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Chapter 8

Operator Overloading

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User-defined Meaning for “built-in” Operators

Natural, suggestive usage Complex number addition,

subtraction, etc.Semantic integrity

Assignment for objects with pointersUniformity with built-in types

<< operator with user defined types

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Format for Overloaded Operators

int& Array::operator+ (int i);

array1 = array2 + 10 array2.operator+(10)

Standard return typeand Class

Operatorkeyword Operator being

overloaded

ParameterParameterParameter

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Example Overloaded Operators for Array Class

class Array {private: int array[20];public: Array(int init = 0); // overload the index operator, [] int& operator[](int i); ~Array();};

int& Array::operator[](int i) { assert(0 <= i && i < 20); return array[i];}

Interface

Implementation

Checks arraybounds

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Using Overloaded Operator

Array array;

array[0] = 1;array[1] = 1;// compute first twenty// Fibonnaci numbersfor (int i = 2; i < 20; i++) array[i] = array[i-1] + array[i-2];

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Mixing User-Defined and Built-In Operations

Array safe; int regular[20];

regular[10] = safe[10];safe[11] = regular[11];safe[0] = safe[0] + regular[0]

“Safe” array in terms ofindex bounds

Typical array, no indexbounds checking

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Implementing Overloaded Operators for Arrays

int& Array::operator[](int i) { assert(0 <= i && i < 20); return array[i];}

Indexoperator

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Additional Overloaded ArrayOperators

class Array {private: int array[20];public: int& operator[](int i);

// addition operator Array operator+(Array& other); // subtraction operator Array operator-(Array& other); ~Array();};

Adding + and- operators

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Implementation of + and -Overloaded Operators

Array Array::operator+(Array& other) { Array result; for(int i=0; i<20; i++) result[i] = array[i] + other[i]; return result;}

Array Array::operator-(Array& other) { Array result; for(int i=0; i<20; i++) result[i] = array[i] - other[i]; return result;}

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Multiply Overloading an Operatorclass Array { private: int array[20];public: // addition operator Array operator+(Array& other); // add increment to all Array operator+(int increment); };

Array Array::operator+(int increment){ Array result; for(int i = 0; i< 20; i++) result[i] = array[i] + increment; return result;}

Both are +operators, but

parameters differ

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Usage of Multiply Overloaded Operators

Array one(1); // all elements are 1

// add 1 to each elementArray two = one + 1;

// pairwise addition of Arrays one and twoArray three = one + two;

Can we do this?Array four = 1 + one;

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When to Overload Operators with Nonmember Functions

Operators on primitive data types E.g., int, float, arrays

Class source not always available E.g., ostream

Type casting

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Operator Lookup Rules

Suppose:

Compiler looks for ‘op’ in this order Member function in X of form

Nonmember function of form

Return type is not part of lookup

Y y;X x; x op y;

operator op(Y)

operator op(X,Y)

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Nonmember Function Operator Overload

Array operator+(int increment, Array array){ Array result;

for(int i=0; i<20; i++)result[i] = array[i] + increment;

return result;}

Now we can do this!// add 1 to each elementArray four = 1 + one;

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Input-Output Using a Method

class Array{private: int array[20];public: // ... void Print(); ~Array();};

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Input-Output Using a Method (cont’d)

void Array::Print() { cout << "[ "; for(int i = 0; i < 19; i++) cout << array[i] << ", "; cout << array[19] << " ]";}

Array test;test.Print();

[0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0]

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Defining Stream Operators

ostream& operator<< (ostream& os, Array& a) { os << "[ "; for(int i = 0; i < 19; i++) os << a[i] << ", "; os << a[19] << " ]";}

ofstream outFile("array.data");

Array a;// write values of a to outfileoutFile << a;

// write to standard outputcout << a;

Cool!We can write

to any ostream!

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Overloading Stream Operators

ofstream& operator<<(ofstream& ofs, Array& a){

ofs.write((char*)(a.array), 20*sizeof(int)); return ofs;};

ofstream binaryFile("array.bin");

Array a;

// write a to file in binary form

binaryFile << a;

binaryFile.close();

How do we know the binary implementation

will be invoked?

But this is private data!

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Declaring a "friend" Stream I/O Operator

class Array { //... friend ofstream& operator<< (ofstream& ofs, Array& a);};

But not:class Array { //...

friend class ofstream;}

Why not?

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How do I make an overloaded << operator for my Parts Class?

class Part {public:

Part(PartType type, int time, int identifier, float quality);int GetTime() {return time;}int GetIdentifier() {return identifier;}PartType GetType() {return type;}float GetQuality() {return quality;}

protected:PartType type;int time;int identifier;float quality;

};

Show definition anddeclaration changesand additions.

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Stream Operator for a Part/Piece Object

/* This is not in the Parts class. */ostream& operator<< (ostream &os, Part&p) {

os << “ (“ << TranslateType(p.type) << “) ID# “;os << p.identifier << “ Quality: “;os << setw(4) << setprecision(2) << p.quality;os << “ Entered: “ << TranslateTime(p.time);os << “ #Recycles: << p.recycles << endl;

}

friend ofstream& operator<< (ofstream& ofs, Part& p);This is in the Parts class.

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Type Conversion

Object of Class Y

Need

Object of Class X

Have

Type Conversion

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Implicit and Explicit Type Conversion

int i; float f;

f = i; // implicit conversion

f = (float)i; // explicit conversion

f = float(i); // explicit conversion

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Type Conversion

Have:Counter count(0);// manipulate countif ( count.Value() > 0) ...if ( count.Value() == 100) ...cout << count.Value();

Want:Counter count(0);// manipulate countif ( count > 0) ...if ( count == 100) ...cout << count;

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Defining a Type Conversion Operator

class Counter: public DisplayableNumber{

...public:

...operator int();...

};

Simple int typeconversion operator

Used when implicit or explicit conversion done on a Counter

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Constructors as Type Conversion Mechanisms

class Counter : public DisplayableNumber {

...public:

Counter(int i);...

};

A constructor canbe viewed as a

way to convert anint to a Counter.

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Type Conversion andOperator Overloading

Given a class XConversion can be used to simplify

X + int int + X X + X

Provide a basic conversion from int to X

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Type Conversion andOperator Overloading: Colors

Color is Red, Green, Blue combinations

Each primary color 0 - 63 Color something(Red(?), Green(?),

Blue(?))

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Type Conversion andOperator Overloading: Colors

Color aqua(Red(0), Green(50), Blue(50));

Color darkRed = Red(63);Color purple = darkRed + aqua + Blue(13);Color peach = purple - Blue(13) - Green(3);

Color test1 = darkRed + peach;Color test2 = peach + darkRed;

cout << aqua << endl;cout << purple << endl;cout << peach << endl;cout << darkRed << endl;