1. . Plab – Tirgul 11 RTTI, Binary files

2. RTTI – why? Problem: u Up-casting works fine.  Treating sub-class as base class Shape * s = new Circle(); u What about down-casting?  might not be safe !  correctness cannot be determined by the compiler. Circle * c = (Circle*) s; Shape Line Circle

3. RTTI RTTI – Run Time Type Information Mechanisms for RTTI: 1. dynamic_cast operator 2. typeid operator (and type_info class)

4. The ‘dynamic_cast‘ operator dynamic_cast (expression) u Enables run-time type checking:  returns a valid pointer if “expression” is of type “T”  NULL otherwise u Cast of a reference type throws an exception when it fails (“bad_cast”)

5. Dynamic_cast : example Shape* s = container.pop(); Circle* c = dynamic_cast (s); if ( c != NULL ) { // c is a circle c->doSomething(); else handle(); //handle unexpected event Note: the actual type of “ s ” is not mentioned!  Can be any sub-class of Circle.

6. Dynamic_cast : example Shape s; Circle c; Line l; ThinLine tl; Shape* arr[4] = {&s, &c, &l, &tl}; for ( int i=0; i<3; i++ ) { Line * pl = dynamic_cast ( arr[i] ); pc == NULL? cout << “cast ok\n” : cout << “cast fail\n”; } Shape LineCircle Output: cast fail // Shape to Line cast fail // Circle to Line cast ok // Line to Line cast ok // ThickLine to Line ThickLine

7. dynamic_cast - more dynamic_cast (expression) Note: u Used only on pointer or reference types. u Can be used for up-cast, down-cast u and also for cross-cast (out of this scope) u Only for types with virtual-functions (“Polymorphic types”) u These object have a space for the information about the type: the virtual function table

8. dyn_cast: only for polymorphics void foo(Shape * s, Time * t) { Circle * c = dynamic_cast ( s ); //ok Date * date = dynamic_cast ( t ); //error } class Circle : public Shape { virtual void draw(); … } class Date : public Time { …// Time has no virtual functions }

9. RTTI : typeid operator Obtains info about an object/expression usage: typeid( obj ) (like “sizeof”) Example: Dog d; Cat c; cout << “d is a “ << typeid(d).name() << “,” << “c is a “ << typeid(c).name() <<endl; Output: d is a Dog, c is a Cat

10. type_info class typeid(expression) returns an object of the type_info class. e.g. type_info& ti = typeid(d); class type_info { public: bool operator==( type_info const&)const; char const* name() const; … private: type_info( type_info const&); //prevent copying type_info& operator=( type_info const& ); }

11. RTTI misuse void rotate( shape const& s ) { if (typeid(s) == typeid(Circle) ) //do nothing else if (typeid(s) == typeid(Triangle) ) //rotate Triangle else if (typeid(s) == typeid(Rectangle) ) //rotate Rectangle … } Use virtual functions (i.e. polymorphism) when you can ! VERY common misuse of RTTI

12. . Binary files

13. Leading example: image files u Images are stored as matrices of numbers (Pixels). u Here, we deal with gray-scale images (“black and white”) u 8 bits per pixel  i.e. each pixel between 0 and 255 u 0 is white, 255 is black, others are gray 255 0 0 0 100

14. storing images u How can we store images on files? u For each image we want to store:  width//integer  height//integer  number of bits per pixel//short  the pixels//matrix of integers u Requirements: read/write easily, save space, save computation, etc.

15. storing images First try: text files (like you did so far) width = 3 height = 2 bits_per_pixel = 8 255, 0, 0 0,255,100 “myImg.txt” cons: u long u needs parsing (e.g. atoi for numbers) pros: u readable by humans u easy to edit u but who handles images like that?

16. Binary files Better solution: Binary files u Save the data the way the computer holds it pros: u Smaller u No parsing (faster) u Widely used ! u For example: BMP files, other data cons: u hard to read for humans u Machine dependant

17. Images as binary files 3 2 8255 00 0 100 width 2 bytes [0,…,0,0,1,1] height 2 bytes [0,…,0,0,1,0]

18. Images as binary files 3 2 8255 00 0 100 bits per pixel 1 byte [0,0,0,0,1,0,0,0]

19. Images as binary files 3 2 8255 00 0 100 pixel 1 byte [1,1,1,1,1,1,1,1] pixel 1 byte [0,0,0,0,0,0,0,0]

20. Binary files 3 2 16 255 0 0 0 0 bits per pixel now value 16

21. Example: writing a binary file Pixel pixs[30]; int width = 100, height = 150; short bitsPerPix = 16; binary open for writing ofstream outfile; outfile.open(“pic.raw", ios::bin | ios::out); outfile.write(&width, sizeof(int)); outfile.write(&height, sizeof(int)); outfile.write(&bitsPerPix, sizeof(short)); outfile.write(pixs, 30*sizeof(Pixel)); outfile.close(); ostream& write( char const* str, streamsize n);

22. . Learn by yourselves: c++ casting (see slides ahead and Stroustrup 15.4 and more)

23. c++ style casting u At the beginning c++ supported two styles of casts:  (typename)expression  typename(expression) u Instead there are now four new casting operators:  static_cast (expression)  const_cast (expression)  reinterpret_cast (expression)  dynamic_cast (expression)

24. The 'static_cast'operator u Works where implicit conversion exists  standard or user-defined conversion  up-casts u Safer that “old-style” casts  e.g. won’t cast int* to float* u Failure causes a compiler error  No dynamic checking is done! int i = static_cast (12.45); static_cast (expression)

25. The ‘const_cast'operator  Is used to remove (or add) const -ness: void g(C * cp); void f(C const* cp) { g(const_cast (cp)); }  Usually, you should design your variables/methods such that you won’t have to use const_cast. u Failure causes compiler errors const_cast (expression)

26. ‘reinterpret_cast'operator u Is used to reinterpret byte patterns. double d(10.2); char* dBytes = reinterpret_cast (&d); u Circumvents the type checking of c++. u Very implementation-dependent. u Rarely used. ! Very dangerous ! reinterpret_cast (expression)