Chapter 7Operator Overloading and Type Conversions
§7.1 Defining Operator Overloading
§7.2 Overloading Unary Operators
§7.3 Overloading Binary Operators
§7.4 Overloading Other Operators
§7.5 Type Conversions
§7.1 Introduction
Operators （运算符）
How about object variables? String operators
+ - * / % ^ & | ~ ! = < > += -= *= /= %= ^= &= |= <<
>> >>= <<= == != <= >= && || ++ --
->* , -> [] () new delete
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string s1 = "ABC";string s2 = s1 + "DEF";cout<<s1<<endl;cout<<(s1>s2)<<endl;...
string s1 = "ABC";string s2 = s1 + "DEF";cout<<s1<<endl;cout<<(s1>s2)<<endl;...
int a = 1;int b = a+1;if(a<b) cout<<a;
int a = 1;int b = a+1;if(a<b) cout<<a;
class C{};C c, a, b;c = a + b;//?
class C{};C c, a, b;c = a + b;//?
The Rational Class （分数） Rational
-numerator: long
-denominator: long
+Rational()
+Rational(numerator: long, denominator: long)
+getNumerator(): long
+getDenominator(): long
+add(secondRational: Rational &): Rational
+subtract(secondRational: Rational &): Rational
+multiply(secondRational: Rational &): Rational
+divide(secondRational: Rational &): Rational +compareTo(secondRational: Rational &): int
+equals(secondRational: Rational &): bool
+intValue(): int
+doubleValue(): double
+toString(): string
-gcd(n: long, d: long): long
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1/2, 1/3, 11/12,…1/2, 1/3, 11/12,…
3 4 23 83 4 23 8
4 7 32 9…4 7 32 9…
bbaa
Rational.cppRational.cpp
RunRun
Rational.hRational.h
TestRationalClass.cppTestRationalClass.cpp
分子分母
Operation of the Rational Class
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Rational Rational::add(Rational &secondRational) {long n = numerator * secondRational.getDenominator() + denominator * secondRational.getNumerator();long d = denominator * secondRational.getDenominator();return Rational(n, d);
}Rational Rational::subtract(Rational &secondRational){
long n = numerator * secondRational.getDenominator() - denominator * secondRational.getNumerator();long d = denominator * secondRational.getDenominator();return Rational(n, d);
}…
Rational Rational::add(Rational &secondRational) {long n = numerator * secondRational.getDenominator() + denominator * secondRational.getNumerator();long d = denominator * secondRational.getDenominator();return Rational(n, d);
}Rational Rational::subtract(Rational &secondRational){
long n = numerator * secondRational.getDenominator() - denominator * secondRational.getNumerator();long d = denominator * secondRational.getDenominator();return Rational(n, d);
}…
cout<<r1.toString()<< " + "<< r2.toString()<< " = "<< r1.add(r2).toString()<< endl; cout<<r1.toString()<< " - "<< r2.toString()<< " = "<< r1.subtract(r2).toString()<< endl; …
cout<<r1.toString()<< " + "<< r2.toString()<< " = "<< r1.add(r2).toString()<< endl; cout<<r1.toString()<< " - "<< r2.toString()<< " = "<< r1.subtract(r2).toString()<< endl; …
cout << r1<< " + " << r2 << " = "<< r1+r2<< endl; cout << r1<< " - " << r2 << " = "<<r1-r2<< endl; …
cout << r1<< " + " << r2 << " = "<< r1+r2<< endl; cout << r1<< " - " << r2 << " = "<<r1-r2<< endl; …
Operator Functions （运算符函数） The functions to overload operators
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bool Rational::operator< (Rational &secondRational){ if (this->compareTo(secondRational) < 0) return true; else return false;}
if(r1 < r2) cout<<“r1 is less than r2.”<<endl;if(r1 < r2) cout<<“r1 is less than r2.”<<endl;
if(r1.operator<(r2)) cout<<“r1 is less than r2.”<<endl;if(r1.operator<(r2)) cout<<“r1 is less than r2.”<<endl;
Defining Operator Overloading
General form
Can also be friends:
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type classname :: operator op(arglis){ Function body // task defined}
vector operator+ (vector); // vector additionvector operator- (); // unary minusfriend vector operator+ (vector, vector);//vector additionfriend vector operator- (vector); // unary minus
§7.2 Overloading Unary Operators
‘+’ and ‘-’ --no parameters
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Overloading Unary MinusOverloading Unary Minus
void space :: operator-(){ x=-x; y=-y; z=-z;}
//declarationfriend void operator-(space &s);//definition void operator-(space &s) { s.x=-s.x; s.y=-s.y; s.z=-s.z;}
Overloading ++ and --
Prefix vs. Postfix
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++r1
r1++
Rational Rational::operator++() { numerator += denominator; return *this;
}
Rational Rational::operator++(){ Rational temp(numerator, denominator); numerator += denominator; return temp;
}
(int dummy){
Same signature,How to distinguish them?
哑元参数 ,the value is never used;It must be “int”.
Same signature,How to distinguish them?
Overloading “[]”
The array subscript [] is an operator Can be overloaded to access the members of an
object
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long Rational::operator[](const int &index){ if (index == 0) return numerator; else if (index == 1) return denominator; else{ cout << "subscript error" << endl; exit(0); }
}
int main(){ Rational r(2,3); cout << r[0]/r[1];
}
[] Accessor and Mutator The [] operator functions as both accessor and
mutator. After adding this operator to the Rational class,
the Rational class is mutable.
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Rational r4(1, 2); r4[0] = 3; r4[1] = 4;
Rational
-numerator: long
-denominator: long
+Rational()
+Rational(numerator: long, denominator: long)
+getNumerator(): long
+getDenominator(): long
+add(secondRational: Rational &): Rational
+subtract(secondRational: Rational &): Rational
+multiply(secondRational: Rational &): Rational
+divide(secondRational: Rational &): Rational +compareTo(secondRational: Rational &): int
+equals(secondRational: Rational &): bool
+intValue(): int
+doubleValue(): double
+toString(): string
-gcd(n: long, d: long): long
long &Rational::operator[](const int &index)
§7.3 Overloading Binary Operators
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Overloading + operatorOverloading + operator
//functional notationC = sum ( A, B);
//arithmetic notationC = A + B;
complex complex :: operator+(complex c){ complex temp; temp.x = x + c.x; temp.y = y + c.y; return(temp); }
Operands of Binary Operators
Features: Only one complex argument Returns a complex type value A member function of complex
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complex complex :: operator+(complex c){ complex temp; temp.x = x + c.x; temp.y = y + c.y; return(temp); } C3 = C1 + C2;
C3 = C1.operator+(C2);
Operands of Binary Operators
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complex complex :: operator+(complex c){ complex temp;
temp.x = c.x + x; temp.y = c.y + y;
return(temp);}
4.10
6.20
temp
4.10 x
6.20 y
C3
2.50 x
3.50 y
C1
1.60 x
2.70 y
C2;+=
Fig.7.1 ⇔ Implementation of the overload + operator
Notes
The left operand is fixed to be the operating object automatically
c1 = c2 + c3 ; c1 = c2.operator+(c3);
The number of parameters is determined by the operator itself You cannot change it
Overloading does not change the operator precedence （优先级） and associativity （结合性）
The (return) type of the operator function can be defined by you Usually the same class type to be operational in complex
operation
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§7.4 Overloading Other Operators
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+=, -=, *=, /=
Rational Rational::operator +=(Rational &secondRational){ *this = this->add(secondRational); return (*this);}
Rational r1(2, 4);Rational r2 = r1 += Rational(2, 3);cout << "r1 is " << r1.toString() << endl;cout << "r2 is " << r2.toString() << endl;
Overloading “=”
By default, “=” performs a memberwise copy For Rational class, it is OK.
How about others?
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Rational r1(1, 2);
Rational r2(4, 5);
r1 = r2; class Person{private: int id; Date* birthDate; };…p1 = p2;
Overloading “=”
“The rule of three” Copy constructor Destructor “=”
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person1: Person
id = 111
birthDate
111
Memory : BirthDate
year = 1963 month = 5 day = 3 reference
person2: Person
id = 111
birthDate
111
Memory
reference
Overloading “=”
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const Person Person::operator=(const Person &p){id = p.id;Date *p=p.getBirthDate();birthDate = new Date(*p);return *this;
}
Friend: the left-hand operand as an argument
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Friend vs. Member
C3 = C1 + C2 C3 = C1.operator+(C2)
C3 = C1 + 2 C3 = C1.operator+(2)
C3 = 2 + C1 C3 = 2.operator+(C1) ???
C3 = 2 + C1 C3 = operator+(2, C1)
friend complex operator+(complex a, complex b);complex operator+(complex a, complex b) { return complex((a.x+b.x),(a.y+b.y)); }
Two versions p = 2 * m;//equivalent to p = operator*(2,m)
q = n * 2;//equivalent to q = operator*(n,2) By overloading >> and << using functions:
friend istream& operator>>(istream&, vector&);
friend ostream& operator<<(ostream&, vector&);
vector variables in input and output statements can be used like simple variables
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Overloading >> and <<
Overloading as friends.cppOverloading as friends.cpp
Manipulation of Strings To define operators to manipulate the strings For example:
string3 = string1 + string2; if( string1 >= string2 ) string = string1;
An example program overloads + and <=
21Mathematical operations on stringsMathematical operations on strings
// overloading + operatorString operator+(const String &s, const String &t){ String temp; temp.len = s.len + t.len; temp.p = new char[temp.len+1]; strcpy(temp.p,s.p); strcat(temp.p,t.p); return temp;}
// overloading <= operatorint operator<=(const String &s, const String &t){
int m = strlen(s.p);int n = strlen(t.p);if(m <= n) return(1);else return(0);
}
Overloadable Operators
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+ - * / % ^ & | ~ ! = < > += -= *= /= %= ^= &= |= <<
>> >>= <<= == != <= >= && || ++ --
->* , -> [] () new delete
Unoverloadable Operators:
:: . .* ? : # sizeof
Unoverloadable Operators:
:: . .* ? : # sizeof
A friend cannot be used:
= ( ) [ ] ->
A friend cannot be used:
= ( ) [ ] ->
§7.5 Type Conversions
Automatic type conversion for built-in types Constants and variables of different types are mixed in
an expression Right type of an assignment operator “=“ to the left
For example,int m;
float x = 3.14159;
m = x; What happens when they are user-defined data
types?
Using Casting operator or Constructor
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Three types of situations
A. Conversion from basic type to class type
B. Conversion from class type to basic type
C. Conversion from one class type to another class type
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Basic to Class Type
Implemented by using constructors with single argument
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string::string (char * a){ length = strlen(a); p = new char[length+1]; strcpy(p, a);}
string s1,s2;char* name1 = “IBM PC”;char* name2 = “Apple”;s1 = string(name1);s2 = name2;
Class to Basic Type
By defining an overloaded casting operator Usually referred to as a conversion function:
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operator typename(){ ...... ......(Function statements)}
Rational::operator double(){ return numerator/denominator;}
int main(){ Rational r(2,3); double d = r;}
string:: operator char*(){ return (p);}
Notes on class basic type
1. It must be a class member
2. It must not specify a return type
3. It must not have any arguments
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One Class to Another Class Type By either a constructor or a conversion function
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objx = objy // Y is a source class
Casting operator function
Data access functions
Constructor function
Class X
Argument of type Y
Class Y
Conversion here (destination class)
Conversion here (source class)
Class Y
Converted value of type X
Fig.7.2 ⇔ Conversion between object
An Example of Class to Class
A data conversion example From invent1 to invent2
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Data conversionsData conversions
//constructor for conversion//invent2(invent1 p){ code = p.getcode(); value =p.getitems() * p.getprice();
}
//operator function for conversionoperator invent2(){ invent2 temp; temp.code = code; temp.value = price * items; return temp; }
Summary of all the three conversions
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Conversion requiredConversion takes place in
Source class Destination class
Basic → Class Not applicable Constructor
Class → Basic Casting operator Not applicable
Class → Class Casting operator Constructor
Summary
Operator functions to overload operators Overloading as member or friend Overloading unary, binary operators
Operands? Overloading other operators Type conversion
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Review Questons
Q7.3 Q7.7 Q7.8
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