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+# CSE332S Lecture 12
+
+## Object-Oriented Programming (OOP) in C++
+
+Today:
+
+1. Type vs. Class
+2. Subtypes and Substitution
+3. Polymorphism
+    a. Parametric polymorphism (generic programming)
+    b. Subtyping polymorphism (OOP)
+4. Inheritance and Polymorphism in C++
+    a. construction/destruction order
+    b. Static vs. dynamic type
+    c. Dynamic binding via virtual functions
+    d. Declaring interfaces via pure virtual functions
+
+## Type vs. Class, substitution
+
+### Type (interface) vs. Class
+
+Each function/operator declared by an object has a signature: name, parameter list, and return value
+
+The set of all public signatures defined by an object makes up the interface to the object, or its type
+
+- An object’s type is known (what can we request of an object?)
+- Its implementation is not - different objects may implement an interface very differently
+- An object may have many types (think interfaces in Java)
+
+An object’s class defines its implementation:
+
+- Specifies its state (internal data and its representation)
+- Implements the functions/operators it declares
+
+### Subtyping: Liskov Substitution Principle
+
+An interface may contain other interfaces!
+
+A type is a **subtype** if it contains the full interface of another type (its **supertype**) as a subset of its own interface. (subtype has more methods than supertype)
+
+**Substitutability**: if S is a subtype of T, then objects of type T may be replaced with objects of type S
+
+Substitutability leads to **polymorphism**: a single interface may have many different implementations
+
+## Polymorphism
+
+Parametric (interface) polymorphism (substitution applied to generic programming)
+
+- Design algorithms or classes using **parameterized types** rather than specific concrete data types.
+- Any class that defines the full interface required of the parameterized type (is a **subtype** of the parameterized type) can be substituted in place of the type parameter **at compile-time**.
+- Allows substitution of **unrelated types**.
+
+### Polymorphism in OOP
+
+Subtyping (inheritance) polymorphism: (substitution applied to OOP)
+
+- A derived class can inherit an interface from its parent (base) class
+  - Creates a subtype/supertype relationship. (subclass/superclass)
+  - All subclasses of a superclass inherit the superclass’s interface and its implementation of that interface.
+  - Function overriding - subclasses may override the superclass’s implementation of an interface
+    - Allows the implementation of an interface to be substituted at run-time via dynamic binding
+
+## Inheritance in C++ - syntax
+
+### Forms of Inheritance in C++
+
+A derived class can inherit from a base class in one of 3 ways:
+
+- Public Inheritance ("is a", creates a subtype)
+  - Public part of base class remains public
+  - Protected part of base class remains protected
+- Protected Inheritance ("contains a", **derived class is not a subtype**)
+  - Public part of base class becomes protected
+  - Protected part of base class remains protected
+- Private Inheritance ("contains a", **derived class is not a subtype**)
+  - Public part of base class becomes private
+  - Protected part of base class becomes private
+
+So public inheritance is the only way to create a **subtype**.
+
+```cpp
+class A {
+public:
+ int i;
+protected:
+ int j;
+private:
+ int k;
+};
+class B : public A {
+// ...
+};
+class C : protected A {
+// ...
+};
+class D : private A {
+// ...
+};
+```
+
+Class B uses public inheritance from A
+
+- `i` remains public to all users of class B
+- `j` remains protected. It can be used by methods in class B or its derived classes
+
+Class C uses protected inheritance from A
+
+- `i` becomes protected in C, so the only users of class C that can access `i` are the methods of class C
+- `j` remains protected. It can be used by methods in class C or its derived classes
+
+Class D uses private inheritance from A
+
+- `i` and `j` become private in D, so only methods of class D can access them.
+
+## Construction and Destruction Order of derived class objects
+
+### Class and Member Construction Order
+
+```cpp
+class A {
+public:
+ A(int i) : m_i(i) {
+ cout << "A" << endl;}
+ ~A() {cout<<"~A"<<endl;}
+private:
+ int m_i;
+};
+class B : public A {
+public:
+ B(int i, int j)
+ : A(i), m_j(j) {
+ cout << "B" << endl;}
+ ~B() {cout << "~B" << endl;}
+private:
+ int m_j;
+};
+int main (int, char *[]) {
+ B b(2,3);
+ return 0;
+};
+```
+
+In the main function, the B constructor is called on object b
+
+- Passes in integer values 2 and 3
+
+B constructor calls A constructor
+
+- passes value 2 to A constructor via base/member initialization list
+
+A constructor initializes `m_i` with the passed value 2
+
+- Body of A constructor runs
+- Outputs "A"
+
+B constructor initializes `m_j` with passed value 3
+
+- Body of B constructor runs
+- outputs "B"
+
+### Class and Member Destruction Order
+
+```cpp
+class A {
+public:
+ A(int i) : m_i(i) {
+ cout << "A" << endl;}
+ ~A() {cout<<"~A"<<endl;}
+private:
+ int m_i;
+};
+class B : public A {
+public:
+ B(int i, int j)
+ : A(i), m_j(j) {
+ cout << "B" << endl;}
+ ~B() {cout << "~B" << endl;}
+private:
+ int m_j;
+};
+int main (int, char *[]) {
+ B b(2,3);
+ return 0;
+};  
+```
+
+B destructor called on object b in main
+
+- Body of B destructor runs
+- outputs "~B"
+
+B destructor calls “destructor” of m_j
+
+- int is a built-in type, so it’s a no-op
+
+B destructor calls A destructor
+
+- Body of A destructor runs
+- outputs "~A"
+
+A destructor calls “destructor” of m_i
+
+- again a no-op
+
+At the level of each class, order of steps is reversed in constructor vs. destructor
+
+- ctor: base class, members, body
+- dtor: body, members, base class
+
+In short, cascading order is called when constructor is called, and reverse cascading order is called when destructor is called.
+
+## Polymorphic function calls - function overriding
+
+### Static vs. Dynamic type
+
+The type of a variable is known statically (at compile time), based on its declaration
+
+```cpp
+int i; int * p;
+Fish f; Mammal m;
+Fish * fp = &f;
+```
+
+However, actual types of objects aliased by references & pointers to base classes vary dynamically (at run-time)
+
+```cpp
+Fish f; Mammal m;
+Animal * ap = &f; // dynamic type is Fish
+ap = &m; // dynamic type is Mammal
+Animal & ar = get_animal(); // dynamic type is the type of the object returned by get_animal()
+```
+
+A base class and its derived classes form a set of types
+
+`type(*ap)` $\in$ `{Animal, Fish, Mammal}`
+`typeset(*fp)` $\subset$ `typeset(*ap)`
+
+Each type set is **open**
+
+- More subclasses can be added
+
+### Supporting Function Overriding in C++: Virtual Functions
+
+Static binding: A function/operator call is bound to an implementation at compile-time
+
+Dynamic binding: A function/operator call is bound to an implementation at run-time. When dynamic binding is used:
+
+1. Lookup the dynamic type of the object the function/operator is called on
+2. Bind the call to the implementation defined in that class
+
+Function overriding requires dynamic binding!
+
+In C++, virtual functions facilitate dynamic binding.
+
+```cpp
+class A {
+public:
+ A () {cout<<" A";}
+ virtual ~A () {cout<<" ~A";} // tells compiler that this destructor might be overridden in a derived class (the destructor of the parent class is usually virtual)
+ virtual void f(int); // tells compiler that this function might be overridden in a derived class
+};
+class B : public A {
+public:
+ B () :A() {cout<<" B";}
+ virtual ~B() {cout<<" ~B";}
+ virtual void f(int) override; // tells compiler that this function might be overridden in a derived class, the parent function is virtual otherwise it will be an error
+//C++11
+};
+int main (int, char *[]) {
+ // prints "A B"
+ A *ap = new B;
+ // prints "~B ~A" : would only
+ // print "~A" if non-virtual
+ delete ap;
+ return 0;
+};
+```
+
+Virtual functions:
+
+- Declared virtual in a base class
+- Can override in derived classes
+- Overriding only happens when signatures are the same
+
+- Otherwise it just overloads the function or operator name
+
+When called through a pointer or reference to a base class:
+
+- function/operator calls are resolved dynamically
+
+Use `final` (C++11) to prevent overriding of a virtual method
+
+Use `override` (C++11) in derived class to ensure that the signatures match (error if not)
+
+```cpp
+class A {
+public:
+ void x() {cout<<"A::x";};
+ virtual void y() {cout<<"A::y";};
+};
+class B : public A {
+public:
+ void x() {cout<<"B::x";};
+ virtual void y() {cout<<"B::y";};
+};
+int main () {
+ B b;
+ A *ap = &b; B *bp = &b;
+ b.x (); // prints "B::x": static binding always calls the x() function of the class of the object
+ b.y (); // prints "B::y": static binding always calls the y() function of the class of the object
+ bp->x (); // prints "B::x": lookup the type of bp, which is B, and x() is non-virtual so it is statically bound
+ bp->y (); // prints "B::y": lookup the dynamic type of bp, which is B (at run-time), and call the overridden y() function
+ ap->x (); // prints "A::x": lookup the type of ap, which is A, and x() is non-virtual so it is statically bound
+ ap->y (); // prints "B::y": lookup the dynamic type of ap, which is B (at run-time), and call the overridden y() function of class B
+ return 0;
+};
+```
+
+Only matter with pointer or reference
+
+- Calls on object itself resolved statically
+- E.g., `b.y();`
+
+Look first at pointer/reference type
+
+- If non-virtual there, resolve statically
+- E.g., `ap->x();`
+- If virtual there, resolve dynamically
+- E.g., `ap->y();`
+
+Note that virtual keyword need not be repeated in derived classes
+
+- But it’s good style to do so
+
+Caller can force static resolution of a virtual function via scope operator
+
+- E.g., `ap->A::y();` prints “A::y”
+
+Potential Problem: Class Slicing
+
+When a derived type may be caught by a catch block, passed into a function, or returned out of a function that expects a base type:
+
+- Be sure to catch by reference
+- Pass by reference
+- Return by reference
+
+Otherwise, a copy is made:
+
+- Loses original object's "dynamic type"
+- Only the base parts of the object are copied, resulting in the class slicing problem
+
+## Class (implementation) Inheritance VS. Interface
+
+Inheritance
+Class is the implementation of a type.
+
+- Class inheritance involves inheriting interface and implementation
+  - Internal state and representation of an object
+
+Interface is the set of operations that can be called on an object.
+
+- Interface inheritance involves inheriting only a common interface
+  - What operations can be called on an object of the type?
+  - Subclasses are related by a common interface
+  - But may have very different implementations
+
+In C++, pure virtual functions make interface inheritance possible.
+
+```cpp
+class A { // the abstract base class
+public:
+ virtual void x() = 0; // pure virtual function, no default implementation
+ virtual void y() = 0; // pure virtual function, no default implementation
+};
+class B : public A { // B is still an abstract class because it still has a pure virtual function y() that is not defined
+public:
+ virtual void x();
+};
+class C : public B { // C is a concrete derived class because it has all the pure virtual functions defined
+public:
+ virtual void y();
+};
+int main () {
+ A * ap = new C; // ap is a pointer to an abstract class type, but it can point to a concrete derived class object, cannot create an object of an abstract class, for example, new A() will be an error.
+ ap->x ();
+ ap->y ();
+ delete ap;
+ return 0;
+};
+```
+
+Pure Virtual Functions and Abstract Base Classes:
+
+A is an **abstract (base) class**
+
+- Similar to an interface in Java
+- Declares pure virtual functions (=0)
+- May also have non-virtual methods, as well as virtual methods that are not pure virtual
+
+Derived classes override pure virtual methods
+
+- B overrides `x()`, C overrides `y()`
+
+Can't instantiate an abstract class
+
+- class that declares pure virtual functions
+- or inherits ones that are not overridden
+
+A and B are abstract, can create a C
+
+Can still have a pointer or reference to an abstract class type
+
+- Useful for polymorphism
+
+## Review of Inheritance and Subtyping Polymorphism in C++
+
+Create related subclasses via public inheritance from a common superclass
+
+- All subclasses inherit the interface and its implementation from the superclass
+
+Override superclass implementation via function overriding
+
+- Relies on virtual functions to support dynamic binding of function/operator calls
+
+Use pure virtual functions to declare a common interface that related subclasses can implement
+
+- Client code uses the common interface, does not care how the interface is defined. Reduces complexity and dependencies between objects in a system.