diff --git a/include/cppjieba/FullSegment.hpp b/include/cppjieba/FullSegment.hpp
index 2f101ee..d859e81 100644
--- a/include/cppjieba/FullSegment.hpp
+++ b/include/cppjieba/FullSegment.hpp
@@ -37,7 +37,7 @@ class FullSegment: public SegmentBase {
     }
     words.clear();
     words.reserve(wrs.size());
-    GetStringsFromWordRanges(wrs, words);
+    GetStringsFromWordRanges(sentence, wrs, words);
   }
   void Cut(RuneStrArray::const_iterator begin, 
         RuneStrArray::const_iterator end, 
diff --git a/include/cppjieba/HMMSegment.hpp b/include/cppjieba/HMMSegment.hpp
index 84e69aa..0038a49 100644
--- a/include/cppjieba/HMMSegment.hpp
+++ b/include/cppjieba/HMMSegment.hpp
@@ -35,7 +35,7 @@ class HMMSegment: public SegmentBase {
     }
     words.clear();
     words.reserve(wrs.size());
-    GetStringsFromWordRanges(wrs, words);
+    GetStringsFromWordRanges(sentence, wrs, words);
   }
   void Cut(RuneStrArray::const_iterator begin, RuneStrArray::const_iterator end, vector<WordRange>& res) const {
     RuneStrArray::const_iterator left = begin;
diff --git a/include/cppjieba/MPSegment.hpp b/include/cppjieba/MPSegment.hpp
index b386ae9..2bffeb8 100644
--- a/include/cppjieba/MPSegment.hpp
+++ b/include/cppjieba/MPSegment.hpp
@@ -38,7 +38,7 @@ class MPSegment: public SegmentBase {
     }
     words.clear();
     words.reserve(wrs.size());
-    GetStringsFromWordRanges(wrs, words);
+    GetStringsFromWordRanges(sentence, wrs, words);
   }
   void Cut(RuneStrArray::const_iterator begin,
            RuneStrArray::const_iterator end,
diff --git a/include/cppjieba/MixSegment.hpp b/include/cppjieba/MixSegment.hpp
index e096815..82f078b 100644
--- a/include/cppjieba/MixSegment.hpp
+++ b/include/cppjieba/MixSegment.hpp
@@ -31,7 +31,7 @@ class MixSegment: public SegmentBase {
     }
     words.clear();
     words.reserve(wrs.size());
-    GetStringsFromWordRanges(wrs, words);
+    GetStringsFromWordRanges(sentence, wrs, words);
   }
 
   void Cut(RuneStrArray::const_iterator begin, RuneStrArray::const_iterator end, vector<WordRange>& res, bool hmm) const {
diff --git a/include/cppjieba/QuerySegment.hpp b/include/cppjieba/QuerySegment.hpp
index 15a684e..0b04ce6 100644
--- a/include/cppjieba/QuerySegment.hpp
+++ b/include/cppjieba/QuerySegment.hpp
@@ -37,7 +37,7 @@ class QuerySegment: public SegmentBase {
     }
     words.clear();
     words.reserve(wrs.size());
-    GetStringsFromWordRanges(wrs, words);
+    GetStringsFromWordRanges(sentence, wrs, words);
   }
   void Cut(RuneStrArray::const_iterator begin, RuneStrArray::const_iterator end, vector<WordRange>& res, bool hmm) const {
     //use mix Cut first
diff --git a/include/cppjieba/Unicode.hpp b/include/cppjieba/Unicode.hpp
index 37813d2..923fcdb 100644
--- a/include/cppjieba/Unicode.hpp
+++ b/include/cppjieba/Unicode.hpp
@@ -9,16 +9,19 @@
 
 namespace cppjieba {
 
+using std::string;
+using std::vector;
+
 typedef uint32_t Rune;
 
 struct RuneStr {
   Rune rune;
-  const char* str;
+  uint32_t offset;
   uint32_t len;
-  RuneStr(): rune(0), str(NULL), len(0) {
+  RuneStr(): rune(0), offset(0), len(0) {
   }
-  RuneStr(Rune r, const char* s, uint32_t l)
-    : rune(r), str(s), len(l) {
+  RuneStr(Rune r, uint32_t o, uint32_t l)
+    : rune(r), offset(o), len(l) {
   }
 }; // struct RuneStr
 
@@ -118,14 +121,14 @@ inline bool DecodeRunesInString(const char* s, size_t len, RuneStrArray& runes)
     if (rp.len == 0) {
       return false;
     }
-    RuneStr x(rp.rune, s + i, rp.len);
+    RuneStr x(rp.rune, i, rp.len);
     runes.push_back(x);
     i += rp.len;
   }
   return true;
 }
 
-inline bool DecodeRunesInString(const std::string& s, RuneStrArray& runes) {
+inline bool DecodeRunesInString(const string& s, RuneStrArray& runes) {
   return DecodeRunesInString(s.c_str(), s.size(), runes);
 }
 
@@ -142,37 +145,38 @@ inline bool DecodeRunesInString(const char* s, size_t len, Unicode& unicode) {
   return true;
 }
 
-inline bool IsSingleWord(const std::string& str) {
+inline bool IsSingleWord(const string& str) {
   RuneStrLite rp = DecodeRuneInString(str.c_str(), str.size());
   return rp.len == str.size();
 }
 
-inline bool DecodeRunesInString(const std::string& s, Unicode& unicode) {
+inline bool DecodeRunesInString(const string& s, Unicode& unicode) {
   return DecodeRunesInString(s.c_str(), s.size(), unicode);
 }
 
-inline Unicode DecodeRunesInString(const std::string& s) {
+inline Unicode DecodeRunesInString(const string& s) {
   Unicode result;
   DecodeRunesInString(s, result);
   return result;
 }
 
 
-//[left, right]
-inline std::string GetStringFromRunes(RuneStrArray::const_iterator left, RuneStrArray::const_iterator right) {
-  assert(right->str >= left->str);
-  return std::string(left->str, right->str - left->str + right->len);
+// [left, right]
+inline string GetStringFromRunes(const string& s, RuneStrArray::const_iterator left, RuneStrArray::const_iterator right) {
+  assert(right->offset >= left->offset);
+  uint32_t len = right->offset - left->offset + right->len;
+  return s.substr(left->offset, len);
 }
 
-inline void GetStringsFromWordRanges(const std::vector<WordRange>& wrs, std::vector<std::string>& words) {
+inline void GetStringsFromWordRanges(const string& s, const vector<WordRange>& wrs, vector<string>& words) {
   for (size_t i = 0; i < wrs.size(); i++) {
-    words.push_back(GetStringFromRunes(wrs[i].left, wrs[i].right));
+    words.push_back(GetStringFromRunes(s, wrs[i].left, wrs[i].right));
   }
 }
 
-inline std::vector<std::string> GetStringsFromWordRanges(const std::vector<WordRange>& wrs) {
-  std::vector<std::string> result;
-  GetStringsFromWordRanges(wrs, result);
+inline vector<string> GetStringsFromWordRanges(const string& s, const vector<WordRange>& wrs) {
+  vector<string> result;
+  GetStringsFromWordRanges(s, wrs, result);
   return result;
 }
 
diff --git a/test/unittest/pre_filter_test.cpp b/test/unittest/pre_filter_test.cpp
index 7a532b9..7ff080e 100644
--- a/test/unittest/pre_filter_test.cpp
+++ b/test/unittest/pre_filter_test.cpp
@@ -20,7 +20,7 @@ TEST(PreFilterTest, Test1) {
     while (filter.HasNext()) {
       PreFilter::Range range;
       range = filter.Next();
-      words.push_back(GetStringFromRunes(range.begin, range.end - 1));
+      words.push_back(GetStringFromRunes(s, range.begin, range.end - 1));
     }
     res = limonp::Join(words.begin(), words.end(), "/");
     ASSERT_EQ(res, expected);
@@ -35,7 +35,7 @@ TEST(PreFilterTest, Test1) {
     while (filter.HasNext()) {
       PreFilter::Range range;
       range = filter.Next();
-      words.push_back(GetStringFromRunes(range.begin, range.end - 1));
+      words.push_back(GetStringFromRunes(s, range.begin, range.end - 1));
     }
     res = limonp::Join(words.begin(), words.end(), "/");
     ASSERT_EQ(res, expected);