adding shared pointer example

lectures/27_garbage_collection/README.md

@@ -325,3 +325,33 @@ int main() {
 - std::weak_ptr: Use with shared_ptr. Memory is destroyed when no more shared_ptrs are pointing to object. So each time a weak_ptr is used you should first “lock” the data by creating a shared_ptr.
 
 - std::scoped_ptr: (Boost) “Remembers” to delete things when they go out of scope. Alternate to auto_ptr. Cannot be copied.
+
+## 27.15 Exercise
+
+In the following program, the use count will be printed 3 times. What exact value will be printed each time?
+
+```cpp
+#include <iostream>
+#include <memory>
+
+int main(){
+        std::shared_ptr<int> age(new int(40));
+        std::cout << "age is " << *age << std::endl;
+
+        // you can never do this, which is assigning a smarter pointer to a raw pointer.
+        // int * temp = age;
+
+        {
+                std::shared_ptr<int> temp = age;
+                std::cout << "age is " << *temp << std::endl;
+                std::cout << "the use count is : " << age.use_count() << std::endl;
+        }
+        std::cout << "the use count is : " << age.use_count() << std::endl;
+
+        // give up my ownership, it decreases the reference count of the managed object by one.
+        // if that shared pointer was the last owner (i.e., reference count becomes zero), the object is deleted.
+        // the shared_ptr itself is now empty (i.e., it holds nullptr).
+        age.reset();
+        std::cout << "the use count is : " << age.use_count() << std::endl;
+}
+```

lectures/27_garbage_collection/shared_ptr1.cpp

@@ -0,0 +1,23 @@
+#include <iostream>
+#include <memory>
+
+int main(){
+	std::shared_ptr<int> age(new int(40));
+	std::cout << "age is " << *age << std::endl;
+
+	// you can never do this, which is assigning a smarter pointer to a raw pointer.
+	// int * temp = age;
+	
+	{
+		std::shared_ptr<int> temp = age;
+		std::cout << "age is " << *temp << std::endl;
+		std::cout << "the use count is : " << age.use_count() << std::endl;
+	}
+	std::cout << "the use count is : " << age.use_count() << std::endl;
+
+	// give up my ownership, it decreases the reference count of the managed object by one.
+	// if that shared pointer was the last owner (i.e., reference count becomes zero), the object is deleted. 
+	// the shared_ptr itself is now empty (i.e., it holds nullptr).
+	age.reset();
+	std::cout << "the use count is : " << age.use_count() << std::endl;
+}