C Constructor Insanity (cont’d)

L11: C Constructor InsanityCSE333, Autumn 2021C Constructor Insanity (cont’d)CSE 333 Autumn 2021Instructor:Chris ThachukTeaching Assistants:Arpad (John) DepaszthoryAngela XuIan HsiaoKhang Vinh PhanLogan GnanapragasamMaruchi KimMengqi (Frank) ChenCosmo Wang

L11: C Constructor InsanityCSE333, Autumn 2021Administrivia Exercise 9 out, due Wed (Oct. 27) @ 10am Operators will be covered in Monday’s lecture Homework 2 due next Thursday (Oct. 28) Midterm exam on Friday, Nov. 5 Take-home (completed independently)Exam will be open for 24 hours on GradescopeCan start / stop / resume all dayMore details will be posted on class website2

L11: C Constructor InsanityCSE333, Autumn 2021Lecture Outline ConstructorsCopy Constructors (restart)AssignmentDestructors3

L11: C Constructor InsanityCSE333, Autumn 2021Copy Constructors STYLETIPC has the notion of a copy constructor (cctor) Used to create a new object as a copy of an existing objectPoint::Point(const int x, const int y) : x (x), y (y) { }// copy constructorPoint::Point(const Point& copyme) {x copyme.x ;y copyme.y ;}void foo() {Point x(1, 2);Point y(x);// invokes the 2-int-arguments constructor// invokes the copy constructor// could also be written as "Point y x;"} Initializer lists can also be used in copy constructors (preferred)4

L11: C Constructor InsanityCSE333, Autumn 2021Synthesized Copy Constructor If you don’t define your own copy constructor, C willsynthesize one for you It will do a shallow copy of all of the fields (i.e., member variables)of your class Sometimes the right thing; sometimes the wrong thing#include "SimplePoint.h". // definitions for Distance() and SetLocation()int main(int argc, char** argv) {SimplePoint x;SimplePoint y(x); // invokes synthesized copy constructor.return EXIT SUCCESS;}5

L11: C Constructor InsanityCSE333, Autumn 2021When Do Copies Happen? The copy constructor is invoked if: You initialize an object fromanother object of the sametype: You pass a non-referenceobject as a value parameterto a function: You return a non-referenceobject value from a function:Point x;Point y(x);Point z y;// default ctor// copy ctor// copy ctorvoid foo(Point x) { . }Point y;foo(y);// default ctor// copy ctorPoint foo() {Point y;// default ctorreturn y;// copy ctor}6

L11: C Constructor InsanityCSE333, Autumn 2021Compiler Optimization The compiler sometimes uses a “return by valueoptimization” or “move semantics” to eliminateunnecessary copies Sometimes you might not see a constructor get invoked when youmight expect itPoint foo() {Point y;return y;}// default ctor// copy ctor? optimized?int main(int argc, char** argv) {Point x(1, 2);// two-ints-argument ctorPoint y x;// copy ctorPoint z foo(); // copy ctor? optimized?}7

L11: C Constructor InsanityCSE333, Autumn 2021Lecture Outline ConstructorsCopy ConstructorsAssignmentDestructors8

L11: C Constructor InsanityCSE333, Autumn 2021Assignment ! Construction “ ” is the assignment operator Assigns values to an existing, already constructed objectPoint w;Point x(1, 2);Point y(x);Point z w;y x;//////////default ctortwo-ints-argument ctorcopy ctorcopy ctorassignment operator9

L11: C Constructor InsanityCSE333, Autumn 2021Overloading the “ ” Operator STYLETIPYou can choose to define the “ ” operator But there are some rules you should follow:Point& Point::operator (const Point& rhs) {if (this ! &rhs) { // (1) always check against thisx rhs.x ;y rhs.y ;}return *this;// (2) always return *this from op }Point a;a b c;a (b c);(a b) c;////////default constructorworks because return *thisequiv. to above ( is right-associative)"works" because returns a non-const10

L11: C Constructor InsanityCSE333, Autumn 2021Synthesized Assignment Operator If you don’t define the assignment operator, C willsynthesize one for you It will do a shallow copy of all of the fields (i.e., member variables)of your class Sometimes the right thing; sometimes the wrong thing#include "SimplePoint.h". // definitions for Distance() and SetLocation()int main(int argc, char** argv) {SimplePoint x;SimplePoint y(x);y x;// invokes synthesized assignment operatorreturn EXIT SUCCESS;}11

L11: C Constructor InsanityCSE333, Autumn 2021Lecture Outline ConstructorsCopy ConstructorsAssignmentDestructors12

L11: C Constructor InsanityCSE333, Autumn 2021Destructors C has the notion of a destructor (dtor) Invoked automatically when a class instance is deleted, goes outof scope, etc. (even via exceptions or other causes!) Place to put your cleanup code – free any dynamic storage orother resources owned by the object Standard C idiom for managing dynamic resources Slogan: “Resource Acquisition Is Initialization” (RAII)Point:: Point() {// destructor// do any cleanup needed when a Point object goes away// (nothing to do here since we have no dynamic resources)}13

L11: C Constructor InsanityCSE333, Autumn 2021Destructor Exampleclass FileDescriptor {public:FileDescriptor(char* file) {fd open(file, O RDONLY);// Error checking omitted} FileDescriptor() { close(fd ); }int get fd() const { return fd ; }private:int fd ; // data member}; // class FileDescriptor// Constructor// Destructor// inline member functionFileDescriptor.h#include "FileDescriptor.h"int main(int argc, char** argv) {FileDescriptor fd(foo.txt);return EXIT SUCCESS;}14

L11: C Constructor InsanityCSE333, Autumn 2021pollev.com/cse333 How many times does the destructor get invoked? Assume Point with everything defined (ctor, cctor, , dtor) Assume no compiler optimizationstest.ccA.B.C.D.E.Point PrintRad(Point& pt) {Point origin(0, 0);double r origin.Distance(pt);double theta atan2(pt.get y(), pt.get x());cout "r " r endl;cout "theta " theta " rad" endl;return pt;}12int main(int argc, char**Point pt(3, 4);3PrintRad(pt);return EXIT SUCCESS;4}We’re lost argv) {15

L11: C Constructor InsanityCSE333, Autumn 2021Class Definition (from previous lecture)Point.h#ifndef POINT H#define POINT Hclass Point {public:Point(int x, int y);// constructorint get x() const { return x ; }// inline memberint get y() const { return y ; }// inline memberdouble Distance(const Point& p) const;// membervoid SetLocation(int x, int y);// memberfunctionfunctionfunctionfunctionprivate:int x ; // data memberint y ; // data member}; // class Point#endif// POINT H16

L11: C Constructor InsanityCSE333, Autumn 2021pollev.com/cse333 How many times does the destructor get invoked?ctorcctorop dtortest.ccPoint PrintRad(Point& pt) {Point origin(0, 0);double r origin.Distance(pt);double theta atan2(pt.get y(), pt.get x());cout "r " r endl;cout "theta " theta " rad" endl;return pt;}int main(int argc, char** argv) {Point pt(3, 4);PrintRad(pt);return EXIT SUCCESS;}17

L11: C Constructor InsanityCSE333, Autumn 2021Extra Exercise #1 Modify your Point3D class from Lec 9 Extra #1 Disable the copy constructor and assignment operator Attempt to use copy & assignment in code and see what error thecompiler generates Write a CopyFrom() member function and try using it instead (See details about CopyFrom() in next lecture)19

L11: C Constructor InsanityCSE333, Autumn 2021Extra Exercise #2 Write a C class that: Is given the name of a file as a constructor argument Has a GetNextWord() method that returns the nextwhitespace- or newline-separated word from the file as a copy ofa string object, or an empty string once you hit EOF Has a destructor that cleans up anything that needs cleaning up20

L11: C Constructor Insanity CSE333, Autumn 2021 Synthesized Copy Constructor If you don’t define your own copy constructor, C will synthesize one for you It will do a shallow copy of all of the fields (i.e., member variables) of your class Sometimes the right th