CMPT 212 Object-Oriented Applications Design in C++ Fall 1997

Classes with Pointers to Dynamic Memory

• If a class instance contains members which point to dynamically allocated memory, constructors and destructors are almost always required in order for the memory to be allocated and released properly.

• Example:
  • A class sentence that we can add words to, find the number of words, and clear the sentence.

       class sentence {
          public:
             sentence();
             sentence(const char * w);
             sentence(const sentence& s);
             ~sentence();
             int wordCount();
             void clear();
             boid appendWord(const char * w);
          private:
             char * words;
             int    count;
          };   
                   

  • The default constructor will set the pointer to NULL and count to 0.

       sentence::sentence() {
          words = NULL;
          count = 0;
       }
                   

  • We can construct a sentence from a word.

       sentence::sentence(const char * w) {
          words = NULL;
          count = 0;
          appendWord(w);
       }
                   

  • The copy constructor makes a "clone" of the given sentence.

       sentence::sentence(const sentence& s) {
          words = new char[strlen(s.words) + 1]
          strcpy(words, s.words);
          count = s.count;
       }
                   

  • The destructor will release any memory pointed to by the words pointer.

       sentence::~sentence() {
          clear();
       }
                   

  • wordCount returns the number of words.

       int sentence::wordCount() {
          return count;
       }
                   

  • clear removes all words from the sentence.

       void sentence::clear() {
          delete [] words;
          words = NULL;
          count = 0;
       }
                   

  • appendWord will allocate a new block of memory big enough to hold the new sentence. (If we will be adding words often, this is an inefficient implementation.)

       void sentence::appendWord(const char * w) {
          if (words == NULL) {
             words = new char[strlen(w) + 1];
             strcpy(words, w);
             count = 1;
             }
          else {
             int    wordsLen = strlen(words);
             char * newWords = new char[wordsLen + 1 + strlen(w) + 1];
             strcpy(newWords, words);
             strcat(newWords, " ");
             strcat(newWords, w);
             delete [] words;
             words = newWords;
             count++;
             }
       }
                   

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