Introduction to Programming

Transcription

1 Introduction to Programming Summer Term 2014 Dr. Adrian Kacso, Univ. Siegen Tel.: 0271/ , Office: H-B 8406 State: June 4, 2014 Betriebssysteme / verteilte Systeme Introduction to Programming (1) i Introduction to Programming Summer Term More on Objects and Classes Betriebssysteme / verteilte Systeme Introduction to Programming (1) 228

2 8 More on Objects and Classes... Contents: An example Initializing member variables Copy constructors Operators Betriebssysteme / verteilte Systeme Introduction to Programming (1) An Example Classes for a 2-D drawing program Assume we want to write a drawing program First, think about the classes / objects we have to deal with: points, lines, rectangles, polygons, circles, text,... Then, think about the operations each object should provide. E.g., for rectangles: constructor: rectangle is aligned horizontally draw, move, movepoint, rotate, scale,... Then, think about the (private) data members needed for each object. E.g., for rectangles: four lines, one for each side (easy drawing!) Then, implement the methods Betriebssysteme / verteilte Systeme Introduction to Programming (1) 20

3 8.1 An Example... Classes for a 2-D drawing program... For brevity, we consider only three classes: Point, Line, Rectangle Operations, data members, and implementation: see the directories DRAW and DRAW2 inside the source code archive chapter8.tgz. the second version prints constructor and destructor calls Betriebssysteme / verteilte Systeme Introduction to Programming (1) Initializing Member Variables References and constants can only be initialized, but not assigned Member variables of a class can be references or constants Question: How can we initialize member variables when creating a new object? C++ provides a special syntax in constructors: Cat::Cat() : itsage(5), itsweight(8) // initialization // body of constructor Normal member variabes can be either initialized or assigned in the body Betriebssysteme / verteilte Systeme Introduction to Programming (1) 22

4 8.2 Initializing Member Variables... Example class Example public: Example(int &avariable); private: const int aconstant; int &areference; Wrong constructor: Example::Example(int &avariable) aconstant = 4711; // > Compiler error areference = avariable; // Assignment to the variable // referenced by areference Betriebssysteme / verteilte Systeme Introduction to Programming (1) Initializing Member Variables... Example class Example public: Example(int &avariable); private: const int aconstant; int &areference; Correct constructor: Example::Example(int &avariable) : aconstant(4711), // initialization areference(avariable) // initialization Betriebssysteme / verteilte Systeme Introduction to Programming (1) 24

5 8. Copy Constructors Reminder: call-by-value A function receives a copy of the actual parameters Objects can be parameters: void myfunction(cat thecat) Cat frisky(5); myfunction(frisky);... how do we copy objects??? Betriebssysteme / verteilte Systeme Introduction to Programming (1) Copy Constructors... Copying objects Copy constructor: Cat::Cat(Cat const &source) itsage = source.itsage; itsweight = source.itsweight; A copy constructor makes an object from another object of the same class in our example, we get a clone cat ;-) The copy constructor is implicitly called when an object is passed to a function / method by value when a function / method returns an object Betriebssysteme / verteilte Systeme Introduction to Programming (1) 26

6 8. Copy Constructors... Quiz: how many Cats are constructed / deconstructed? Cat functionone(cat thecat) cout << "functionone. Returning..." << endl; return thecat; int main() cout << "Making a Cat" << endl; Cat frisky(10); cout << "Calling functionone" << endl; int age = functionone(frisky).getage(); cout << "age is " << age << endl; return 0; Betriebssysteme / verteilte Systeme Introduction to Programming (1) Copy Constructors and with call-by-reference? Cat &functiontwo(cat &thecat) cout << "functiontwo. Returning..." << endl; return thecat; int main() cout << "Making a Cat" << endl; Cat frisky(10); cout << "Calling functiontwo" << endl; int age = functiontwo(frisky).getage(); cout << "age is " << age << endl; return 0; Betriebssysteme / verteilte Systeme Introduction to Programming (1) 28

7 8. Copy Constructors... What if a class does not implement a copy constructor? The compiler provides a default copy constructor, performing a member-wise copy (shallow copy) Example: Cat::Cat(Cat const &thecat) itsage = thecat.itsage; itsweight = thecat.itsweight; So why do we ever have to implement a copy constructor ourselves? Betriebssysteme / verteilte Systeme Introduction to Programming (1) Copy Constructors... Shallow copy vs. deep copy Consider an AristoCat class for aristocratic cats, which can have many names: class AristoCat public: AristoCat(unsigned int numnames); ~AristoCat(); void setname(unsigned int number, string const &name); string const &getname(unsigned int number) const; private: string *; unsigned int ; Betriebssysteme / verteilte Systeme Introduction to Programming (1) 240

8 8. Copy Constructors... Shallow copy vs. deep copy... AristoCat::AristoCat(unsigned int numnames) = numnames; = new string[numnames]; AristoCat::~AristoCat() delete[] ; = NULL; = 0; void AristoCat::setName(unsigned int number, string const &name) if (number < ) [number] = name; Betriebssysteme / verteilte Systeme Introduction to Programming (1) Copy Constructors... A copy constructor for AristoCats The standard, shallow copy AristoCat::AristoCat(AristoCat const &thecat) = thecat.; = thecat.; results in "Frisby" Betriebssysteme / verteilte Systeme Introduction to Programming (1) 242

9 8. Copy Constructors... (Animierte Folie) Obvious problems with shallow copy setname(2,"twinky") "Twinky" dangling t pointer!! "Twinky" delete 0 NULL Betriebssysteme / verteilte Systeme Introduction to Programming (1) Copy Constructors... Deep copy For objects with pointers / references, we (usually) need a deep copy: "Frisby" "Frisby" Betriebssysteme / verteilte Systeme Introduction to Programming (1) 244

10 8. Copy Constructors... Deep copy constructor for AristoCats AristoCat::AristoCat(AristoCat const &thecat) = thecat.; = new string[]; for (int i=0; i<; i++) [i] = thecat.[i]; Betriebssysteme / verteilte Systeme Introduction to Programming (1) Operators Data types and operators A data type defines a range of data values, and a set of operations on instances of this type Operations on basic data types are expressed by operators e.g., for int we have + - * / %... Classes define data types so far, we only used methods to express operations but we also can use operators, e.g. assignment: Cat mycat;...; mycat = yourcat; arithmetic operators: string x;...; x += "hi"; Betriebssysteme / verteilte Systeme Introduction to Programming (1) 246

11 8.4 Operators... The assignment operator Like with the copy constructor, the compiler generates an assigment operator, if it is not implemented in the class: AristoCat & AristoCat::operator=(AristoCat const &thecat) = thecat.; = thecat.; return *this; What is the difference to a copy constructor? Is this the assignment we want to have? Betriebssysteme / verteilte Systeme Introduction to Programming (1) Operators... Shallow assignment: problems Assume we have two AristoCats: mycat yourcat 1 "Frisby" "Twinky" Betriebssysteme / verteilte Systeme Introduction to Programming (1) 248

12 8.4 Operators... Shallow assignment: problems... After mycat = yourcat; we would have: mycat 1 memory leak! "Frisby" yourcat 1 "Twinky" Whenever you implement your own (deep) copy constructor, you also should implement a (deep) assignment operator Betriebssysteme / verteilte Systeme Introduction to Programming (1) Operators... Deep Assignment operator for AristoCats AristoCat & AristoCat::operator=(AristoCat const &thecat) = thecat.; delete ; = new string[]; for (int i=0; i<; i++) [i] = thecat.[i]; return *this; Betriebssysteme / verteilte Systeme Introduction to Programming (1) 250

13 8.4 Operators... Defining other operators (operator overloading) A class can also define other operators however, the set of operators in C++ is fixed: = = * *= / /= % << >> < <= &&... Using operators can make the code more clear, e.g. Complex x, y, z;... x = x * y + z; // instead ofx = x.mult(y).add(z) But use operators only when their meaning is obvious! Betriebssysteme / verteilte Systeme Introduction to Programming (1) Summing up Initializing member variables class Example public: Example(int &avariable); private: const int aconstant; int &areference; Correct constructor: Example::Example(int &avariable) : aconstant(4711), // initialization areference(avariable) // initialization Betriebssysteme / verteilte Systeme Introduction to Programming (1) 252

14 8.5 Summing up... Reminder: Copy Constructor Copy constructor: Cat::Cat(Cat const &source) itsage = source.itsage; itsweight = source.itsweight; A copy constructor makes an object from another object of the same class in our example, we get a clone cat ;-) The copy constructor is implicitly called when an object is passed to a function / method by value when a function / method returns an object Betriebssysteme / verteilte Systeme Introduction to Programming (1) Summing up... Shallow copy setname(2,"twinky") "Twinky" delete 0 NULL dangling t pointer!! "Twinky" Betriebssysteme / verteilte Systeme Introduction to Programming (1) 254

15 8.5 Summing up... Deep copy For objects with pointers / references, we (usually) need a deep copy: "Frisby" "Frisby" Betriebssysteme / verteilte Systeme Introduction to Programming (1) Summing up... Assignment operator AristoCat & AristoCat::operator=(AristoCat const &thecat) = thecat.; delete ; = new string[]; for (int i=0; i<; i++) [i] = thecat.[i]; return *this; This is a deep assignment, like a deep copy Betriebssysteme / verteilte Systeme Introduction to Programming (1) 256