CIS 250 - Pointers, virtual functions, abstract classes

Objectives

• use pointers
• use virtual functions
• use pure virtual functions
• explain what makes a class abstract

Pointers

• declaring a pointer: int *p;
• initializing a pointer: p = &n; // assumes n is an int variable
• using a pointer: *p = 55;
• using a pointer to a struct:

     struct obj { int i; };
     obj* pobj = new obj;
     (*pobj).i = 32;
     pobj->i = 16;

Dynamic memory allocation

• allocating memory dynamically: int *p = new int;
• deallocating memory: delete p;
• allocating an array dynamically: int *p2 = new int[10];
• deallocating an array: delete[] p2;
• allocating an object's memory dynamically: obj *p3 = new obj;
• deallocating an object's memory: delete p3;
• if a class dynamically allocates memory, you should write a destructor for it to deallocate the memory (to replace the default one written for it)
• if a class dynamically allocates memory, you should probably write an assignment operator overload for it (to replace the default one written for it)
• if a class dynamically allocates memory, you should write a copy constructor for it (to replace the default one written for it)
• if a class dynamically allocates memory, you should write a default constructor if one is wanted (to replace the default constructor provided)
• a base class with a destructor to deallocate memory should make the destructor virtual

Virtual functions

• compile-time binding is also called static binding and early binding
• early binding sets what functions will be called at compilation
• run-time binding is also called dynamic binding and late binding
• late binding decides which functions to call (base or derived class) at run time
• the keyword virtual makes a function late binding
• the keyword virtual is only used in the base class
• a pure virtual function has no body; it is specified with this in place of the body: =0;
• it is up to derived classes to implement the pure virtual functions of the base class
• classes with one or more virtual functions are called abstract classes
• abstract classes can not be instantiated