Inheritance

Lecture contents Inheritance Class hierarchy Types of Inheritance Derived and Base classes derived class constructors protected access identifier multiple inheritance

Introduction Inheritance –Software reusability –Create new class from existing class Absorb existing class’s data and behaviors Enhance with new capabilities –Derived class inherits from base class Derived class –More specialized group of objects –Behaviors inherited from base class »Can customize –Additional behaviors

Class hierarchy –Direct base class Inherited explicitly (one level up hierarchy) –Indirect base class Inherited two or more levels up hierarchy –Single inheritance Inherits from one base class –Multiple inheritance Inherits from multiple base classes –Base classes possibly unrelated

Types of inheritance Three types of inheritance –public Every object of derived class also object of base class –Base-class objects not objects of derived classes Can access non- private members of base class –private Alternative to composition –protected Rarely used

Abstraction –Focus on commonalities among objects in system “is-a” vs. “has-a” –“is-a” Inheritance Derived class object treated as base class object Example: Car is a vehicle –Vehicle properties/behaviors also car properties/behaviors –“has-a” Composition Object contains one or more objects of other classes as members Example: Car has a steering wheel Vehicle Car

Base Classes and Derived Classes –Object of one class “is an” object of another class Example: Rectangle is quadrilateral. –Class Rectangle inherits from class Quadrilateral –Quadrilateral : base class –Rectangle : derived class –Base class typically represents larger set of objects than derived classes Example: –Base class: Vehicle »Cars, trucks, boats, bicycles, … –Derived class: Car »Smaller, more-specific subset of vehicles

What a derived class inherits Every data member defined in the parent class (although such members may not always be accessible in the derived class!) Every ordinary member function of the parent class (although such members may not always be accessible in the derived class!) The same initial data layout as the base class

What a derived class doesn't inherit The base class's constructors and destructor The base class's assignment operator The base class's friends

What a derived class can add –New data members –New member functions –New constructors and destructor –New friends

What happens when a derived-class object is created and destroyed Space is allocated (on the stack or the heap) for the full object (that is, enough space to store the data members inherited from the base class plus the data members defined in the derived class itself) The base class's constructor is called to initialize the data members inherited from the base class The derived class's constructor is then called to initialize the data members added in the derived class The derived-class object is then usable When the object is destroyed (goes out of scope or is deleted) the derived class's destructor is called on the object first Then the base class's destructor is called on the object Finally the allocated space for the full object is reclaimed

Base Classes and Derived Classes Inheritance examples

Base Classes and Derived Classes Inheritance hierarchy –Inheritance relationships: tree-like hierarchy structure –Each class becomes Base class –Supply data/behaviors to other classes OR Derived class –Inherit data/behaviors from other classes

Single inheritance CommunityMember EmployeeStudent AdministratorTeacher AdministratorTeacher StaffFaculty Alumnus Single inheritance Multiple inheritance Inheritance hierarchy for university CommunityMember s.

Shape TwoDimensionalShapeThreeDimensionalShape CircleSquareTriangleSphereCubeTetrahedron Inheritance hierarchy for Shapes.

Base Classes and Derived Classes public inheritance –Specify with: Class TwoDimensionalShape : public Shape Class TwoDimensionalShape inherits from class Shape –Base class private members Not accessible directly Still inherited –Manipulate through inherited member functions –Base class public and protected members Not inherited

protected Members protected access –Intermediate level of protection between public and private –protected members accessible to Base class members Base class friend s Derived class members Derived class friend s

#include using namespace std; class CPolygon { protected: int width, height; public: void set_values (int a, int b) { width=a; height=b;} }; class CRectangle: public CPolygon { public: int area () { return (width * height); } }; Example

class CTriangle: public CPolygon { public: int area () { return (width * height / 2); } }; int main () { CRectangle rect; CTriangle trgl; rect.set_values (4,5); trgl.set_values (4,5); cout << rect.area() << endl; cout << trgl.area() << endl; return 0; }

Output 20 10

Relationship between Base Classes and Derived Classes Using protected data members –Advantages Derived classes can modify values directly Slight increase in performance –Avoid set/get function call overhead –Disadvantages No validity checking –Derived class can assign illegal value Implementation dependent –Derived class member functions more likely dependent on base class implementation –Base class implementation changes may result in derived class modifications

Constructors and Destructors in Derived Classes Instantiating derived-class object –Chain of constructor calls Derived-class constructor invokes base class constructor –Implicitly or explicitly Base of inheritance hierarchy –Last constructor called in chain –First constructor body to finish executing –Example: Point3 / Circle4 / Cylinder hierarchy »Point3 constructor called last »Point3 constructor body finishes execution first Initializing data members –Each base-class constructor initializes data members »Inherited by derived class

Constructors and Destructors in Derived Classes Destroying derived-class object –Chain of destructor calls Reverse order of constructor chain Destructor of derived-class called first Destructor of next base class up hierarchy next –Continue up hierarchy until final base reached »After final base-class destructor, object removed from memory

Example // constructors and derived classes #include using namespace std; class mother { public: mother () { cout << "mother: no parameters\n"; } mother (int a) { cout << "mother: int parameter\n"; } }; class daughter : public mother { public: daughter (int a) { cout << "daughter: int parameter\n\n"; } };

class son : public mother { public: son (int a) : mother (a) { cout << "son: int parameter\n\n"; } }; int main () { daughter cynthia (0); son daniel(0); return 0; }

Output mother: no parameters daughter: int parameter mother: int parameter son: int parameter

Multiple Inheritance A class inherits members from more than one class. This is done by simply separating the different base classes with commas in the derived class declaration. For example, if we had a specific class to print on screen (COutput) and we wanted our classes CRectangle and CTriangle to also inherit its members in addition to those of CPolygon we could write: class CRectangle: public CPolygon, public COutput; class CTriangle: public CPolygon, public COutput;

Example // multiple inheritance #include using namespace std; class CPolygon { protected: int width, height; public: void set_values (int a, int b) { width=a; height=b;} }; class COutput { public: void output (int i); };

void COutput::output (int i) { cout << i << endl; } class CRectangle: public CPolygon, public COutput { public: int area () { return (width * height); } }; class CTriangle: public CPolygon, public COutput { public: int area () { return (width * height / 2); } };

int main () { CRectangle rect; CTriangle trgl; rect.set_values (4,5); trgl.set_values (4,5); rect.output (rect.area()); trgl.output (trgl.area()); return 0; }

Output 20 10