Case Study: String Class

Case Study String Class

As a capstone exercise to our study of overloading, we will build our own String class to handle the creation and manipulation of strings (Figs. 11.911.11). The C++ standard library provides a similar, more robust class string as well. We present an example of the standard class string in Section 11.13 and study class string in detail in Chapter 18. For now, we will make extensive use of operator overloading to craft our own class String.

Figure 11.9. String class definition with operator overloading.

(This item is displayed on pages 595 - 596 in the print version)

 1 // Fig. 11.9: String.h
 2 // String class definition.
 3 #ifndef STRING_H
 4 #define STRING_H
 5
 6 #include 
 7 using std::ostream;
 8 using std::istream;
 9
10 class String
11 {
12 friend ostream &operator<<( ostream &, const String & );
13 friend istream &operator>>( istream &, String & );
14 public:
15 String( const char * = "" ); // conversion/default constructor
16 String( const String & ); // copy constructor 
17 ~String(); // destructor 
18
19 const String &operator=( const String & ); // assignment operator 
20 const String &operator+=( const String & ); // concatenation operator
21
22 bool operator!() const; // is String empty? 
23 bool operator==( const String & ) const; // test s1 == s2
24 bool operator<( const String & ) const; // test s1 < s2 
25
26 // test s1 != s2 
27 bool operator!=( const String &right ) const
28 { 
29  return !( *this == right ); 
30 } // end function operator!= 
31
32 // test s1 > s2 
33 bool operator>( const String &right ) const
34 { 
35  return right < *this; 
36 } // end function operator> 
37
38 // test s1 <= s2 
39 bool operator<=( const String &right ) const
40 { 
41  return !( right < *this ); 
42 } // end function operator <= 
43
44 // test s1 >= s2 
45 bool operator>=( const String &right ) const
46 { 
47  return !( *this < right ); 
48 } // end function operator>= 
49
50 char &operator[]( int ); // subscript operator (modifiable lvalue)
51 char operator[]( int ) const; // subscript operator (rvalue) 
52 String operator()( int, int = 0 ) const; // return a substring 
53 int getLength() const; // return string length
54 private:
55 int length; // string length (not counting null terminator)
56 char *sPtr; // pointer to start of pointer-based string
57
58 void setString( const char * ); // utility function
59 }; // end class String
60
61 #endif

First, we present the header file for class String. We discuss the private data used to represent String objects. Then we walk through the class's public interface, discussing each of the services the class provides. We discuss the member-function definitions for the class String. For each of the overloaded operator functions, we show the code in the program that invokes the overloaded operator function, and we provide an explanation of how the overloaded operator function works.

String Class Definition

Now let us walk through the String class header file in Fig. 11.9. We begin with the internal pointer-based representation of a String. Lines 5556 declare the private data members of the class. Our String class has a length field, which represents the number of characters in the string, not including the null character at the end, and has a pointer sPtr that points to the dynamically allocated memory representing the character string.

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Overloading the Stream Insertion and Stream Extraction Operators as friends

Lines 1213 (Fig. 11.9) declare the overloaded stream insertion operator function operator<< (defined in Fig. 11.10, lines 170174) and the overloaded stream extraction operator function operator>> (defined in Fig. 11.10, lines 177183) as friends of the class. The implementation of operator<< is straightforward. Note that operator>> restricts the total number of characters that can be read into array temp to 99 with setw (line 180); the 100th position is reserved for the string's terminating null character. [ Note: We did not have this restriction for operator>> in class Array (Figs. 11.611.7), because that class's operator>> read one array element at a time and stopped reading values when the end of the array was reached. Object cin does not know how to do this by default for input of character arrays.] Also, note the use of operator= (line 181) to assign the C-style string temp to the String object to which s refers. This statement invokes the conversion constructor to create a temporary String object containing the C-style string; the temporary String is then assigned to s. We could eliminate the overhead of creating the temporary String object here by providing another overloaded assignment operator that receives a parameter of type const char *.

Figure 11.10. String class member-function and friend-function definitions.

(This item is displayed on pages 597 - 600 in the print version)

 1 // Fig. 11.10: String.cpp
 2 // Member-function definitions for class String.
 3 #include 
 4 using std::cerr;
 5 using std::cout;
 6 using std::endl;
 7
 8 #include 
 9 using std::setw;
10
11 #include  // strcpy and strcat prototypes
12 using std::strcmp;
13 using std::strcpy;
14 using std::strcat;
15
16 #include  // exit prototype
17 using std::exit;
18
19 #include "String.h" // String class definition
20
21 // conversion (and default) constructor converts char * to String
22 String::String( const char *s )
23 : length( ( s != 0 ) ? strlen( s ) : 0 )
24 {
25 cout << "Conversion (and default) constructor: " << s << endl;
26 setString( s ); // call utility function
27 } // end String conversion constructor
28
29 // copy constructor
30 String::String( const String © )
31 : length( copy.length )
32 {
33 cout << "Copy constructor: " << copy.sPtr << endl;
34 setString( copy.sPtr ); // call utility function
35 } // end String copy constructor
36
37 // Destructor
38 String::~String()
39 {
40 cout << "Destructor: " << sPtr << endl;
41 delete [] sPtr; // release pointer-based string memory
42 } // end ~String destructor
43
44 // overloaded = operator; avoids self assignment
45 const String &String::operator=( const String &right )
46 {
47 cout << "operator= called" << endl;
48
49 if ( &right != this ) // avoid self assignment
50 {
51 delete [] sPtr; // prevents memory leak
52 length = right.length; // new String length
53 setString( right.sPtr ); // call utility function
54 } // end if
55 else
56 cout << "Attempted assignment of a String to itself" << endl;
57
58 return *this; // enables cascaded assignments
59 } // end function operator=
60
61 // concatenate right operand to this object and store in this object
62 const String &String::operator+=( const String &right )
63 {
64 size_t newLength = length + right.length; // new length
65 char *tempPtr = new char[ newLength + 1 ]; // create memory
66
67 strcpy( tempPtr, sPtr ); // copy sPtr
68 strcpy( tempPtr + length, right.sPtr ); // copy right.sPtr
69
70 delete [] sPtr; // reclaim old space
71 sPtr = tempPtr; // assign new array to sPtr
72 length = newLength; // assign new length to length
73 return *this; // enables cascaded calls
74 } // end function operator+=
75
76 // is this String empty?
77 bool String::operator!() const
78 {
79 return length == 0;
80 } // end function operator!
81
82 // Is this String equal to right String?
83 bool String::operator==( const String &right ) const
84 {
85 return strcmp( sPtr, right.sPtr ) == 0;
86 } // end function operator==
87
88 // Is this String less than right String?
89 bool String::operator<( const String &right ) const
90 {
91 return strcmp( sPtr, right.sPtr ) < 0;
92 } // end function operator<
93
94 // return reference to character in String as a modifiable lvalue
95 char &String::operator[]( int subscript )
96 {
97 // test for subscript out of range
98 if ( subscript < 0 || subscript >= length )
99 {
100 cerr << "Error: Subscript " << subscript
101 << " out of range" << endl;
102 exit( 1 ); // terminate program
103 } // end if
104
105 return sPtr[ subscript ]; // non-const return; modifiable lvalue
106 } // end function operator[]
107
108 // return reference to character in String as rvalue
109 char String::operator[]( int subscript ) const
110 {
111 // test for subscript out of range
112 if ( subscript < 0 || subscript >= length )
113 {
114 cerr << "Error: Subscript " << subscript
115 << " out of range" << endl;
116 exit( 1 ); // terminate program
117 } // end if
118
119 return sPtr[ subscript ]; // returns copy of this element
120 } // end function operator[]
121
122 // return a substring beginning at index and of length subLength
123 String String::operator()( int index, int subLength ) const
124 {
125 // if index is out of range or substring length < 0,
126 // return an empty String object
127 if ( index < 0 || index >= length || subLength < 0 )
128 return ""; // converted to a String object automatically
129
130 // determine length of substring
131 int len;
132
133 if ( ( subLength == 0 ) || ( index + subLength > length ) )
134 len = length - index;
135 else
136 len = subLength;
137
138 // allocate temporary array for substring and
139 // terminating null character
140 char *tempPtr = new char[ len + 1 ];
141
142 // copy substring into char array and terminate string
143 strncpy( tempPtr, &sPtr[ index ], len );
144 tempPtr[ len ] = '';
145
146 // create temporary String object containing the substring
147 String tempString( tempPtr );
148 delete [] tempPtr; // delete temporary array
149 return tempString; // return copy of the temporary String
150 } // end function operator()
151
152 // return string length
153 int String::getLength() const
154 {
155 return length;
156 } // end function getLength
157
158 // utility function called by constructors and operator=
159 void String::setString( const char *string2 )
160 {
161 sPtr = new char[ length + 1 ]; // allocate memory
162
163 if ( string2 != 0 ) // if string2 is not null pointer, copy contents
164 strcpy( sPtr, string2 ); // copy literal to object
165 else // if string2 is a null pointer, make this an empty string
166 sPtr[ 0 ] = ''; // empty string
167 } // end function setString
168
169 // overloaded output operator
170 ostream &operator<<( ostream &output, const String &s )
171 {
172 output << s.sPtr;
173 return output; // enables cascading
174 } // end function operator<<
175
176 // overloaded input operator
177 istream &operator>>( istream &input, String &s )
178 {
179 char temp[ 100 ]; // buffer to store input
180 input >> setw( 100 ) >> temp;
181 s = temp; // use String class assignment operator
182 return input; // enables cascading
183 } // end function operator>>

String Conversion Constructor

Line 15 (Fig. 11.9) declares a conversion constructor. This constructor (defined in Fig. 11.10, lines 2227) takes a const char * argument (that defaults to the empty string; Fig. 11.9, line 15) and initializes a String object containing that same character string. Any single-argument constructor can be thought of as a conversion constructor. As we will see, such constructors are helpful when we are doing any String operation using char * arguments. The conversion constructor can convert a char * string into a String object, which can then be assigned to the target String object. The availability of this conversion constructor means that it is not necessary to supply an overloaded assignment operator for specifically assigning character strings to String objects. The compiler invokes the conversion constructor to create a temporary String object containing the character string; then the overloaded assignment operator is invoked to assign the temporary String object to another String object.

Software Engineering Observation 11.8

When a conversion constructor is used to perform an implicit conversion, C++ can apply only one implicit constructor call (i.e., a single user-defined conversion) to try to match the needs of another overloaded operator. The compiler will not match an overloaded operator's needs by performing a series of implicit, user-defined conversions.

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The String conversion constructor could be invoked in such a declaration as String s1( "happy" ). The conversion constructor calculates the length of its character-string argument and assigns it to data member length in the member-initializer list. Then, line 26 calls utility function setString (defined in Fig. 11.10, lines 159167), which uses new to allocate a sufficient amount of memory to private data member sPtr and uses strcpy to copy the character string into the memory to which sPtr points.[4]

[4] There is a subtle issue in the implementation of this conversion constructor. As implemented, if a null pointer (i.e., 0 ) is passed to the constructor, the program will fail. The proper way to implement this constructor would be to detect whether the constructor argument is a null pointer, then "throw an exception." Chapter 16 discusses how we can make classes more robust in this manner. Also, note that a null pointer (0) is not the same as the empty string (""). A null pointer is a pointer that does not point to anything. An empty string is an actual string that contains only a null character ('').

String Copy Constructor

Line 16 in Fig. 11.9 declares a copy constructor (defined in Fig. 11.10, lines 3035) that initializes a String object by making a copy of an existing String object. As with our class Array (Figs. 11.611.7), such copying must be done carefully to avoid the pitfall in which both String objects point to the same dynamically allocated memory. The copy constructor operates similarly to the conversion constructor, except that it simply copies the length member from the source String object to the target String object. Note that the copy constructor calls setString to create new space for the target object's internal character string. If it simply copied the sPtr in the source object to the target object's sPtr, then both objects would point to the same dynamically allocated memory. The first destructor to execute would then delete the dynamically allocated memory, and the other object's sPtr would be undefined (i.e., sPtr would be a dangling pointer), a situation likely to cause a serious runtime error.

String Destructor

Line 17 of Fig. 11.9 declares the String destructor (defined in Fig. 11.10, lines 3842). The destructor uses delete [] to release the dynamic memory to which sPtr points.

Overloaded Assignment Operator

Line 19 (Fig. 11.9) declares the overloaded assignment operator function operator= (defined in Fig. 11.10, lines 4559). When the compiler sees an expression like string1 = string2, the compiler generates the function call

 string1.operator=( string2 );

The overloaded assignment operator function operator= tests for self-assignment. If this is a self-assignment, the function does not need to change the object. If this test were omitted, the function would immediately delete the space in the target object and thus lose the character string, such that the pointer would no longer be pointing to valid dataa classic example of a dangling pointer. If there is no self-assignment, the function deletes the memory and copies the length field of the source object to the target object. Then operator= calls setString to create new space for the target object and copy the character string from the source object to the target object. Whether or not this is a self-assignment, operator= returns *this to enable cascaded assignments.

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Overloaded Addition Assignment Operator

Line 20 of Fig. 11.9 declares the overloaded string-concatenation operator += (defined in Fig. 11.10, lines 6274). When the compiler sees the expression s1 += s2 (line 40 of Fig. 11.11), the compiler generates the member-function call

 s1.operator+=( s2 )

 

Figure 11.11. String class test program.

(This item is displayed on pages 600 - 603 in the print version)

 1 // Fig. 11.11: fig11_11.cpp
 2 // String class test program.
 3 #include 
 4 using std::cout;
 5 using std::endl;
 6 using std::boolalpha;
 7
 8 #include "String.h"
 9
10 int main()
11 {
12 String s1( "happy" );
13 String s2( " birthday" );
14 String s3;
15
16 // test overloaded equality and relational operators
17 cout << "s1 is "" << s1 << ""; s2 is ""  << s2
18 << ""; s3 is "" << s3 << '"'
19 << boolalpha << "

The results of comparing s2 and s1:"
20 << "
s2 == s1 yields " << ( s2 == s1 )
21 << "
s2 != s1 yields " << ( s2 != s1 )
22 << "
s2 > s1 yields " << ( s2 > s1 )
23 << "
s2 < s1 yields " << ( s2 < s1 )
24 << "
s2 >= s1 yields " << ( s2 >= s1 )
25 << "
s2 <= s1 yields " << ( s2 <= s1 );
26
27
28 // test overloaded String empty (!) operator
29 cout << "

Testing !s3:" << endl;
30
31 if ( !s3 )
32 {
33 cout << "s3 is empty; assigning s1 to s3;" << endl;
34 s3 = s1; // test overloaded assignment
35 cout << "s3 is "" << s3 << """;
36 } // end if
37
38 // test overloaded String concatenation operator
39 cout << "

s1 += s2 yields s1 = ";
40 s1 += s2; // test overloaded concatenation
41 cout << s1;
42
43 // test conversion constructor
44 cout << "

s1 += " to you" yields" << endl;
45 s1 += " to you"; // test conversion constructor
46 cout << "s1 = " << s1 << "

";
47
48 // test overloaded function call operator () for substring
49 cout << "The substring of s1 starting at
"
50 << "location 0 for 14 characters, s1(0, 14), is:
"
51 << s1( 0, 14 ) << "

";
52
53 // test substring "to-end-of-String" option
54 cout << "The substring of s1 starting at
"
55 << "location 15, s1(15), is: "
56 << s1( 15 ) << "

";
57
58 // test copy constructor
59 String *s4Ptr = new String( s1 );
60 cout << "
*s4Ptr = " << *s4Ptr << "

";
61
62 // test assignment (=) operator with self-assignment
63 cout << "assigning *s4Ptr to *s4Ptr" << endl;
64 *s4Ptr = *s4Ptr; // test overloaded assignment
65 cout << "*s4Ptr = " << *s4Ptr << endl;
66
67 // test destructor
68 delete s4Ptr;
69
70 // test using subscript operator to create a modifiable lvalue
71 s1[ 0 ] = 'H'; 
72 s1[ 6 ] = 'B'; 
73 cout << "
s1 after s1[0] = 'H' and s1[6] = 'B' is: " 
74  << s1 << "

"; 
75
76 // test subscript out of range
77 cout << "Attempt to assign 'd' to s1[30] yields:" << endl;
78 s1[ 30 ] = 'd'; // ERROR: subscript out of range
79 return 0;
80 } // end main

 

 Conversion (and default) constructor: happy
 Conversion (and default) constructor: birthday
 Conversion (and default) 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 (and default) constructor: to you
 Destructor: to you
 s1 = happy birthday to you

 Conversion (and default) 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 (and default) constructor: to you
 Copy constructor: to you
 Destructor: to you
 The substring of s1 starting at
 location 15, s1(15), 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:
 Error: Subscript 30 out of range

 

Function operator+= calculates the combined length of the concatenated string and stores it in local variable newLength, then creates a temporary pointer (tempPtr) and allocates a new character array in which the concatenated string will be stored. Next, operator+= uses strcpy to copy the original character strings from sPtr and right.sPtr into the memory to which tempPtr points. Note that the location into which strcpy will copy the first character of right.sPtr is determined by the pointer-arithmetic calculation tempPtr + length. This calculation indicates that the first character of right.sPtr should be placed at location length in the array to which tempPtr points. Next, operator+= uses delete [] to release the space occupied by this object's original character string, assigns tempPtr to sPtr so that this String object points to the new character string, assigns newLength to length so that this String object contains the new string length and returns *this as a const String & to enable cascading of += operators.

Do we need a second overloaded concatenation operator to allow concatenation of a String and a char *? No. The const char * conversion constructor converts a C-style string into a temporary String object, which then matches the existing overloaded concatenation operator. This is exactly what the compiler does when it encounters line 44 in Fig. 11.11. Again, C++ can perform such conversions only one level deep to facilitate a match. C++ can also perform an implicit compiler-defined conversion between fundamental types before it performs the conversion between a fundamental type and a class. Note that, when a temporary String object is created in this case, the conversion constructor and the destructor are called (see the output resulting from line 45, s1 += "to you", in Fig. 11.11). This is an example of function-call overhead that is hidden from the client of the class when temporary class objects are created and destroyed during implicit conversions. Similar overhead is generated by copy constructors in call-by-value parameter passing and in returning class objects by value.

Performance Tip 11.2

Overloading the += concatenation operator with an additional version that takes a single argument of type const char * executes more efficiently than having only a version that takes a String argument. Without the const char * version of the += operator, a const char * argument would first be converted to a String object with class String's conversion constructor, then the += operator that receives a String argument would be called to perform the concatenation.

Software Engineering Observation 11.9

Using implicit conversions with overloaded operators, rather than overloading operators for many different operand types, often requires less code, which makes a class easier to modify, maintain and debug.

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Overloaded Negation Operator

Line 22 of Fig. 11.9 declares the overloaded negation operator (defined in Fig. 11.10, lines 7780). This operator determines whether an object of our String class is empty. For example, when the compiler sees the expression !string1, it generates the function call

 string1.operator!()

This function simply returns the result of testing whether length is equal to zero.

Overloaded Equality and Relational Operators

Lines 2324 of Fig. 11.9 declare the overloaded equality operator (defined in Fig. 11.10, lines 8386) and the overloaded less-than operator (defined in Fig. 11.10, lines 8992) for class String. These are similar, so let us discuss only one example, namely, overloading the == operator. When the compiler sees the expression string1 == string2, the compiler generates the member-function call

 string1.operator==( string2 )

which returns true if string1 is equal to string2. Each of these operators uses function strcmp (from ) to compare the character strings in the String objects. Many C++ programmers advocate using some of the overloaded operator functions to implement others. So, the !=, >, <= and >= operators are implemented (Fig. 11.9, lines 2748) in terms of operator== and operator<. For example, overloaded function operator>= (implemented at lines 4548 in the header file) uses the overloaded < operator to determine whether one String object is greater than or equal to another. Note that the operator functions for !=, >, <= and >= are defined in the header file. The compiler inlines these definitions to eliminate the overhead of the extra function calls.

Software Engineering Observation 11.10

By implementing member functions using previously defined member functions, the programmer reuses code to reduce the amount of code that must be written and maintained.

 

Overloaded Subscript Operators

Lines 5051 in the header file declare two overloaded subscript operators (defined in Fig. 11.10, lines 95106 and 109120, respectively)one for non-const Strings and one for const Strings. When the compiler sees an expression like string1[ 0 ], the compiler generates the member-function call

 string1.operator[]( 0 )

(using the appropriate version of operator[] based on whether the String is const). Each implementation of operator[] first validates the subscript to ensure that it is in range. If the subscript is out of range, each function prints an error message and terminates the program with a call to exit.[5] If the subscript is in range, the non-const version of operator[] returns a char & to the appropriate character of the String object; this char & may be used as an lvalue to modify the designated character of the String object. The const version of operator[] returns the appropriate character of the String object; this can be used only as an rvalue to read the value of the character.

[5] Again, it is more appropriate when a subscript is out of range to "throw an exception" indicating the out-of-range subscript.

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Error-Prevention Tip 11.2

Returning a non-const char reference from an overloaded subscript operator in a String class is dangerous. For example, the client could use this reference to insert a null ('') anywhere in the string.

 

Overloaded Function Call Operator

Line 52 of Fig. 11.9 declares the overloaded function call operator (defined in Fig. 11.10, lines 123150). We overload this operator to select a substring from a String. The two integer parameters specify the start location and the length of the substring being selected from the String. If the start location is out of range or the substring length is negative, the operator simply returns an empty String. If the substring length is 0, then the substring is selected to the end of the String object. For example, suppose string1 is a String object containing the string "AEIOU". For the expression string1( 2, 2 ), the compiler generates the member-function call

 string1.operator()( 2, 2 )

When this call executes, it produces a String object containing the string "IO" and returns a copy of that object.

Overloading the function call operator () is powerful, because functions can take arbitrarily long and complex parameter lists. So we can use this capability for many interesting purposes. One such use of the function call operator is an alternate array-subscripting notation: Instead of using C's awkward double-square-bracket notation for pointer-based two-dimensional arrays, such as in a[ b ][ c ], some programmers prefer to overload the function call operator to enable the notation a( b, c ). The overloaded function call operator must be a non-static member function. This operator is used only when the "function name" is an object of class String.

String Member Function getLength

Line 53 in Fig. 11.9 declares function getLength (defined in Fig. 11.10, lines 153156), which returns the length of a String.

Notes on Our String Class

At this point, you should step through the code in main, examine the output window and check each use of an overloaded operator. As you study the output, pay special attention to the implicit constructor calls that are generated to create temporary String objects throughout the program. Many of these calls introduce additional overhead into the program that can be avoided if the class provides overloaded operators that take char * arguments. However, additional operator functions can make the class harder to maintain, modify and debug.