[PDF] Introduction to C++ Operator Overloading - Computer Action Team

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CS202 6-1

Introduction to C++

OperatorOverloading

Topic #6

CS202 6-2

"Intro to Operator Overloading

ÊCopy Constructors, Issues of Memberwise Copy

ÊConstant Objects and Constant Member Functions

ÊFriend Functions

ÊWhen to define operators as Members vs. Non-

Members

ÊLvalue vs. Rvalue Expressions

ÊReturn by Value vs. Return by Reference

CS202 6-3

"Designing Effective User Defined Data Types

ÊHow to design User Defined Types that

behave as expected

ÊPractical Rules for Operator Overloading

CS202 6-4

Copy Constructors

"Shallow Copy: ÊThe data members of one object are copied into the data members of another object without taking any dynamic memory pointed to by those data members

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"Deep Copy:

ÊAny dynamic memory pointed to by the data

members is duplicated and the contents of that memory is copied (via copy constructors and assignment operators --when overloaded)

CS202 6-5

Copy Constructors

"In every class, the compiler automatically supplies both a copy constructor and an assignment operator if we don't explicitly provide them. "Both of these member functions perform copy operations by performing a memberwise copy from one object to another. "In situations where pointers are not members of a class, memberwise copy is an adequate operation for copying objects. "However, it is not adequate when data members point to memory dynamically allocated within the class.

CS202 6-6

Copy Constructors

"Problems occur with shallow copying when we: Êinitialize an object with the value of another object:name s1; name s2(s1);

Êpass an object by value to a function or when

we return by value: name function_proto (name)

Êassign one object to another:

s1 = s2;

CS202 6-7

Copy Constructors

"If name had a dynamically allocated array of characters (i.e., one of the data members is a pointer to a char),

Êthe following shallow copy is disastrous!

smith ptr length=10 clone ptr length=10 '\0' S u e S m i t hname smith("Sue Smith"); //one arg constructor used name clone(smith); //default copy constructor used

CS202 6-8

Copy Constructors

"To resolve the pass by value and the initialization issues, we mustwrite a copy constructor whenever dynamic member is allocated on an object-by-object basis. "They have the form: class_name(const class_name &class_object); "1RPLŃH POH QMPH RI POH ´IXQŃPLRQµ LV POH VMPH QMPH MV the class, and has noreturn type "7OH MUJXPHQP·V GMPM P\SH LV POMP RI POH ŃOMVV SMVVHG MV a constant reference (think about what would happen if this was passed by value?!)

CS202 6-9

Copy Constructors

//name.h interface class name {public:name(char* = ""); //default constructorname(const name &); //copy constructor~name(); //destructorname &operator=(name &); //assignment op private:char* ptr; //pointer to nameint length; //length of name including nul char};

#include "name.h"//name.c implementationname::name(char* name_ptr) { //constructorlength = strlen(name_ptr); //get name lengthptr = new char[length+1]; //dynamically allocatestrcpy(ptr, name_ptr); //copy name into new space}name::name(const name &obj) { //copy constructorlength = obj.length; //get lengthptr = new char[length+1]; //dynamically allocatestrcpy(ptr, obj.ptr); //copy name into new space}

CS202 6-10

Copy Constructors

"Now, when we use the following constructors for initialization, the two objects no longer share memory but have their own allocated name smith("Sue Smith"); //one arg constructor used name clone(smith); //default copy constructor used smith ptr length=10 clone ptr length=10 '\0' S u e S m i t h '\0' S u e S m i t h

CS202 6-11

Copy Constructors

"Copy constructors are also used whenever passing an object of a class by value: (get_name returns a ptr to a char for the current object) int main() {name smith("Sue Smith"); //constructor with arg used

//call function by value & display from object returnedcout < name function(name obj) {cout <CS202 6-12

Copy Constructors

"Using a copy constructor avoids

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memory --but causes this behavior "This should convince us to avoid pass by value whenever possible -- when passing or returning objects of a class! ptr length=10 '\0' S u e S m i t h program stack ptr length=10 ptr length=10 return address smith call by value return by value '\0' S u e S m i t h '\0' S u e S m i t h

CS202 6-13

Copy Constructors

"Using the reference operator instead, we change the function to be: (the function callremains the same) name &function(name &obj) {cout <CS202 6-14

Operator

Overloading

Introduction to C++

CS202 6-15

What is..Operator Overloading

"Operator Overloading: ÊAllows us to define the behavior of operators when applied to objects of a class Ê([MPLQH ROMP RSHUMPRUV PMNH VHQVH IRU M ´QHR GMPM from last lecture) and implement those that make sense as operators:

Êinput_data is replaced by >>

Êdisplay is replaced by <<

Êassign or copy is replaced by =

CS202 6-16

Operator Overloading

"Operator Overloading does not allow us to alter the meaning of operators when applied to built-in types Êone of the operands mustbe an object of a class "Operator Overloading does not allow us to define new operator symbols

Êwe overload those provided for in the

language to have meaning for a new type of data...and there are veryspecific rules!

CS202 6-17

Operator Overloading

"It is similar to overloading functions

Êexcept the function name is replaced by the

keyword operatorIROORRHG N\ POH RSHUMPRU·V symbol

Êthe return type represents the type of the

residual valueresulting from the operation

²rvalue?-lvalue?

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