Question

I thought I was good at this programming thing, until I hit this devil of a walkthrough that is (conveniently) absent from my textbook.

My guess is something about a copy constructor.

It involves two classes, Paragraph and Word. There is a dynamic array of Word objects inside each Paragraph class. The trouble is when the Paragraph class += operator overload method is called in main() – a professor comments that it calls the Word constructor, then the Word = operator.

Correct me if I’m wrong, but shouldn’t there be another operator overload to handle a Word object on the left and a string on the right??

#include <iostream>
 using namespace std;
 #define WIDTH 8

 class Word {
     char* text;
     int nletters;
   public:
     Word() {
         text = NULL;
         nletters = 0;
         cout << 'W';
     }
     Word(const char* s) {
         nletters = 0;
         while(s[nletters] != '\0')
             nletters++;
         text = new char[nletters];
         for (int i = 0; i < nletters; i++)
             text[i] = s[i];
         cout << 'X';
     }
     Word(const Word& p) {
         text = NULL;
         *this = p;  // calls assignment operator below
         cout << 'Y';
     }
     ~Word() {
         if (text != NULL) delete [] text;
         cout << "~W" << nletters;
     }
     Word& operator=(const Word& p) {
         if (this != &p) {
             if (text != NULL) delete [] text;
             nletters = 0;
             if (p.text != NULL) {
                 text = new char[p.nletters];
                 while(nletters < p.nletters) {
                     text[nletters] = p.text[nletters];
                     nletters++;
                 }
             }
             else
                 text = NULL;
         }
         cout << 'V';
         return *this;
     }
     int nLetters() const { return nletters; }
     friend ostream& operator<<(ostream& os, const Word& w) {
         if (w.text != NULL)
             for (int i = 0; i < w.nletters; i++)
                 os << w.text[i];
         else
             os << "***";
         os << ' ';
         return os;
     }
 };

 class Paragraph {
     Word* word;
     int mwords;
     int nwords;
     int width;
   public:
     Paragraph() {
         word  = NULL;
         width = WIDTH;
         nwords = 0;
         mwords = 0;
         cout << 'P' << endl;
     }
     Paragraph(const Paragraph& c) {
         word  = NULL;
         *this = c;  // calls assignment operator below
         cout << "cP";
     }
     Paragraph& operator=(const Paragraph& c) {
         if (this != &c) {
             if (word != NULL) delete [] word;
             if (c.word != NULL) {
                 word = new Word[c.mwords];
                 cout << endl;
                 for (int i = 0; i < c.nwords; i++)
                     word[i] = c.word[i];  // calls Word assignment operator
             }
             else
                 word = NULL;
             width  = c.width;
             nwords = c.nwords;
             mwords = c.mwords;
         }
         cout << "=P";
         return *this;
     }
     ~Paragraph() {
         if (word != NULL) delete [] word;
         cout << '\n' << nwords << "~P" << endl;
     }
     void make(int m) {
         if (word == NULL) {
             word = new Word[m];
             mwords = m;
             nwords = 0;
             width  = WIDTH;
         }
     }
     void setWidth(int w) { width = w; }
     friend ostream& operator<<(ostream& os, const Paragraph& p) {
         int nextWord = 0;
         for (int i = 0; i < p.nwords; i++) {
             if (nextWord + p.word[i].nLetters() > p.width) {
                 os << '\n';
                 nextWord = 0;
             }
             cout << p.word[i];
             nextWord += p.word[i].nLetters() + 1;
         }
         return os;
     }
     Paragraph& operator+=(const char* w) {
         if (nwords < mwords) {
             cout << "\n+=";
             word[nwords] = w;  // calls Word constructor, then Word = operator
             nwords++;
         }
         return *this;
     }
 };

 int main() {

     Paragraph p;
     cout << "--------\n";
     p.make(5);
     p += "This";
     p += "is";
     p += "hard";
     cout << "\n--------\n";
     cout << p << endl;
     cout << "--------\n";
     Paragraph q = p;
     q.setWidth(6);
     q += "too";
     cout << "\n--------\n";
     cout << q << endl;
     cout << "---------------\n";
     return 0;
 }

Answer 1

Your professor is right. You don’t need another operator since a const char* is directly (and implicitly) convertible to a Word, through:

Word(const char* s)

This is called a conversion constructor. If you declare it as explicit, it should no longer compile. See here – http://ideone.com/brjIi

So on the line

word[nwords] = w;

a temporary Word is indeed created from w, and then the assignment operator for Word is called to assign the temporary to words[nwords].

Some other notes on the code:

• since your professor teaches you C++, you should learn about std::string and std::vector. He doesn’t need C++ nor classes to demonstrate the power of pointers or dynamic memory allocation.
• statements like if (word != NULL) delete [] word; are redundant. delete or delete[] on a NULL pointer is guaranteed to work (and not do anything).
• the logic in the constructor is wrong (wouldn’t be if you didn’t manage memory yourself). After you construct a Paragraph, it’s unusable until you call make(), which is bad design.