diff --git a/src/storage/repo-storage.hpp b/src/storage/repo-storage.hpp
index ed0ce9d..bc2ecbe 100644
--- a/src/storage/repo-storage.hpp
+++ b/src/storage/repo-storage.hpp
@@ -84,15 +84,14 @@
   /**
    *  @brief  read data from repo
    *  @param   interest  used to request data
-   *  @return  std::pair<bool,shared_ptr<Data> >
+   *  @return  std::shared_ptr<Data>
    */
-  shared_ptr<Data>
+  std::shared_ptr<Data>
   readData(const Interest& interest) const;
 
 private:
   Index m_index;
   Storage& m_storage;
-
 };
 
 } // namespace repo
diff --git a/src/storage/skiplist.hpp b/src/storage/skiplist.hpp
index 34c7c57..e6dd6a0 100644
--- a/src/storage/skiplist.hpp
+++ b/src/storage/skiplist.hpp
@@ -397,7 +397,7 @@
     m_head->prevs.resize(newLevel + 1, m_head);
     insertPositions.resize(newLevel + 1, m_head);
   }
-  for (int i = 0; i <= newLevel; i++) {
+  for (size_t i = 0; i <= newLevel; i++) {
     newNode->nexts[i] = insertPositions[i]->nexts[i];
     newNode->prevs[i] = insertPositions[i];
     insertPositions[i]->nexts[i] = newNode;
diff --git a/src/storage/sqlite-storage.cpp b/src/storage/sqlite-storage.cpp
index 78b1792..7edcda7 100644
--- a/src/storage/sqlite-storage.cpp
+++ b/src/storage/sqlite-storage.cpp
@@ -25,6 +25,8 @@
 
 namespace repo {
 
+using std::string;
+
 SqliteStorage::SqliteStorage(const string& dbPath)
   : m_size(0)
 {
diff --git a/src/storage/sqlite-storage.hpp b/src/storage/sqlite-storage.hpp
index 73579c7..168cc41 100755
--- a/src/storage/sqlite-storage.hpp
+++ b/src/storage/sqlite-storage.hpp
@@ -48,7 +48,7 @@
   };
 
   explicit
-  SqliteStorage(const string& dbPath);
+  SqliteStorage(const std::string& dbPath);
 
   virtual
   ~SqliteStorage();
@@ -72,7 +72,7 @@
    *  @brief  get the data from database
    *  @para   id   id number of each entry in the database, used to find the data
    */
-  virtual shared_ptr<Data>
+  virtual std::shared_ptr<Data>
   read(const int64_t id);
 
   /**
@@ -86,7 +86,7 @@
    *         insertItemToIndex to reubuild index from database
    */
   void
-  fullEnumerate(const ndn::function<void(const Storage::ItemMeta)>& f);
+  fullEnumerate(const std::function<void(const Storage::ItemMeta)>& f);
 
 private:
   void
@@ -94,7 +94,7 @@
 
 private:
   sqlite3* m_db;
-  string m_dbPath;
+  std::string m_dbPath;
   int64_t m_size;
 };
 
diff --git a/src/storage/storage.hpp b/src/storage/storage.hpp
index 04f366f..bf75306 100755
--- a/src/storage/storage.hpp
+++ b/src/storage/storage.hpp
@@ -76,7 +76,7 @@
    *  @brief  get the data from database
    *  @param  id   id number of each entry in the database, used to find the data
    */
-  virtual shared_ptr<Data>
+  virtual std::shared_ptr<Data>
   read(const int64_t id) = 0;
 
   /**
@@ -90,7 +90,7 @@
    *         insertItemToIndex to reubuild index from database
    */
   virtual void
-  fullEnumerate(const ndn::function<void(const Storage::ItemMeta)>& f) = 0;
+  fullEnumerate(const std::function<void(const Storage::ItemMeta)>& f) = 0;
 
 };