C++ Alexander Baltatzis

Transcription

1 C++ Alexander Baltatzis

2 Recap Undefined behaviour int * q1 = new int[2]; delete q1; int * q2 = new int; delete [] q2; Contract with the compiler constructor How would you implement delete [] Classes with dynamic memory The rule of three

3 Class design How should the class be initiated/assigned Constructors and destructors What operators and functions make sense to implement? What functions should be disallowed?

4 Postponing variable definitions Basically the same idea as operator= (see narrated lecture on construction) Don t declare until you can initialize the variable string encrypted(password);

5 Classical problem Widget w; for (int i = 0; w = foo(i); for (int i = 0; Widget w(foo(i)); What costs more assignment or constructor/destructor? Sometimes it s better to measure afterwards In this case, if you ve written the Widget class you should probably be able to answer

6 Exercise How should a singleton class be designed? Make your mind about construction, assignment, copying and inheritance Singleton s1, s2; s1 = s2; Singleton s3 = s1; class BetterSingleton : Singelton {

7 Factories A method constructs the objects Complex createcomplexfrompolar (double r, double fi) { Complex createcomplexfromcartesian (double x, double y) {

8 What about returning pointers? BigC * p1 = makebigc(); // delete? auto_ptr<bigc> p2(makebigc()); auto_ptr<bigc> p3 = p2; // How to copy? tr1::shared_ptr<bigc> p4(makebigc()); Sometimes you want to return a null (0) value Database value To indicate fault

9 Using references Efficiency int calc(matrix m) { int calc(const Matrix & m) { int calc(int a, const int & b) { //? Avoid slicing void draw(baseclass b) { void draw(const BaseClass & b) Use as return values bool dostuff(customers & c, Agents & a) { Avoid pointer arithmetics

10 What about returning referencs Almost always wrong Except when forcing a static construction DBInit & create() { static DBInit x; return x; } DBInit & ref = create(); Code Static data Heap Stack

11 Class design

12 Constructing a date class Date today; Date d1(25, 12, 2007); Date d2(12, 25, 2007); Date d3 = d1 + 3; // OK int daysgone = d3 d1; // OK d1 + d2 //?? (d1 + 5) d3 //?? d1.addyear(3); //?? d1.addmonth(2); //?? Date d(13); //??

13 Class design The functions should be reasonable Allowed operations should make sense Can I use inheritance? Chinese d2; // today Persian d1; // today int zero = d1 d2; Gregorian d3 = d2 + 10;

14 Date as base class Recap make the destructor virtual? There is no dynamic memory Rule of three does not apply (but beware) What do we know of future subclasses?

15 How to inherit? Date Date Date? Persian Persian Gregorian Gregorian Persian Gregorian Where do I put the code?

16 Beware of rule of three Gregorian (Date &) { do_something_important(); } // Discuss the following lines Persian p; Gregorian g = p; Persian p2 = g; Gregorian g2 = g;

17 How to construct a date Suppose we allow the following constructions Date d3; Date d4(5, 3); Date d5(25);

18 How to construct a date Suppose we allow the following constructions Date d3; // today Date d4(5, 3); // 5/3 current year Date d5(25); // 25th current month Date(int d=0, int m=0, int y=0) { // Why is this constructor a bad idea?

19 Compile time vs runtime Do you prefer compile time error or runtime error? Date d(day(1), Month(2), Year(2003)); Date d(month(2), Day(1),, Year(2003)); Date d(day(1), Month::Jan(), Year(2007));

20 struct Month { static Month Jan() { return Month(1); } static Month Jan() { return Month(1); } private: explicit Month(int x) : month(x) {} int month; } static methods to make sure it is initialized Explicit constructor to avoid implicit conversions Always consider explicit on one-argument constructors!

21 Compile time vs runtime Date d(1, 1, 2007); Date d(day(1), Month::Jan(), Year(2007)); What are the benefits? The first one is easier to type The first one is seamingly easier to guess The second one catches errors in compile time!! Always prefer compile time error checking

22 Exposing data Sometimes a struct is a struct struct coord { float x; float y; }; Sometimes you need to change it struct coord { double x; double y; }; Sometimes it s not what you think struct coord { double q1; double q2; }; // radius, fi

23 Exposing data Prefer day() instead of m_day Exposing m_day, month, year commits! We cannot change internal representation! date as number of days from a fixed date , , Julian day Or as day. month, year?

24 Encapsulate data Hide data from other programmers Let them access via accessors/inspectors Let them change via mutator functions Incedently that s one of the cornerstones of object oriented programming Another cornerstone being organizing the code, put functions that handle the data together with the data (in the same class)

25 Class design of a calendar Calendar c; c.setdate(24, 12, 2007); c.add_event( Buy gifts, if you haven t already!!! ); c.add_event( Meeting, 2007, 5); // when is // that? void setdate(int d = -1, m = -1, y = -1) { Still a very bad idea, discuss!

26 Why a bad idea The programmer did not have to explicitly write 4 functions (12 code lines?) He probably saved 6 minutes (2 lines/minute) The debugging programmer is looking for a valid date that shouldn t be in the calendar in runtime probably had to spend more than 6 minutes looking for the cause

27 Exercise - Design a calendar class How to construct the class? What operations is allowed (+ - * etc)? What functions do I want? Compare with what I can do with a simple calendar paperback booklet? How do I handle a Persian calendar? Can I convert my scheduled meetings in Iran? Can I merge persian and chinese appointments?

28 Generic programming Sometimes your functions apply to more than one type What do you need to know when sorting a container? You need to know how to compare the items inside STL uses iterators to point into containers. std::sort assumes operator< is implemented If it is not implemented we get a compile error

29 Generic programming benefits Reusing code one sort implemetation Compile time checking operator< missing Optimization in runtime Can optimize containers for simple types

30 Generic programming Instantiate your calendar with the type it should handle Calendar<Persian> persian_calendar; Calendar<Gregorian> gregorian_calendar; gregorian_calendar += persian_calendar; //!!

31 Dependencies

32 Event Event e (Date (Day(1), Month::Jan(), Year(2008)), Happy new year ); Let s define an event consisting of a date and some description string. How shall I store the string and the date internally?

33 On the stack As an instance member the date will be allocated on the stack It might improve performance class Event { Date date; string description; } Code Static data Heap Stack date desc

34 Compile dependancy Constructing the date object on the stack requires knowledge in compile time. How big is the object to put on the stack? It requires a compilation dependancy. Build time will increase An alternative is to use a pointer

35 Compile time dependancies class Event { auto_ptr<date> date; To remove the compile dependancy make the member variable a pointer A pointer can always be allocated on the stack without knowing the size of the object it points to.

36 Performance The actual date object will be on the heap It might take a while to fetch it in runtime On the other hand it might be cached Do not make decisions on assumptions Test performance with a profiler

37 Until next time Object oriented programming Important concepts is a has a Object modelling Several solutions possible fuzzy (flummigt) subject, sometimes religous Extra thinking in lab2 Apply is a and has a on Date, Calendar, Event