3. More About Data Structures and ADTs: C++ Data Types ( Read §3.1 & §3.2) - PowerPoint PPT Presentation

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  1. 3. More About Data Structures and ADTs:C++ Data Types(Read §3.1 & §3.2) • C++ types an be classified as: • Fundamental(or simple or scalar): • A data object of one of these types is a single object. • - int, double, char, bool, complex, and the related types (unsigned, short, etc.) - enumerations • Structured: • Collections of data • arrays, structs, unions, classes, valarrays, bitsets, the containers and adapters in STL

  2. Structs vs. Classes Similarities 1. Essentially the same syntax 2. Both are used to model objectswith multiple attributes(characteristics) represented as data members (also called fields or instance or attribute variables). Thus, both are used to process non-homogeneous data sets.

  3. Differences • 1. C does not provide classes; C++ provides both structs and classes • 2. Members of a struct by default are public (can be accessed outside the struct by using the dot operator.)In C++ they can be declared to be private(cannot be accessed outside the struct.) • 3. Members of a class by default are private(cannot be accessed outside the class) but can be explicitlydeclared to be public.

  4. C structs C Structs vs. C++ Classes (& Structs)("traditional" vs "OOP") C++'s structs and classes model objects that have:  Attributes (characteristics) represented as and data members  Operations (behaviors) represented as (also called methods). function members Operations This leads to a whole new style of programming: object-oriented. Objects are self-contained, possessing their own operations — commonly called the I can do it myself principle — rather than being passed as a parameter to an external function that operates on them and sends them back. Function Members Data Members Attributes

  5. Declaring a Class Common Forms: class ClassName { // private by default Declarations of private members public: Declarations of public members }; class ClassName { public: //"interface" of class given first Declarations of public members private: Declarations of private members };

  6. Notes: Data members are normally placed in the private section of a class; Operations public: Function Members function members in the public section. private: • Some programmers prefer to put the private section first: • - Using the default access for classes • - Can omit the private: specifier • Other programmers put the public interface of the class first and the hidden private details last. (In the text, the latter approach is used.) • Although not commonly done, a class may have several private and public sections; the keywords private: and public: mark the beginning of each. Data Members Attributes

  7. Access to Class Members A particular instance of a class is called an object:ClassNameobject_name; Private members can be accessed only within the class (object)(except by friend functions to be described later). Public members can be accessed within or outside the class(object); to access them outside the class (object), one must use the dot operator: object_name.public_member_name A class declaration is usually placed in a header file whose name is ClassName.h . The library is then called a class library.

  8. Note the doc! Header file for class Time — Version 1 /** Time.h -------------------------------------------------------- This header file defines the data type Time for processing time. Basic operations are: Set: To set the time Display: To display the time ----------------------------------------------------------------------*/ #include <iostream>using namespace std; class Time{/******** Member functions ********/public: /* Set sets the data members of a Time object to specified values. * * Receive: hours, the number of hours in standard time * minutes, the number of minutes in standard time * AMPM ('A' if AM, 'P' if PM) * Postcondition: The Time object containing this function has its * myHours, myMinutes, and myAMorPM members set to hours, * minutes, and am_pm, respectively, and myMilTime to * the equivalent military time ********************************************************************/ Plus: Name, Course #, Lab/Project #, etc. void Set(unsigned hours, unsigned minutes, char am_pm);

  9. /* Display displays time in standard and military format using • * output stream out. • * • * Receive: ostream out • * Output: The time represented by the Time object containing • * this function • * Passes back: The ostream out with time inserted into it *****************************************************************/ void Display(ostream & out) const; /********** Data Members **********/ private: unsigned myHours,myMinutes; char myAMorPM; // 'A' or 'P' unsigned myMilTime; // military time equivalent}; // end of class declaration Notes: 1. "my" in data members names is a reminder of internal ("I can do it myself") perspective. 2. const at end of Display()'s prototype makes it a const functionwhich means it cannot modify any of the data members.Good idea to protect data members from accidental modification. "Accessors"vs. "Mutators"

  10. 3. Why make data members private? (p. 91) "Hidden" data members:  Cannot be accessed outside class  Application programs must interact with an object through its interface Public member functions control interaction between programs and class.  Application programs need not know about implementation!  Implementation maychange (improve storage, simpler algorithms. etc.) If interface is constant, programs using an object need not be changed. What's wrong with tying application code to implementation details?  A change in implementation forces a change in the application code Increased upgrade time Increased programmer cost Decreased programmer productivity Reduced profits due to — Delayed releases/upgrades — Loss of customer confidence in software reliability Always define data members of a class as private.

  11. Implementation of a Class Class declaration contains: Declarations of data members Prototypes (declarations) of function members Definitions of function members are not usually placed in class declaration Avoid cluttering up the interfaceDefinitions placed outside the class declaration must tell compiler where the corresponding declaration/prototype is : Use the scope operator :: which has the formClassName::ItemName (the qualified or full name of ItemName.) This applies also to constants and types declared within a class. e.g., Lab 3

  12. Use of static locals inside functions See slide 19 Example: class Something{ public:static const int CAPACITY = 100; typedef double ArrayType[CAPACITY];void Print(ArrayType a, int itsSize); . . .}; . . .Something::ArrayType x = {0}; for (int i = 0; i < Something::CAPACITY; i++) . . . void Something::Print(Something::ArrayType a, int itsSize){ . . . }

  13. Traditionally, definitions of member functions have been put in an implementation file ClassName.cpp corresponding to the class' header file. This is done to enforce data abstraction — separating theinterfaceof the ADT from itsimplementation details.(Unfortunately, the class data members, which store data and are therefore part of the implementation, must be in the .h file.) With the increasing use of templates, however, this practice is becoming less common because current compiler technology doesn't permit this split for templates — everything has to be in the same file. Thus the reason for dropping the ".h" from standard class libraries. They're really class-template libraries, and there are therefore no corresponding ".cpp" files.

  14. Implementation of class Time — Version 1 // Time.cpp -- implements the Time member functions #include "Time.h" /*** Utility functions ***/ /* ToMilitary converts standard time to military time. Receive: hours, minutes, am_pm Return: The military time equivalent Could implement this as a private class method */ int ToMilitary (unsigned hours, unsigned minutes, char am_pm) { if (hours == 12) hours = 0; return hours * 100 + minutes + (am_pm == 'P' ? 1200 : 0); } Note the doc

  15. Implementation of class Time — cont. Note the doc //--- Function to implement the Set operation void Time::Set(unsigned hours, unsigned minutes, char am_pm) { // Check class invariant if (hours >= 1 && hours <= 12 && minutes >= 0 && minutes <= 59 && (am_pm == 'A' || am_pm == 'P')) { myHours = hours; myMinutes = minutes; myAMorPM = am_pm; myMilTime = ToMilitary(hours, minutes, am_pm); } else cerr << "*** Can't set time with these values ***\n"; // Object's data members remain unchanged } See slide 21

  16. Implementation of class Time — cont. //--- Function to implement the Display operation void Time::Display(ostream & out) const { out << myHours << ':' << (myMinutes < 10 ? "0" : "") << myMinutes << ' ' << myAMorPM << ".M. (" << myMilTime << " mil. time)"; }

  17. Test driver for class Time Develop and test classes incrementally! #include "Time.h"#include <iostream>using namespace std;int main(){ Time mealTime; mealTime.Set(5, 30, 'P'); cout << "We'll be eating at ";mealTime.Display(cout); cout << endl;} Execution:We'll be eating at 5:30 P.M. (1730 mil. time)

  18. Object-Oriented Perspective Procedural: Send object off to some function for processing OOP: Send a message to the object to operate on itself. To set my digital watch to 5:30 P.M., I don't wrap it up and mail it off to Casio. I push a button! To display the time on my watch, I don't wrap it up and mail it off to Casio and have them tell me what time it is. I have it display the time itself, perhaps pushing a button to turn on the backlight so I can see it .

  19. Notes: 1. Member functions: "Inside" an object, so don't pass object to them as a parameter. (They receive the object to be operated on implicitly, rather than explicitly via a parameter.)Non-member functions: "Outside" an object, so to operate on an object, they must receive it via a parameter. 2. Public items must be qualified when referred to outside the class declaration:ClassName::ItemNamePublic constants are usually declared static so they are global class properties that can be accessed by all objects of that class type rather than each object having its own copy. 3. Simple member functions: Usually specified as inline functions. This suggests to compiler to replace a function call with actual code of the function with parameters replaced by arguments— saves overhead of function call. Danger:"Code Bloat"

  20. Two ways to inline a class function member: 1. Prototype in class declaration; inline definition below the class declaration in the header file,qualifying the name as usual. InClassName.h:class ClassName{ public: RetType SimpleFun(param_list); //prototype . . . };inlineRetType ClassName::SimpleFun(param_list) { . . . } // definition Interface clutter 2.Put function’s definition (instead of prototype) inside class declaration — compiler will treat it as inlined.

  21. Class Invariant: A condition (boolean expression) that ensures that the data members always contain valid values. Example: 1 <myHours< 12 && 0 <myMinutes< 59 &&myAMorPM == 'A' or 'P' && 0 <myMilTime< 2359 Operations that modify data members must check the class invariant. Mutators Then other operations can be sure data members have valid values. How? 1. Use an if statement — see Set() 2. Use assert(class_invar); (must#include <cassert>) true false continue execution halt execution and display error message

  22. App. D.4 3. Throw an exception that the calling function can catch and take appropriate action: //----- Function to implement the Set operation -----void Time::Set(unsigned hours, unsigned minutes, char am_pm){ // Check class invariant if (hours >= 1 && hours <= 12 && minutes >= 0 && minutes <= 59 && (am_pm == 'A' ||am_pm == 'P')) { . . . } else { char error[] = "*** Illegal initializer values ***\n"; throw error; }}

  23. To catch this exception, a calling function might contain: try{ mealTime.Set(13, 30, 'P'); cout << "This is a valid time\n";} catch (char badTime[]){ cerr << "ERROR: " << badTime << endl; exit(-1);} cout << "Proceeding. . .\n";When executed, the output produced will be ERROR: *** Illegal initializer values ***

  24. Class Constructors Constructing an object consists of: (1) allocating memory for the object, and (2) initializing the object. In our example, after the declarationTime mealTime;memory has been allocated for object mealTime and it's datamembers have some default (perhaps "garbage") initial values. Need to:  Specify initial values for mealTime  Provide default values to be used if no initial values are specified. This is the role of a class' constructor. (Later, it will also allocate memory.)

  25. Class Constructors - Properties 1. Names are always the same as the class name. 2. Initialize the data members of an object with default values or with values provided as arguments. 3. Do not return a value; have no return type (not even void). 4. Often simple and can be inlined. 5. Called whenever an object is declared. 6. If no constructor is given in the class, compiler supplies adefault constructor which allocates memory and initializes it with some default (possibly garbage) value. A default constructor is one that is used when the declaration ofan object contains no initial values:ClassName object_name; Anexplicit-value constructoris used for declarations with initial values:ClassName object_name(list-of-init-values); 7. If we supply a class constructor, we must also provide a default constructor or we can't use first kind of declaration.

  26. Constructors for Time class In Time.h class Time{ public: /* --- Construct a class object (default). * Precondition: A Time object has been declared. * Postcondition: Data members initialized to 12, 0, 'A', and 0. ****************************************************************/ Time(); /* --- Construct a class object (explicit values). * Precondition: A Time object has been declared. * Receive: Initial values initHours, initMinutes,and * initAMPM * Postcondition: Data members initialized to initHours,initMinutes, * initAMPM, & correspoding military time **********************************************************/ Time(unsigned initHours, unsigned initMinutes, char initAMPM); . . .// other member function prototypesprivate:/********** Data Members **********/ . . .}; // end of class declaration

  27. Constructors for Time class - Implementation In Time.h, after class declaration:inline Time::Time(){ myHours = 12;myMinutes = 0;myAMorPM = 'A';myMilTime = 0;} In Time.cpp:Time::Time(unsigned initHours, unsigned initMinutes, char initAMPM) { // Check class invariant assert(initHours >= 1 && initHours <= 12 && initMinutes >= 0 && initMinutes <= 59 && (initAMPM == 'A' || initAMPM == 'P')); myHours = initHours; myMinutes = initMinutes; myAMorPM = initAMPM; myMilTime = ToMilitary(initHours, initMinutes, initAMPM);} Remember... exceptions

  28. Testing #1: Time mealTime, //default constructor bedTime(11,30,'P'); //explicit-value constructor Create and initialize 2 Time objects: Time(); Time(11,30,'P'); 12 11 0 30 A P 0 2330 mealTime.Display(cout); cout << endl;bedTime.Display(cout); cout << endl; Execution: 12:00 A.M. (0 mil. time) 11:30 P.M. (2330 mil. time)

  29. App. 1 from "Other Links" Constructors for Time class — Default Arguments Can combine both constructors into a single constructor function by using default arguments: Replace constructors in Time.h with: /*--- Construct a class object. Precondition: A Time object has been declared. Receive: Initial values initHours, initMinutes, and initAMPM (defaults 12, 0, 'A') Postcondition: Data members initialized to initHours, initMinutes, initAMPM, & correspoding military time.*/ Time(unsigned initHours = 12, unsigned initMinutes = 0, char initAMPM = 'A'); Note: Any parameter with default argument must appear after all parameters without default arguments.

  30. 5 5 12 5 30 30 0 0 P A A A 530 1730 500 0 Testing: Time mealTime, t1(5), t2(5, 30), t3(5, 30, 'P'); Creates 4 Time objects: Execution: 12:00 A.M. (0 mil. time)5:00 A.M. (500 mil. time)5:30 A.M. (530 mil. time)5:30 P.M. (1730 mil. time) mealTime.Display(cout); cout << endl;t1.Display(cout); cout << endl;t2.Display(cout); cout << endl;t3.Display(cout); cout << endl;

  31. Note: Type t = value; Type t; t = value;  Copy Operations Two default copy operations are provided: 1. Copy ininitialization (via ) 2. Copy in assignment (via ) copy constructor assignment operator Each makes a raw (bitwise) copy of the memory allocated to the data members of the object. This is OK for now

  32. 11 11 30 30 P P 2330 2330 Examples: Time t = bedTime; Time t(bedTime); Both: 1. Allocate memory for t 2. Copy data members of bedTime so t is a copy of bedTime In contrast: Time t = Time(11, 30, 'P'); calls the explicit-value constructor to construct a (temporary) Time object and then copies it into t. Note: These are not assignments; a default copy constructor is called. What aboutTime t = 3;? In mixed-mode operations, compiler will look for a constructor to convert one value to the type of the other.

  33. This is OK for now There is a default copy operation for assignment. Example: t = mealTime; copies the members of mealTime into t, replacing any previous values: 12 12 0 0 A A 0 0 It returnsthis Time object as the value of this expression. Why? So we can chain =

  34. Access Member Functions Data members are private: they cannot be accessed outside the class. To make the values stored in some or all of these members accessible, provide accessor(or extractor) member functions. (They are non-mutators; mutators change data members.) Example: To provide access to myHours of class Time.(Access for the remaining members is analogous.)  simply retrieves and returns the value stored in a data member  inline, because it's simple  prototype (and define) as a const function Add in Time class declaration/* Hour Accessor * Return: value stored in myHours data member of * Time object containing this function */ unsigned Hour() const;

  35. Add below Time class declaration inline unsigned Time::Hour() const { return myHours; } Testing: Time mealTime; . . . cout << "Hour: " << mealTime.Hour() << endl; Execution: Hour: 12

  36. Output and Input Add output operation(s) to a class early so it can be used for debugging other operations. Example: overload operator<<() for a Time object Instead of: cout << "We'll be eating at " ; mealTime.Display(cout); cout << endl; we can write:cout << "We'll be eating at " << mealTime << endl;

  37. Overloading operators In C++, operator D can be implemented with the functionoperatorD(). If a member function of a class C, and a is of type C, the compiler treats a D b as a.operatorD(b) If not a member function of a class C, the compiler treats a D b as operatorD(a, b)

  38. Overloading Output Operator << Can operator<<() be a member function? No, because the compiler will treatcout << tas cout.operator<<(t) which means that operator<<(const Time &) would have to be amember of class ostream(orcoutbe oftypeTime)and we can't (or don't want to) modify standard C++ classes. Putting the prototypeostream& operator<<(ostream & out, const Time& t);inside our class declaration produces a compiler error like:'Time::operator <<(ostream &, const Time &)' must take exactly one argument

  39. Why is out a reference parameter? Socorresponding actual ostream argument gets modifiedwhen out does. Why is t a const reference parameter? Avoidoverhead of copying a class object. Why is return type ostream & (a reference to an ostream)? Else a copy of out is returned. Why return out? So we can chain output operators. Since << is - associative: left operator<<(cout, t1) << endl << t2 << endl; first function must return cout cout << endl << t2 << endl; operator<<(cout, endl) << t2 << endl; cout << t2 << endl; . . . ( ) ( ) ( ) cout << t1 << endl << t2 << endl;

  40. Overloading << for a Class Method 1: Put definition in .h file, after class declaration, and have it call an output function member. Inline it, because it's simple. . . .} // end of class declaration. . ./* operator<< displays time in standard and military format. Receive: ostream out and Time object t Output: time represented by Time object t Pass back: ostream out with t inserted into it Return: out*/ inlineostream&operator<<(ostream&out,const Time&t) { } t.Display(out); return out; Note: No Time:: It's not a member function!

  41. Note: No friend Method 2: Define operator<<()outside the class declaration as a non-member function and: (i) use accessors to display data members, or (ii) declare it to be a friend in the class declaration. In class declaration /* doc. as before */friend ostream & operator<<(ostream & out, const Time & t); Outside class declaration (in .cpp file) ostream & operator<<(ostream & out, const Time & t){ out << t.myHours << ':' << (t.myMinutes < 10 ? "0" : "") << t.myMinutes << ' ' << t.myAMorPM << ".M. (" << t.myMilTime << " mil. time)"; return out;}

  42. Friend Functions A function that a class declares as a friend is a non-member function to which the class has granted permission to access members in its private sections. Note:Because a friend function is not a function member:  Don't qualify its definition with class name and scope operator (::).  Don't putfriend in definition.  It receives the object on which it operates as a parameter.  It uses the dot operator to access the data members.

  43. To add an input operator to our Time class, we proceed in much the same way as for output. We could either: 1. Add a member function ReadTime() that reads values and stores them in the data members of a Time object; then call it from non-member function operator>>() 2. Declare operator>>() to be a friend function so that it can access the data members of a Time object and store input values in them. Is one of two methods for input/output preferred? We'll see later when we study inheritance and polymorphism that the first method is preferred (or perhaps required).

  44. Relational Operators (<) Specification: Receives: Two Time objects Returns: True if the first object is less than the second; false otherwise. Should operator<() be a member function? Internal perspective: I compare myself with another Time object and determine if I am less than that other object External perspective: Two Time objects are compared by an external function to determine if the first is less than the second. OOP: "I-can-do-it-myself" principle ( objects self-contained): Use member functions whenever possible. Rephrased specification: Receives: A Time object (and the current object implicitly) Returns: True if I (the Time object containing this function) am less than the Time object received; false otherwise.

  45. // Internal perspective : Add to Time.h class Time{ public: // member functions . . ./***** Relational operators *****//* operator< determines if one Time is less than another Time * Receive: A Time t (and the current object implicitly) * Return: True if time represented by current object is < t. */ bool operator<(const Time & t) const; . . .}; // end of class declaration inline bool Time::operator<(const Time & t) const { return myMilTime < t.myMilTime; } However . . .

  46. Caveat re Operator Overloading Internal perspective may lead to seeming inconsistencies: Example: Suppose a and b are objects of type class C. a < b okay? Yes, equivalent to a.operator<(b) b < a okay? Yes, equivalent to b.operator<(a) a < 2 okay? Yes, if there is a constructor that promotes 2 to type C, since this is then equivalent to a.operator<(C(2)) 2 < a okay? No, equivalent to 2.operator<(a), which is meaningless. May confuse an application programmer to support a < 2 but disallow 2 < a.  probably best to use friends here.

  47. Overloaded Operator as Friend External perspective: (Permits a < 2 and 2 < a , if 2 can be promoted) class Time{public: // member functions . . . /* operator< * Receive: Two Times t1 and t2 * Return: True if time t1 is less than time t2/ */ friend bool operator<(const Time & t1, const Time & t2); . . .}; // end of class declarationinline bool Time::operator< (const Time & t1, const Time & t2) { return t1.myMilTime < t2.myMilTime; } Or don't use friend and compare values obtained by accessors.

  48. Adding Increment/Decrement Operators Specification: Receives: A Time object (perhaps implicitly) Returns: The Time object with minutes incremented by 1 minute. Question: Should it be a member function? Yes Add to Time.h: /***** Increment operator *****//* --- Advance() increments a Time by 1 minute. Postcondition: The Time object has its minutes incremented by 1.-----------------------------------------------*/ void Advance();

  49. Add to Time.cpp: //----- Function to implement Advance() ----- void Time::Advance(){ myMinutes++; myHours += myMinutes / 60; myMinutes %= 60; myHours %= 12; if (myMilTime == 1159) myAMorPM = 'P'; else if (myMilTime == 2359) myAMorPM = 'A'; // else no change myMilTime = ToMilitary(myHours, myMinutes, myAMorPM);}

  50. ++ We could replace Advance() with overloaded operator++(). Question: How do we distinguish between prefix ++ and postfix ++? What's the difference? Solution: In C++, when theCompiler encounters:It looks for: Prefix ++: operator++() with no parameters Postfix ++: operator++(int) with one int parameter (which is not used in the definition.)