string.h

  1 // Listing 16.1 The String Class
  2 #include <iostream>
  3 #include <string.h>
  4 using namespace std;
  5 class String
  6 {
  7   public:
  8      // constructors
  9     String();
 10     String(const char *const);
 11     String(const String &);
 12     ~String();
 13 
 14     // overloaded operators
 15     char & operator[](int offset);
 16     char operator[](int offset) const;
 17     String operator+(const String&);
 18     void operator+=(const String&);
 19     String & operator= (const String &);
 20 
 21     // General accessors
 22     int GetLen()const { return itsLen; }
 23     const char * GetString() const { return itsString; }
 24     static int ConstructorCount;
 25 
 26   private:
 27     String (int);         // private constructor
 28     char * itsString;
 29     unsigned short itsLen;
 30 
 31 };
 32 
 33 // default constructor creates string of 0 bytes
 34 String::String()
 35 {
 36    itsString = new char[1];
 37    itsString[0] = '\0';
 38    itsLen=0;
 39    cout << "\tDefault string constructor\n";
 40    ConstructorCount++;
 41 }
 42 
 43 // private (helper) constructor, used only by
 44 // class methods for creating a new string of
 45 // required size.  Null filled.
 46 String::String(int len)
 47 {
 48    itsString = new char[len+1];
 49    for (int i = 0; i<=len; i++)
 50       itsString[i] = '\0';
 51    itsLen=len;
 52    cout << "\tString(int) constructor\n";
 53    ConstructorCount++;
 54 }
 55 
 56 // Converts a character array to a String
 57 String::String(const char * const cString)
 58 {
 59    itsLen = strlen(cString);
 60    itsString = new char[itsLen+1];
 61    for (int i = 0; i<itsLen; i++)
 62       itsString[i] = cString[i];
 63    itsString[itsLen]='\0';
 64    cout << "\tString(char*) constructor\n";
 65    ConstructorCount++;
 66 }
 67 
 68 // copy constructor
 69 String::String (const String & rhs)
 70 {
 71    itsLen=rhs.GetLen();
 72    itsString = new char[itsLen+1];
 73    for (int i = 0; i<itsLen;i++)
 74       itsString[i] = rhs[i];
 75    itsString[itsLen] = '\0';
 76    cout << "\tString(String&) constructor\n";
 77    ConstructorCount++;
 78 }
 79 
 80 // destructor, frees allocated memory
 81 String::~String ()
 82 {
 83    delete [] itsString;
 84    itsLen = 0;
 85    cout << "\tString destructor\n";
 86 }
 87 
 88 // operator equals, frees existing memory
 89 // then copies string and size
 90 String& String::operator=(const String & rhs)
 91 {
 92    if (this == &rhs)
 93       return *this;
 94    delete [] itsString;
 95    itsLen=rhs.GetLen();
 96    itsString = new char[itsLen+1];
 97    for (int i = 0; i<itsLen;i++)
 98       itsString[i] = rhs[i];
 99    itsString[itsLen] = '\0';
100    return *this;
101    // cout << "\tString operator=\n";
102 }
103 
104 //non constant offset operator, returns
105 // reference to character so it can be
106 // changed!
107 char & String::operator[](int offset)
108 {
109    if (offset > itsLen)
110       return itsString[itsLen-1];
111    else
112       return itsString[offset];
113 }
114 
115 // constant offset operator for use
116 // on const objects (see copy constructor!)
117 char String::operator[](int offset) const
118 {
119    if (offset > itsLen)
120       return itsString[itsLen-1];
121    else
122       return itsString[offset];
123 }
124 
125 // creates a new string by adding current
126 // string to rhs
127 String String::operator+(const String& rhs)
128 {
129    int  totalLen = itsLen + rhs.GetLen();
130    String temp(totalLen);
131    int i, j;
132    for (i = 0; i<itsLen; i++)
133       temp[i] = itsString[i];
134    for (j = 0; j<rhs.GetLen(); j++, i++)
135       temp[i] = rhs[j];
136    temp[totalLen]='\0';
137    return temp;
138 }
139 
140 // changes current string, returns nothing
141 void String::operator+=(const String& rhs)
142 {
143    unsigned short rhsLen = rhs.GetLen();
144    unsigned short totalLen = itsLen + rhsLen;
145    String  temp(totalLen);
146    int i, j;
147    for (i = 0; i<itsLen; i++)
148       temp[i] = itsString[i];
149    for (j = 0; j<rhs.GetLen(); j++, i++)
150       temp[i] = rhs[i-itsLen];
151    temp[totalLen]='\0';
152    *this = temp;
153 }
154 
155  int String::ConstructorCount = 0; 

帮忙注释
156