CMPT 212 Object-Oriented Applications Design in C++ Spring 1998

Classes with Pointers to Dynamic Memory (continued)

• (continued)
  • The copy constructor solves this problem:

       sentence first = oldSentence;      //  These two lines do the
       sentence first(oldSentence);       //  same thing.
                   

    but it does not solve this one:

       sentence first;
       first = oldSentence;      //  Initialize an existing instance.       
                   

    For this we need operator overloading.

Operator Overloading

• Many operators in C++ are already overloaded.
  • Examples:
    • There is a + for ints and a + for floats.

    • There is a = for almost every type.

    • There is a << for many types.

• We can overload many of the operators with our own definitions. (See Table 10-1, page 413, in Prata.)
  • We cannot create new operator symbols.

  • We cannot overload sizeof, ., .*, ::, or ?:.

  • At least one operand must be a user-defined type, so we cannot redefine operators for standard types.

  • We cannot change the precedence of operators.

• The most common operators to overload are =, <<, and >>.

Overloading the Assignment Operator

• We can overload the = operator for our own classes.

• Example:

     const sentence& sentence::operator=(const sentence& s) {
        if (this != &s) {
           delete [] words;
           words = new char[strlen(s.words) + 1];
           strcpy(words, s.words);
           count = s.count;
           }
        return *this;
     }
           

• The assignment operator function looks like the copy constructor.
  • They both set the value of an instance by using an existing instance.

  • The copy constructor is used when a new instance is being created.

  • The assignment operator is used to set the value of an existing instance.

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