Classes

• C++'s implementation of an ADT
• Syntax

     class class_name {
         public:
            variable declarations;
            function prototypes;
         private
            variable declarations;
            function prototypes;
     };
  
  

• If you don't declare things to be public, they are private by default
• Public: anyone using the software can use (access modify) these bits
• Private: these parts can only be messed with by the implementation part of the class.
• The class definition is like the struct definition. It only declares what the thing looks like. It does not declare memory.
• An example:

  
  class BigNum {
     public: 
         const int MAXSIZE=10;
  
         void print() const ;
         void read();
  
         void add(bignum other);
         int compare(bignum other);
         ...
     private:
         bool sign;
         int length;
         int digits[MAXSIZE];
  };
  

• This declares a class
• BigNum a; declares an instance or class object
• print_bignum is a member function
• sign is member data
• A program that uses a bignum is a client.

• 
  BigNum a, b;  // declare two instances of a bignum
  
  a.read();    // read a value into a;
  b.read();    // read a value into b;
  
  if (0 == a.compare(b)) {
      a.print();
      cout <<  " = " ;
      b.print();
      cout << endl;
  }
  

• We are using Information Hiding
  • The encapsulation and hiding of implementation details to keep the user of an abstraction from depending on or incorrectly manipulating these details.
• or using an abstraction barrier
  • The invisible wall around a class object that encapsulates implementation details.
• Class Constructors:
  • initializes the data structure
  • Can take no paramaters
  • Or can take any number of parameters
  • They are given the name of the class
  • They don't have a return type.
  • They are executed automaticaly when an instance of the class is declared.

  • 
    class BigNum {
       public:
          BigNum();  // default constructor, no arguments, set the value to 0
          BigNum(int num); // second constructor, set the value to int
          ...
    }
    
    ...
       BigNum a, b; // initialize a and b to 0
       BigNum c(-10); // initialize c to -10