including shared ptr implementation

lectures/27_garbage_collection/README.md

@@ -410,3 +410,49 @@ int main(){
         return 0;
 }
 ```
+
+## 27.17 Smart Pointer Implementation
+
+This [program](shared_ptr2.cpp) implements a simple and incomplete version of shared_ptr, it does not have reference count; but it does demonstrate why there is no memory leak.
+
+```cpp
+#include <iostream>
+#include <string>
+
+template <class T>
+class shared_ptr {
+public:
+	shared_ptr(T* ptr){
+		ptr_ = ptr;
+	}
+
+	~shared_ptr(){
+		if(ptr_ != nullptr){
+			delete ptr_;
+		}
+	}
+	T& operator*(){
+		return *ptr_;
+	}
+	// operator->() is a unary operator — it only uses the left-hand operand (s1) to get access to the underlying object.
+	T* operator->(){
+		return ptr_;
+	}
+private:
+	T* ptr_;
+};
+
+int main(){
+	shared_ptr<int> age(new int(20));
+
+	std::cout << "age is " << *age << std::endl;
+
+	shared_ptr<std::string> s1(new std::string("test"));
+
+	// compiler will interpret s1->length() as (s1.operator->())->length(), and that’s just how C++ handles overloaded operator->()
+	// and s1.operator->() returns a std::string*
+	// therefore we now have: (std::string*)->length(), which is just normal pointer behavior — calling length() on the string.
+	std::cout << "length is " << s1->length() << std::endl;
+	return 0;
+}
+```