diff --git a/src/sys_file_impl.cpp b/src/sys_file_impl.cpp
index 9fab76a..7aadfba 100644
--- a/src/sys_file_impl.cpp
+++ b/src/sys_file_impl.cpp
@@ -25,7 +25,7 @@
     sys(sys)
 {
     fd_ = sys->open(path.c_str(), O_RDWR);
-    offset_ = offset ? *offset : 0;
+    offset_ = offset.value_or(0);
 
     if (fd_ < 0)
     {