Lecture 19

Constant Data Members

• Data members of a class can be made constant.

• Example:

     class sentence {
        ...
        private:
  	 const int maxLength;
  	 ...
        };
  	 

  • maxLength is a constant, but each instance of a sentence can have a different value for its maxLength.

  • We could have put maxLength in the public area if we wanted anyone to be able to access it.

• Problem: How are constant data members initialized?

• Solution: Use initialization lists.

• By using constant instances, constant member functions, and constant data members, we can be very precise about which parts of an instance can and cannot be changed.
  • Remember that many bugs in programs are caused by values being changed incorrectly. If a value should not change, make it a constant.

Initialization Lists

• An initialization list is used to initialize the data member of an instance before the constructor is executed. (The initialization list is also used to initialize the base classes from which the instance is derived. More on this when we cover class inheritance.)

• Example:

     class sentence {
        public:
  	 sentence();
  	 ...
        private:
  	 const int maxLength;
  	 char *    words;
  	 int       count;
        }:
  
     sentence::sentence()
        : maxLength(10), words(NULL), count(0) {
        // body is now empty.
     }
  	 

• Example:

     class myClass {
        public:
  	 myClass();
  	 myClass(char * w);
  	 myClass(char * a, char * b);
  	 ...
        private:
  	 sentence one;
  	 sentence two;
  	 ...
        };
  
     myClass::myClass() {
     }
  
     myClass::myClass(char * w) 
        : one(w) {
     }
  
     mytClass::myClass(char * a, char * b)
        : one(a), two(b) {
     }
  	 

• Initialization lists appear only the constructor's definition, not in its prototype.

• Initialization lists are the only way to initialize members that are references or constants. They can also be used to initialize normal data members, as the examples show.

• A constructor can initialize members of a constant instance, but cannot initialize constant members of an instance. For that, we must use initialization lists.

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