Defining Data Types in C++ Part 2: classes

Quick review of OOP Object: combination of: –data structures (describe object attributes) –functions (describe object behaviors) Class: C++ mechanism used to represent an object Class definition includes: –member functions –member variables

Information Hiding Principle says we should only know what we need to know (to prevent getting bogged down in irrelevant detail) A class that uses information hiding in its design is an Abstract Data Type (ADT –member variables hidden –access to hidden members available only through member functions

Class declaration promotes information hiding class Name { public: //member functions //declared here private: //member variables //declared here }; “Name” is a valid identifier public section includes class parts accessible to outside world private section includes hidden parts Note semicolon!

Example object: the Nesper sign Member variables: –message –time & temperature Member functions: –display message –display time & temp –change message –change time & temp

class Nesper { public: void display_message ( ) const; void display_tnt ( ) const; void change_message (char[] text); void change_tnt (int temp, clock current); private: char message[25]; int temperature; clock time; };

Class definition: public section class Nesper { public: void display_message ( ) const; void display_tnt ( ) const;... Public section contains member function prototypes Keyword “const” indicates that the functions declared here do not change the object

Class definition: public section... void change_message (char[] text); void change_tnt (int temp, clock current); “Change” functions are examples of modifiers - - member functions that alter the calling object change_tnt includes a parameter (current) that is an example of an ADT variable

Class definition: private section … private: char message[25]; int temperature; clock time; }; Member variables declared here Can be variables of built-in types, arrays or pointer, or instances of objects defined elsewhere

Where do they go? Class declaration: header file (xxxx.h) Member function definitions: definition file –xxxx.cpp (same name as header -- different extension) –should not include a main( ) function Class instances: user program

Implementing member functions #include void Nesper::change_message (char[] text) { assert (strlen(text)<25); message = text; } Preprocessor directives for all needed library routines Class name and scope operator (::) indicate this is a member function

Notes on member functions Every instance of a class (object) has its own copies of all class members, including member functions A member function is called by the object that owns it (can have several objects of same class in a program) Member functions may call other member functions

Using a class in a program #include “nesper.h” int main( ) { Nesper sign1, sign2;... Preprocessor directive indicates class definition should be included Declaring Nesper variable instantiates the class sign1 and sign2 are objects of type Nesper

Using a class in a program … sign1.change_message (“This is my sign”); sign1.display_message( );... Member function is activated by calling object Syntax: calling_object. function_name (argument(s));

Constructors Special member function Automatically called when an object is instantiated Unique characteristics: –Constructor name is same as class name –Constructor has no return value (not even void) Can have multiple constructors for a class -- example of function overloading

Why define constructors? Can define a class without one; in this case, compiler uses automatic default constructor –memory allocated for member variables –constructors for individual members are called, if they exist With defined constructor, can do more -- including initializing member variables

Constructor Prototypes class Nesper { public: Nesper (string msg); // initializes message Nesper (int temp); // initializes temperature Nesper (clock tm); // initializes time Nesper ( ); // default constructor

Notes on Function Overloading Can have as many functions with the same name in a class as you wish Compiler differentiates between the functions by their parameter lists A constructor that requires no arguments is the default constructor -- will be called under most circumstances

Calling a constructor Constructor is called when an object is declared: Nesper stopsign (“Stop! Stop, I say!”); // calls first ctor in class Nesper midnight (12:00); // calls ctor with clock parameter Nesper sign; // calls default constructor

Constructor implementation Nesper::Nesper (string msg) { message = msg; }

Another example: default ctor Nesper::Nesper( ) { message = “Your message here”; temperature = 32; time = 12:00; }

Default arguments Functions can be declared with default values listed for their parameters Provides flexibility for constructors: –can be called with or without arguments –can be called with some, but not all arguments specified –if no arguments are specified, default values are used

Nesper constructor with default arguments class Nesper { public: Nesper (string msg = “Your message here”, int temp = 32, clock tm = 12:00);... Default arguments are specified in the function prototype, not the implementation When function is called, can omit some or all arguments -- may be omitted starting from right

Examples of function calls Nesper mysign (“Taurus”, 65, 4:30); // all defaults replaced by actual arguments Nesper yoursign (“Pisces”); // msg replaced by argument; use defaults for // remaining values Nesper sign; // most common -- defaults used for all values

Implementation of function with default arguments Nesper::Nesper (string msg, int temp, clock tm) { message = msg; temperature = temp; time = tm; } Implementation is identical to version that called for the same parameters but didn’t use default arguments Default arguments appear only in prototype

One more variation: inline functions Inline functions are defined (implemented) within the class definition –Saves a little execution time (no function call, no return) –Can be inefficient in terms of memory (can end up with many copies of same compiled code Best for extremely simple, “one-liner” functions

Inline constructor example class Nesper { public: Nesper( ) { message = “Your message”; temperature = 32; time = 12:00; }... Inline functions aren’t usually used for constructors, unless the object is very small This is still just one function declaration within class definition

Value semantics Operations that determine how values are copied from one object to another object of the same class type Assignment operator Copy constructor: constructor that instantiates an object which is an exact copy of its argument

Automatic assignment example Nesper sign1(“Hi”, 32, 1:00), sign2; // sign2 has default values sign2 = sign1; // sign2 now has same // values as sign1 Can use automatic assignment when the object doesn’t use dynamic memory Later we’ll see how to define the assignment operation for classes that require it

Automatic copy constructor Nesper sign1; … Nesper sign2(sign1);... Nesper sign3 = sign1; First object uses default constructor Both second and third objects use copy constructor, even though third example looks like assignment Like automatic assignment, must be explicitly defined for some classes

Assignment vs. copy constructor Assignment copies information from one existing object into another existing object Copy constructor declares and initializes a new object, which is a copy of an existing object

One more look at Nesper.h #ifndef NESPER_H// macro guard -- use to ensure that class declaration #define NESPER_H// only appears once in a program -- safeguard // to prevent linking errors in large programs class Nesper { public: Nesper (string msg=“Your message”, int temp=32, clock tm=12:00); void display_message( ) const; void display_tnt( ) const; void change_message (string); void change_tnt (int temp, clock tm); private: string message; int temperature; clock time; }; #endif