lecture12.ppt

Introduction
• Operator overloading
– Enabling C++’s operators to work with class objects
– Using traditional operators with user-defined objects
– Requires great care; when overloading is misused,
program difficult to understand
– Examples of already overloaded operators
• Operator << is both the stream-insertion operator and the
bitwise left-shift operator
• + and -, perform arithmetic on multiple types
– Compiler generates the appropriate code based on the
manner in which the operator is used

Introduction
• Overloading an operator
– Write function definition as normal
– Function name is keyword operator followed by the
symbol for the operator being overloaded
– operator+ used to overload the addition operator (+)
• Using operators
– To use an operator on a class object it must be overloaded
unless the assignment operator(=)or the address
operator(&)
• Assignment operator by default performs memberwise assignment
• Address operator (&) by default returns the address of an object

Restrictions on Operator Overloading
• C++ operators that can be overloaded
• C++ Operators that cannot be overloaded
Operators that cannot be overloaded
. .* :: ?: sizeof
Operators that can be overloaded
+ - * / % ^ & |
~ ! = < > += -= *=
/= %= ^= &= |= << >> >>=
<<= == != <= >= && || ++
-- ->* , -> [] () new delete
new[] delete[]

Restrictions on Operator Overloading
• Overloading restrictions
– Precedence of an operator cannot be changed
– Associativity of an operator cannot be changed
– Arity (number of operands) cannot be changed
• Unary operators remain unary, and binary operators remain binary
• Operators &, *, + and - each have unary and binary versions
• Unary and binary versions can be overloaded separately
• No new operators can be created
– Use only existing operators
• No overloading operators for built-in types
– Cannot change how two integers are added
– Produces a syntax error

Operator Functions as Class Members
vs. as friend Functions
• Member vs non-member
– Operator functions can be member or non-member functions
– When overloading ( ), [ ], -> or any of the assignment
operators, must use a member function
• Operator functions as member functions
– Leftmost operand must be an object (or reference to an object)
of the class
• If left operand of a different type, operator function must be a non-
member function
• Operator functions as non-member functions
– Must be friends if needs to access private or protected
members
– Enable the operator to be commutative

Overloading Stream-Insertion and
Stream-Extraction Operators
• Overloaded << and >> operators
– Overloaded to perform input/output for user-
defined types
– Left operand of types ostream & and istream
&
– Must be a non-member function because left
operand is not an object of the class
– Must be a friend function to access private data
members

1 // Fig. 8.3: fig08_03.cpp
2 // Overloading the stream-insertion and
3 // stream-extraction operators.
4 #include <iostream>
5
6 using std::cout;
7 using std::cin;
8 using std::endl;
9 using std::ostream;
10 using std::istream;
11
12 #include <iomanip>
13
14 using std::setw;
15
16 class PhoneNumber {
17 friend ostream &operator<<( ostream&, const PhoneNumber & );
18 friend istream &operator>>( istream&, PhoneNumber & );
19
20 private:
21 char areaCode[ 4 ]; // 3-digit area code and null
22 char exchange[ 4 ]; // 3-digit exchange and null
23 char line[ 5 ]; // 4-digit line and null
24 };
25
26 // Overloaded stream-insertion operator (cannot be
27 // a member function if we would like to invoke it with
28 // cout << somePhoneNumber;).
29 ostream &operator<<( ostream &output, const PhoneNumber &num )
30 {

31 output << "(" << num.areaCode << ") "
32 << num.exchange << "-" << num.line;
33 return output; // enables cout << a << b << c;
34 }
35
36 istream &operator>>( istream &input, PhoneNumber &num )
37 {
38 input.ignore(); // skip (
39 input >> setw( 4 ) >> num.areaCode; // input area code
40 input.ignore( 2 ); // skip ) and space
41 input >> setw( 4 ) >> num.exchange; // input exchange
42 input.ignore(); // skip dash (-)
43 input >> setw( 5 ) >> num.line; // input line
44 return input; // enables cin >> a >> b >> c;
45 }
46
47 int main()
48 {
49 PhoneNumber phone; // create object phone
50
51 cout << "Enter phone number in the form (123) 456-7890:n";
52
53 // cin >> phone invokes operator>> function by
54 // issuing the call operator>>( cin, phone ).
55 cin >> phone;
56
57 // cout << phone invokes operator<< function by
58 // issuing the call operator<<( cout, phone ).
59 cout << "The phone number entered was: " << phone << endl;
60 return 0;
61 }

Program Output
Enter phone number in the form (123) 456-7890:
(800) 555-1212
The phone number entered was: (800) 555-1212

Overloading Unary Operators
• Overloading unary operators
– Can be overloaded with no arguments or one argument
– Should usually be implemented as member functions
• Avoid friend functions and classes because they violate the
encapsulation of a class
– Example declaration as a member function:
class String {
public:
bool operator!() const;
...
};

Overloading Unary Operators
– Example declaration as a non-member function
class String {
friend bool operator!( const
String & )
...
}

Overloading Binary Operators
• Overloaded Binary operators
– Non-static member function, one argument
– Example:
class String {
public:
const String &operator+=(
const String & );
...
};
– y += z is equivalent to y.operator+=( z )

Overloading Binary Operators
– Non-member function, two arguments
– Example:
class String {
friend const String &operator+=(
String &, const String & );
...
};
– y += z is equivalent to operator+=( y, z )

Case Study: An Array class
• Implement an Array class with
– Range checking
– Array assignment
– Arrays that know their size
– Outputting/inputting entire arrays with << and >>
– Array comparisons with == and !=

1 // Fig. 8.4: array1.h
2 // Simple class Array (for integers)
3 #ifndef ARRAY1_H
4 #define ARRAY1_H
5
7
10
11 class Array {
12 friend ostream &operator<<( ostream &, const Array & );
13 friend istream &operator>>( istream &, Array & );
14 public:
15 Array( int = 10 ); // default constructor
16 Array( const Array & ); // copy constructor
17 ~Array(); // destructor
18 int getSize() const; // return size
19 const Array &operator=( const Array & ); // assign arrays
20 bool operator==( const Array & ) const; // compare equal
21
22 // Determine if two arrays are not equal and
23 // return true, otherwise return false (uses operator==).
24 bool operator!=( const Array &right ) const
25 { return ! ( *this == right ); }
26
27 int &operator[]( int ); // subscript operator
28 const int &operator[]( int ) const; // subscript operator
29 static int getArrayCount(); // Return count of
30 // arrays instantiated.
31 private:
32 int size; // size of the array
33 int *ptr; // pointer to first element of array
34 static int arrayCount; // # of Arrays instantiated

35 };
36
37 #endif
38 // Fig 8.4: array1.cpp
39 // Member function definitions for class Array
41
42 using std::cout;
43 using std::cin;
44 using std::endl;
45
47
48 using std::setw;
49
50 #include <cstdlib>
51 #include <cassert>
52 #include "array1.h"
53
54 // Initialize static data member at file scope
55 int Array::arrayCount = 0; // no objects yet
56
57 // Default constructor for class Array (default size 10)
58 Array::Array( int arraySize )
59 {
60 size = ( arraySize > 0 ? arraySize : 10 );
61 ptr = new int[ size ]; // create space for array
62 assert( ptr != 0 ); // terminate if memory not allocated
63 ++arrayCount; // count one more object
64
65 for ( int i = 0; i < size; i++ )
66 ptr[ i ] = 0; // initialize array

67 }
68
69 // Copy constructor for class Array
70 // must receive a reference to prevent infinite recursion
71 Array::Array( const Array &init ) : size( init.size )
72 {
73 ptr = new int[ size ]; // create space for array
74 assert( ptr != 0 ); // terminate if memory not allocated
75 ++arrayCount; // count one more object
76
77 for ( int i = 0; i < size; i++ )
78 ptr[ i ] = init.ptr[ i ]; // copy init into object
79 }
80
81 // Destructor for class Array
82 Array::~Array()
83 {
84 delete [] ptr; // reclaim space for array
85 --arrayCount; // one fewer object
86 }
87
88 // Get the size of the array
89 int Array::getSize() const { return size; }
90
91 // Overloaded assignment operator
92 // const return avoids: ( a1 = a2 ) = a3
93 const Array &Array::operator=( const Array &right )
94 {
95 if ( &right != this ) { // check for self-assignment
96
97 // for arrays of different sizes, deallocate original
98 // left side array, then allocate new left side array.
99 if ( size != right.size ) {
100 delete [] ptr; // reclaim space

101 size = right.size; // resize this object
102 ptr = new int[ size ]; // create space for array copy
103 assert( ptr != 0 ); // terminate if not allocated
104 }
105
106 for ( int i = 0; i < size; i++ )
107 ptr[ i ] = right.ptr[ i ]; // copy array into object
108 }
109
110 return *this; // enables x = y = z;
111}
112
113// Determine if two arrays are equal and
114// return true, otherwise return false.
115bool Array::operator==( const Array &right ) const
116{
117 if ( size != right.size )
118 return false; // arrays of different sizes
119
120 for ( int i = 0; i < size; i++ )
121 if ( ptr[ i ] != right.ptr[ i ] )
122 return false; // arrays are not equal
123
124 return true; // arrays are equal
125}
126
127// Overloaded subscript operator for non-const Arrays
128// reference return creates an lvalue
129int &Array::operator[]( int subscript )
130{
131 // check for subscript out of range error
132 assert( 0 <= subscript && subscript < size );

133
134 return ptr[ subscript ]; // reference return
135}
136
137// Overloaded subscript operator for const Arrays
138// const reference return creates an rvalue
139const int &Array::operator[]( int subscript ) const
140{
141 // check for subscript out of range error
142 assert( 0 <= subscript && subscript < size );
143
144 return ptr[ subscript ]; // const reference return
145}
146
147// Return the number of Array objects instantiated
148// static functions cannot be const
149int Array::getArrayCount() { return arrayCount; }
150
151// Overloaded input operator for class Array;
152// inputs values for entire array.
153istream &operator>>( istream &input, Array &a )
154{
155 for ( int i = 0; i < a.size; i++ )
156 input >> a.ptr[ i ];
157
158 return input; // enables cin >> x >> y;
159}
160
161// Overloaded output operator for class Array
162ostream &operator<<( ostream &output, const Array &a )
163{

164 int i;
165
166 for ( i = 0; i < a.size; i++ ) {
167 output << setw( 12 ) << a.ptr[ i ];
168
169 if ( ( i + 1 ) % 4 == 0 ) // 4 numbers per row of output
170 output << endl;
171 }
172
173 if ( i % 4 != 0 )
174 output << endl;
175
176 return output; // enables cout << x << y;
177}
178// Fig. 8.4: fig08_04.cpp
179// Driver for simple class Array
180#include <iostream>
181
182using std::cout;
183using std::cin;
184using std::endl;
185
186#include "array1.h"
187
188int main()
189{
190 // no objects yet
191 cout << "# of arrays instantiated = "
192 << Array::getArrayCount() << 'n';
193
# of arrays instantiated = 0

194 // create two arrays and print Array count
195 Array integers1( 7 ), integers2;
196 cout << "# of arrays instantiated = "
197 << Array::getArrayCount() << "nn";
198
199 // print integers1 size and contents
200 cout << "Size of array integers1 is "
201 << integers1.getSize()
202 << "nArray after initialization:n"
203 << integers1 << 'n';
204
205 // print integers2 size and contents
206 cout << "Size of array integers2 is "
209 << integers2 << 'n';
210
211 // input and print integers1 and integers2
212 cout << "Input 17 integers:n";
213 cin >> integers1 >> integers2;
214 cout << "After input, the arrays contain:n"
215 << "integers1:n" << integers1
216 << "integers2:n" << integers2 << 'n';
217
218 // use overloaded inequality (!=) operator
219 cout << "Evaluating: integers1 != integers2n";
220 if ( integers1 != integers2 )
221 cout << "They are not equaln";
222
223 // create array integers3 using integers1 as an
224 // initializer; print size and contents
225 Array integers3( integers1 );
226

227 cout << "nSize of array integers3 is "
230 << integers3 << 'n';
231
232 // use overloaded assignment (=) operator
233 cout << "Assigning integers2 to integers1:n";
234 integers1 = integers2;
235 cout << "integers1:n" << integers1
236 << "integers2:n" << integers2 << 'n';
237
238 // use overloaded equality (==) operator
239 cout << "Evaluating: integers1 == integers2n";
240 if ( integers1 == integers2 )
241 cout << "They are equalnn";
242
243 // use overloaded subscript operator to create rvalue
244 cout << "integers1[5] is " << integers1[ 5 ] << 'n';
245
246 // use overloaded subscript operator to create lvalue
247 cout << "Assigning 1000 to integers1[5]n";
248 integers1[ 5 ] = 1000;
249 cout << "integers1:n" << integers1 << 'n';
250
251 // attempt to use out of range subscript
252 cout << "Attempt to assign 1000 to integers1[15]" << endl;
253 integers1[ 15 ] = 1000; // ERROR: out of range
254
255 return 0;
256}

Size of array integers1 is 7
Array after initialization:
0 0 0 0
0 0 0
0 0 0 0
0 0 0 0
0 0
Input 17 integers:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
After input, the arrays contain:
integers1:
1 2 3 4
5 6 7
integers2:
8 9 10 11
12 13 14 15
16 17
Evaluating: integers1 != integers2
They are not equal
1 2 3 4
5 6 7

Evaluating: integers1 == integers2
They are equal
integers1[5] is 13
Assigning 1000 to integers1[5]
integers1:
8 9 10 11
12 1000 14 15
16 17
Attempt to assign 1000 to integers1[15]
Assertion failed: 0 <= subscript && subscript < size, file Array1.cpp,
line 95 abnormal program termination
Assigning integers2 to integers1:
integers1:
8 9 10 11
12 13 14 15
16 17
integers2:
8 9 10 11
12 13 14 15
16 17

Converting between Types
• Cast operator
– Forces conversions among built-in types
– Specifies conversions between user defined and built-in
types
– Conversion operator must be a non-static member
function
– Cannot be a friend function
– Do not specify return type
• Return type is the type to which the object is being converted
– For user-defined class A
A::operator char *() const;
• Declares an overloaded cast operator function for creating a char
* out of an A object

Converting between Types
A::operator int() const;
• Declares an overloaded cast operator function for converting an
object of A into an integer
A::operator otherClass() const;
• Declares an overloaded cast operator function for converting an
object of A into an object of otherClass
• Compiler and casting
– Casting can prevent the need for overloading
– If an object s of user-defined class String appears in a
program where an ordinary char * is expected, such as
cout << s;
The compiler calls the overloaded cast operator function
operator char * to convert the object into a char * and
uses the resulting char * in the expression

Case Study: A String Class
• Build a class to handle strings
– Class string in standard library
• Conversion constructor
– Single-argument constructors that turn objects
of other types into class objects

1 // Fig. 8.5: string1.h
2 // Definition of a String class
3 #ifndef STRING1_H
4 #define STRING1_H
5
7
10
11 class String {
12 friend ostream &operator<<( ostream &, const String & );
13 friend istream &operator>>( istream &, String & );
14
15 public:
16 String( const char * = "" ); // conversion/default ctor
17 String( const String & ); // copy constructor
18 ~String(); // destructor
19 const String &operator=( const String & ); // assignment
20 const String &operator+=( const String & ); // concatenation
21 bool operator!() const; // is String empty?
22 bool operator==( const String & ) const; // test s1 == s2
23 bool operator<( const String & ) const; // test s1 < s2
24
25 // test s1 != s2
26 bool operator!=( const String & right ) const
27 { return !( *this == right ); }
28
29 // test s1 > s2
30 bool operator>( const String &right ) const
31 { return right < *this; }
32
33 // test s1 <= s2

34 bool operator<=( const String &right ) const
35 { return !( right < *this ); }
36
37 // test s1 >= s2
38 bool operator>=( const String &right ) const
39 { return !( *this < right ); }
40
41 char &operator[]( int ); // subscript operator
42 const char &operator[]( int ) const; // subscript operator
43 String operator()( int, int ); // return a substring
44 int getLength() const; // return string length
45
46 private:
47 int length; // string length
48 char *sPtr; // pointer to start of string
49
50 void setString( const char * ); // utility function
51 };
52
53 #endif
54 // Fig. 8.5: string1.cpp
55 // Member function definitions for class String
57
58 using std::cout;
59 using std::endl;
60
62
63 using std::setw;
64

65 #include <cstring>
66 #include <cassert>
67 #include "string1.h"
68
69 // Conversion constructor: Convert char * to String
70 String::String( const char *s ) : length( strlen( s ) )
71 {
72 cout << "Conversion constructor: " << s << 'n';
73 setString( s ); // call utility function
74 }
75
76 // Copy constructor
77 String::String( const String &copy ) : length( copy.length )
78 {
79 cout << "Copy constructor: " << copy.sPtr << 'n';
80 setString( copy.sPtr ); // call utility function
81 }
82
83 // Destructor
84 String::~String()
85 {
86 cout << "Destructor: " << sPtr << 'n';
87 delete [] sPtr; // reclaim string
88 }
89
90 // Overloaded = operator; avoids self assignment
91 const String &String::operator=( const String &right )
92 {
93 cout << "operator= calledn";
94
95 if ( &right != this ) { // avoid self assignment
Conversion constructor: char * to String.
Constructors and destructors
will print when called.

96 delete [] sPtr; // prevents memory leak
97 length = right.length; // new String length
98 setString( right.sPtr ); // call utility function
99 }
100 else
101 cout << "Attempted assignment of a String to itselfn";
102
103 return *this; // enables cascaded assignments
104}
105
106// Concatenate right operand to this object and
107// store in this object.
108const String &String::operator+=( const String &right )
109{
110 char *tempPtr = sPtr; // hold to be able to delete
111 length += right.length; // new String length
112 sPtr = new char[ length + 1 ]; // create space
113 assert( sPtr != 0 ); // terminate if memory not allocated
114 strcpy( sPtr, tempPtr ); // left part of new String
115 strcat( sPtr, right.sPtr ); // right part of new String
116 delete [] tempPtr; // reclaim old space
117 return *this; // enables cascaded calls
118}
119
120// Is this String empty?
121bool String::operator!() const { return length == 0; }
122
123// Is this String equal to right String?
124bool String::operator==( const String &right ) const
125 { return strcmp( sPtr, right.sPtr ) == 0; }
126
127// Is this String less than right String?

128bool String::operator<( const String &right ) const
129 { return strcmp( sPtr, right.sPtr ) < 0; }
130
131// Return a reference to a character in a String as an lvalue.
132char &String::operator[]( int subscript )
133{
134 // First test for subscript out of range
135 assert( subscript >= 0 && subscript < length );
136
137 return sPtr[ subscript ]; // creates lvalue
138}
139
140// Return a reference to a character in a String as an rvalue.
141const char &String::operator[]( int subscript ) const
142{
143 // First test for subscript out of range
144 assert( subscript >= 0 && subscript < length );
145
146 return sPtr[ subscript ]; // creates rvalue
147}
148
149// Return a substring beginning at index and
150// of length subLength
151String String::operator()( int index, int subLength )
152{
153 // ensure index is in range and substring length >= 0
154 assert( index >= 0 && index < length && subLength >= 0 );
155
156 // determine length of substring
157 int len;
158
Notice the overloaded
function call operator.

159 if ( ( subLength == 0 ) || ( index + subLength > length ) )
160 len = length - index;
161 else
162 len = subLength;
163
164 // allocate temporary array for substring and
165 // terminating null character
166 char *tempPtr = new char[ len + 1 ];
167 assert( tempPtr != 0 ); // ensure space allocated
168
169 // copy substring into char array and terminate string
170 strncpy( tempPtr, &sPtr[ index ], len );
171 tempPtr[ len ] = '0';
172
173 // Create temporary String object containing the substring
174 String tempString( tempPtr );
175 delete [] tempPtr; // delete the temporary array
176
177 return tempString; // return copy of the temporary String
178}
179
180// Return string length
181int String::getLength() const { return length; }
182
183// Utility function to be called by constructors and
184// assignment operator.
185void String::setString( const char *string2 )
186{
187 sPtr = new char[ length + 1 ]; // allocate storage
188 assert( sPtr != 0 ); // terminate if memory not allocated
189 strcpy( sPtr, string2 ); // copy literal to object
190}

191
192// Overloaded output operator
193ostream &operator<<( ostream &output, const String &s )
194{
195 output << s.sPtr;
196 return output; // enables cascading
197}
198
199// Overloaded input operator
200istream &operator>>( istream &input, String &s )
201{
202 char temp[ 100 ]; // buffer to store input
203
204 input >> setw( 100 ) >> temp;
205 s = temp; // use String class assignment operator
206 return input; // enables cascading
207}
208// Fig. 8.5: fig08_05.cpp
209// Driver for class String
211
212using std::cout;
213using std::endl;
214
215#include "string1.h"
216
217int main()
218{
219 String s1( "happy" ), s2( " birthday" ), s3;
220
Conversion constructor: happy
Conversion constructor: birthday
Conversion constructor:

221 // test overloaded equality and relational operators
222 cout << "s1 is "" << s1 << ""; s2 is "" << s2
223 << ""; s3 is "" << s3 << '"'
224 << "nThe results of comparing s2 and s1:"
225 << "ns2 == s1 yields "
226 << ( s2 == s1 ? "true" : "false" )
227 << "ns2 != s1 yields "
228 << ( s2 != s1 ? "true" : "false" )
229 << "ns2 > s1 yields "
230 << ( s2 > s1 ? "true" : "false" )
231 << "ns2 < s1 yields "
232 << ( s2 < s1 ? "true" : "false" )
233 << "ns2 >= s1 yields "
234 << ( s2 >= s1 ? "true" : "false" )
235 << "ns2 <= s1 yields "
236 << ( s2 <= s1 ? "true" : "false" );
237
238 // test overloaded String empty (!) operator
239 cout << "nnTesting !s3:n";
240 if ( !s3 ) {
241 cout << "s3 is empty; assigning s1 to s3;n";
242 s3 = s1; // test overloaded assignment
243 cout << "s3 is "" << s3 << """;
244 }
245
246 // test overloaded String concatenation operator
247 cout << "nns1 += s2 yields s1 = ";
248 s1 += s2; // test overloaded concatenation
249 cout << s1;
250
251 // test conversion constructor
252 cout << "nns1 += " to you" yieldsn";
253 s1 += " to you"; // test conversion constructor

254 cout << "s1 = " << s1 << "nn";
255
256 // test overloaded function call operator () for substring
257 cout << "The substring of s1 starting atn"
258 << "location 0 for 14 characters, s1(0, 14), is:n"
259 << s1( 0, 14 ) << "nn";
260
261 // test substring "to-end-of-String" option
262 cout << "The substring of s1 starting atn"
263 << "location 15, s1(15, 0), is: "
264 << s1( 15, 0 ) << "nn"; // 0 is "to end of string"
265
266 // test copy constructor
267 String *s4Ptr = new String( s1 );
268 cout << "*s4Ptr = " << *s4Ptr << "nn";
269
270 // test assignment (=) operator with self-assignment
271 cout << "assigning *s4Ptr to *s4Ptrn";
272 *s4Ptr = *s4Ptr; // test overloaded assignment
273 cout << "*s4Ptr = " << *s4Ptr << 'n';
274
275 // test destructor
276 delete s4Ptr;
277
278 // test using subscript operator to create lvalue
279 s1[ 0 ] = 'H';
280 s1[ 6 ] = 'B';
281 cout << "ns1 after s1[0] = 'H' and s1[6] = 'B' is: "
282 << s1 << "nn";
283
s1 = happy birthday to you

284 // test subscript out of range
285 cout << "Attempt to assign 'd' to s1[30] yields:" << endl;
286 s1[ 30 ] = 'd'; // ERROR: subscript out of range
287
288 return 0;
289}
Conversion constructor: happy
Conversion constructor: birthday
Conversion constructor:
s1 is "happy"; s2 is " birthday"; s3 is ""
The results of comparing s2 and s1:
s2 == s1 yields false
s2 != s1 yields true
s2 > s1 yields false
s2 < s1 yields true
s2 >= s1 yields false
s2 <= s1 yields true
Testing !s3:
s3 is empty; assigning s1 to s3;
operator= called
s3 is "happy"
s1 += s2 yields s1 = happy birthday
s1 += " to you" yields
Conversion constructor: to you
Destructor: to you
s1 = happy birthday to you
Attempt to assign 'd' to s1[30] yields:
Assertion failed: subscript >= 0 && subscript <
length, file string1.cpp, line 82
Abnormal program termination

Program Output
Conversion constructor: happy birthday
Copy constructor: happy birthday
Destructor: happy birthday
The substring of s1 starting at
location 0 for 14 characters, s1(0, 14), is:
happy birthday
Destructor: happy birthday
Conversion constructor: to you
Copy constructor: to you
Destructor: to you
The substring of s1 starting at
location 15, s1(15, 0), is: to you
Destructor: to you
Copy constructor: happy birthday to you
*s4Ptr = happy birthday to you
assigning *s4Ptr to *s4Ptr
operator= called
Attempted assignment of a String to itself
*s4Ptr = happy birthday to you
Destructor: happy birthday to you
s1 after s1[0] = 'H' and s1[6] = 'B' is: Happy Birthday to you
Attempt to assign 'd' to s1[30] yields:
Assertion failed: subscript >= 0 && subscript < length, file
string1.cpp, line 82
Abnormal program termination

Overloading ++ and --
• Pre/post incrementing/decrementing operators
– Allowed to be overloaded
– Distinguishing between pre and post operators
• prefix versions are overloaded the same as other prefix
unary operators
d1.operator++(); // for ++d1
• convention adopted that when compiler sees
postincrementing expression, it will generate the
member-function call
d1.operator++( 0 ); // for d1++
•0 is a dummy value to make the argument list of
operator++ distinguishable from the argument list
for ++operator

Case Study: A Date Class
• The following example creates a Date class
with
– An overloaded increment operator to change
the day, month and year
– An overloaded += operator
– A function to test for leap years
– A function to determine if a day is last day of a
month

1 // Fig. 8.6: date1.h
2 // Definition of class Date
3 #ifndef DATE1_H
4 #define DATE1_H
6
8
9 class Date {
10 friend ostream &operator<<( ostream &, const Date & );
11
12 public:
13 Date( int m = 1, int d = 1, int y = 1900 ); // constructor
14 void setDate( int, int, int ); // set the date
15 Date &operator++(); // preincrement operator
16 Date operator++( int ); // postincrement operator
17 const Date &operator+=( int ); // add days, modify object
18 bool leapYear( int ) const; // is this a leap year?
19 bool endOfMonth( int ) const; // is this end of month?
20
21 private:
22 int month;
23 int day;
24 int year;
25
26 static const int days[]; // array of days per month
27 void helpIncrement(); // utility function
28 };
29
30 #endif

31 // Fig. 8.6: date1.cpp
32 // Member function definitions for Date class
34 #include "date1.h"
35
36 // Initialize static member at file scope;
37 // one class-wide copy.
38 const int Date::days[] = { 0, 31, 28, 31, 30, 31, 30,
39 31, 31, 30, 31, 30, 31 };
40
41 // Date constructor
42 Date::Date( int m, int d, int y ) { setDate( m, d, y ); }
43
44 // Set the date
45 void Date::setDate( int mm, int dd, int yy )
46 {
47 month = ( mm >= 1 && mm <= 12 ) ? mm : 1;
48 year = ( yy >= 1900 && yy <= 2100 ) ? yy : 1900;
49
50 // test for a leap year
51 if ( month == 2 && leapYear( year ) )
52 day = ( dd >= 1 && dd <= 29 ) ? dd : 1;
53 else
54 day = ( dd >= 1 && dd <= days[ month ] ) ? dd : 1;
55 }
56
57 // Preincrement operator overloaded as a member function.
58 Date &Date::operator++()
59 {
60 helpIncrement();
61 return *this; // reference return to create an lvalue
62 }
63

64 // Postincrement operator overloaded as a member function.
65 // Note that the dummy integer parameter does not have a
66 // parameter name.
67 Date Date::operator++( int )
68 {
69 Date temp = *this;
70 helpIncrement();
71
72 // return non-incremented, saved, temporary object
73 return temp; // value return; not a reference return
74 }
75
76 // Add a specific number of days to a date
77 const Date &Date::operator+=( int additionalDays )
78 {
79 for ( int i = 0; i < additionalDays; i++ )
80 helpIncrement();
81
82 return *this; // enables cascading
83 }
84
85 // If the year is a leap year, return true;
86 // otherwise, return false
87 bool Date::leapYear( int y ) const
88 {
89 if ( y % 400 == 0 || ( y % 100 != 0 && y % 4 == 0 ) )
90 return true; // a leap year
91 else
92 return false; // not a leap year
93 }
94
95 // Determine if the day is the end of the month
96 bool Date::endOfMonth( int d ) const
97 {
postincrement operator
has a dummy int value.

98 if ( month == 2 && leapYear( year ) )
99 return d == 29; // last day of Feb. in leap year
100 else
101 return d == days[ month ];
102}
103
104// Function to help increment the date
105void Date::helpIncrement()
106{
107 if ( endOfMonth( day ) && month == 12 ) { // end year
108 day = 1;
109 month = 1;
110 ++year;
111 }
112 else if ( endOfMonth( day ) ) { // end month
113 day = 1;
114 ++month;
115 }
116 else // not end of month or year; increment day
117 ++day;
118}
119
120// Overloaded output operator
121ostream &operator<<( ostream &output, const Date &d )
122{
123 static char *monthName[ 13 ] = { "", "January",
124 "February", "March", "April", "May", "June",
125 "July", "August", "September", "October",
126 "November", "December" };
127
128 output << monthName[ d.month ] << ' '
129 << d.day << ", " << d.year;
130
131 return output; // enables cascading
132}

133// Fig. 8.6: fig08_06.cpp
134// Driver for class Date
136
137using std::cout;
138using std::endl;
139
140#include "date1.h"
141
142int main()
143{
144 Date d1, d2( 12, 27, 1992 ), d3( 0, 99, 8045 );
145 cout << "d1 is " << d1
146 << "nd2 is " << d2
147 << "nd3 is " << d3 << "nn";
148
149 cout << "d2 += 7 is " << ( d2 += 7 ) << "nn";
150
151 d3.setDate( 2, 28, 1992 );
152 cout << " d3 is " << d3;
153 cout << "n++d3 is " << ++d3 << "nn";
154
155 Date d4( 3, 18, 1969 );
156
157 cout << "Testing the preincrement operator:n"
158 << " d4 is " << d4 << 'n';
159 cout << "++d4 is " << ++d4 << 'n';
160 cout << " d4 is " << d4 << "nn";
161
162 cout << "Testing the postincrement operator:n"
163 << " d4 is " << d4 << 'n';
164 cout << "d4++ is " << d4++ << 'n';
165 cout << " d4 is " << d4 << endl;
166
167 return 0;
168}

d1 is January 1, 1900
d2 is December 27, 1992
d3 is January 1, 1900
d2 += 7 is January 3, 1993
d3 is February 28, 1992
++d3 is February 29, 1992
Testing the preincrement operator:
d4 is March 18, 1969
++d4 is March 19, 1969
Testing the postincrement operator:
d4++ is March 19, 1969

lecture12.ppt

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