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Published byJustin Barton Modified over 8 years ago
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Defining Data Types in C++ Part 2: classes
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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
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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
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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!
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Example object: the Nesper sign Member variables: –message –time & temperature Member functions: –display message –display time & temp –change message –change time & temp
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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; };
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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
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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
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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
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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
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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
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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
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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
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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));
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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
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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
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Constructor Prototypes class Nesper { public: Nesper (string msg); // initializes message Nesper (int temp); // initializes temperature Nesper (clock tm); // initializes time Nesper ( ); // default constructor
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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
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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
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Constructor implementation Nesper::Nesper (string msg) { message = msg; }
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Another example: default ctor Nesper::Nesper( ) { message = “Your message here”; temperature = 32; time = 12:00; }
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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