diff --git a/ch12/ex12_19.h b/ch12/ex12_19.h
index 7c470803..43d55aba 100644
--- a/ch12/ex12_19.h
+++ b/ch12/ex12_19.h
@@ -42,11 +42,22 @@ class StrBlob {
         check(0, "pop_back on empty StrBlob");
         data->pop_back();
     }
-    const std::string& front() {
+
+    std::string& front() {
+        check(0, "front on empty StrBlob");
+        return data->front();
+    }
+
+    std::string& back() {
+        check(0, "back on empty StrBlob");
+        return data->back();
+    }
+
+    const std::string& front() const {
         check(0, "front on empty StrBlob");
         return data->front();
     }
-    const std::string& back() {
+    const std::string& back() const {
         check(0, "back on empty StrBlob");
         return data->back();
     }
diff --git a/ch12/ex12_22.h b/ch12/ex12_22.h
index 57bdcd35..d3c8835d 100644
--- a/ch12/ex12_22.h
+++ b/ch12/ex12_22.h
@@ -28,35 +28,46 @@ class StrBlob {
 public:
     using size_type = vector<string>::size_type;
     friend class ConstStrBlobPtr;
-    
+
     ConstStrBlobPtr begin() const; // should add const
     ConstStrBlobPtr end() const; // should add const
-    
+
     StrBlob():data(std::make_shared<vector<string>>()) { }
     StrBlob(std::initializer_list<string> il):data(std::make_shared<vector<string>>(il)) { }
-    
+
     size_type size() const { return data->size(); }
     bool empty() const { return data->empty(); }
-    
+
     void push_back(const string &t) { data->push_back(t); }
     void pop_back() {
         check(0, "pop_back on empty StrBlob");
         data->pop_back();
     }
-    const std::string& front() {
+
+    std::string& front() {
+        check(0, "front on empty StrBlob");
+        return data->front();
+    }
+
+    std::string& back() {
+        check(0, "back on empty StrBlob");
+        return data->back();
+    }
+
+    const std::string& front() const {
         check(0, "front on empty StrBlob");
         return data->front();
     }
-    const std::string& back() {
+    const std::string& back() const {
         check(0, "back on empty StrBlob");
         return data->back();
     }
-    
+
 private:
     void check(size_type i, const string &msg) const {
         if (i >= data->size()) throw std::out_of_range(msg);
     }
-    
+
 private:
     std::shared_ptr<vector<string>> data;
 };
@@ -75,7 +86,7 @@ class ConstStrBlobPtr {
         ++curr;
         return *this;
     }
-    
+
 private:
     std::shared_ptr<vector<string>> check(size_t i, const string &msg) const {
         auto ret = wptr.lock();
@@ -84,7 +95,7 @@ class ConstStrBlobPtr {
         return ret;
     }
     std::weak_ptr<vector<string>> wptr;
-    size_t curr; 
+    size_t curr;
 };
 
-#endif
\ No newline at end of file
+#endif
diff --git a/ch13/README.md b/ch13/README.md
index 93b69c60..8816802d 100644
--- a/ch13/README.md
+++ b/ch13/README.md
@@ -93,7 +93,7 @@ The compiler defines a synthesized destructor for any class that does not define
 
 When a `StrBlob` object destroyed, the `use_count` of the dynamic object will decrement. It will be freed if no `shared_ptr` to that dynamic object.
 
-When a `StrBlobPter` object is destroyed the object dynamicaly allocated will not be freed.
+When a `StrBlobPter` object is destroyed the object dynamically allocated will not be freed.
 
 ## [Exercise 13.11](ex13_11.h)
 
@@ -169,16 +169,16 @@ QueryResult& operator=(const QueryResult) = delete;
 Check 13.22.
 
 ## Exercise 13.24:
->What would happen if the version of HasPtr in this section didn’t define a destructor? What if HasPtr didn’t define the copy constructor?
+>What would happen if the version of `HasPtr` in this section didn’t define a destructor? What if `HasPtr` didn’t define the copy constructor?
 
 If `HasPtr` didn't define a destructor, memory leak will happened. If `HasPtr` didn't define the copy constructor, when assignment happened, just points copied, the string witch `ps` points haven't been copied.
 
 ## Exercise 13.25:
->Assume we want to define a version of StrBlob that acts like a value. Also assume that we want to continue to use a shared_ptr so that our StrBlobPtr class can still use a weak_ptr to the vector. Your revised class will need a copy constructor and copy-assignment operator but will not need a destructor. Explain what the copy constructor and copyassignment operators must do. Explain why the class does not need a destructor.
+>Assume we want to define a version of `StrBlob` that acts like a value. Also assume that we want to continue to use a shared_ptr so that our `StrBlobPtr` class can still use a weak_ptr to the vector. Your revised class will need a copy constructor and copy-assignment operator but will not need a destructor. Explain what the copy constructor and copy-assignment operators must do. Explain why the class does not need a destructor.
 
-Copy constructor and copy-assignment operator should dynamicly allocate memory for its own , rather than share the object with the right hand operand.
+Copy constructor and copy-assignment operator should dynamically allocate memory for its own , rather than share the object with the right hand operand.
 
-`StrBlob` is using smart pointers which can be managed with synthesized destructor, If an object of `StrBlob` is out of scope, the destructor for std::shared_ptr will be called automaticaly to free the memory dynamically allocated when the `use_count` goes to 0.
+`StrBlob` is using smart pointers which can be managed with synthesized destructor, If an object of `StrBlob` is out of scope, the destructor for std::shared_ptr will be called automatically to free the memory dynamically allocated when the `use_count` goes to 0.
 
 ## Exercise 13.26 [hpp](ex13_26.h) | [cpp](ex13_26.cpp)
 
diff --git a/ch13/ex13_26.h b/ch13/ex13_26.h
index 07734d3d..70b3f4ef 100644
--- a/ch13/ex13_26.h
+++ b/ch13/ex13_26.h
@@ -5,7 +5,7 @@
 //  Created by pezy on 1/19/15.
 //  Copyright (c) 2014 pezy. All rights reserved.
 //
-//  Write your own version of the StrBlob class described in the previous exercise. 
+//  Write your own version of the StrBlob class described in the previous exercise.
 //
 //  @See    ex12_22 and ex13_25
 
@@ -26,40 +26,51 @@ class StrBlob {
 public:
     using size_type = vector<string>::size_type;
     friend class ConstStrBlobPtr;
-    
+
     ConstStrBlobPtr begin() const;
     ConstStrBlobPtr end() const;
-    
+
     StrBlob():data(std::make_shared<vector<string>>()) { }
     StrBlob(std::initializer_list<string> il):data(std::make_shared<vector<string>>(il)) { }
-    
+
     // copy constructor
     StrBlob(const StrBlob& sb) : data(std::make_shared<vector<string>>(*sb.data)) {}
     // copyassignment operators
     StrBlob& operator=(const StrBlob& sb);
-    
+
     size_type size() const { return data->size(); }
     bool empty() const { return data->empty(); }
-    
+
     void push_back(const string &t) { data->push_back(t); }
     void pop_back() {
         check(0, "pop_back on empty StrBlob");
         data->pop_back();
     }
-    const std::string& front() {
+
+    std::string& front() {
         check(0, "front on empty StrBlob");
         return data->front();
     }
-    const std::string& back() {
+
+    std::string& back() {
         check(0, "back on empty StrBlob");
         return data->back();
     }
-    
+
+    const std::string& front() const {
+        check(0, "front on empty StrBlob");
+        return data->front();
+    }
+    const std::string& back() const {
+        check(0, "back on empty StrBlob");
+        return data->back();
+    }
+
 private:
     void check(size_type i, const string &msg) const {
         if (i >= data->size()) throw std::out_of_range(msg);
     }
-    
+
 private:
     std::shared_ptr<vector<string>> data;
 };
@@ -78,7 +89,7 @@ class ConstStrBlobPtr {
         ++curr;
         return *this;
     }
-    
+
 private:
     std::shared_ptr<vector<string>> check(size_t i, const string &msg) const {
         auto ret = wptr.lock();
@@ -87,7 +98,7 @@ class ConstStrBlobPtr {
         return ret;
     }
     std::weak_ptr<vector<string>> wptr;
-    size_t curr; 
+    size_t curr;
 };
 
 #endif