Interface Versus Implementation in C++

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Clients are the parts of the program that create and use objects of your class. You can think of the interface to your class--the class declaration--as a contract with these clients. The contract tells what data your class has available and how your class will behave.

For example, in the Cat class declaration, you create a contract that every Cat will have a member variable itsAge that can be initialized in its constructor, assigned to by its SetAge() accessor function, and read by its GetAge() accessor. You also promise that every Cat will know how to Meow().

If you make GetAge() a const function--as you should--the contract also promises that GetAge() won't change the Cat on which it is called.

C++ is strongly typed, which means that the compiler enforces these contracts by giving you a compiler error when you violate them. The following program demonstrates a program that doesn't compile because of violations of these contracts.


WARNING: The following program does not compile!

A demonstration of violations of the interface.

1:   // Demonstrates compiler errors
4:   #include <iostream.h>          // for cout
6:  class Cat
7:  {
8:   public:
9:      Cat(int initialAge);
10:     ~Cat();
11:     int GetAge() const;          // const accessor function
12:     void SetAge (int age);
13:     void Meow();
14:  private:
15:    int itsAge;
16: };
18:    // constructor of Cat,
19:    Cat::Cat(int initialAge)
20:    {
21:       itsAge = initialAge;
21:       cout << "Cat Constructor\n";
22:    }
24:    Cat::~Cat()                   // destructor, takes no action
25:    {
26:       cout << "Cat Destructor\n";
27:    }
28:   // GetAge, const function
29:   // but we violate const!
30:   int Cat::GetAge() const
31:   {
32:      return (itsAge++);         // violates const!
33:   }
35:   // definition of SetAge, public
36:   // accessor function
38:   void Cat::SetAge(int age)
39:   {
40:      // set member variable its age to
41:      // value passed in by parameter age
42:      itsAge = age;
43:   }
45:  // definition of Meow method
46:  // returns: void
47:  // parameters: None
48:  // action: Prints "meow" to screen
49:  void Cat::Meow()
50:  {
51:     cout << "Meow.\n";
52:  }
54:  // demonstrate various violations of the
55   // interface, and resulting compiler errors
56:  int main()
57:  {
58:     Cat Frisky;                 // doesn't match declaration
59:     Frisky.Meow();
60:     Frisky.Bark();              // No, silly, cat's can't bark.
61:     Frisky.itsAge = 7;          // itsAge is private
62:      return 0;
63: } 

Analysis: As it is written, this program doesn't compile. Therefore, there is no output.
This program was fun to write because there are so many errors in it.

Line 11 declares GetAge() to be a const accessor function--as it should be. In the body of GetAge(), however, in line 32, the member variable itsAge is incremented. Because this method is declared to be const, it must not change the value of itsAge. Therefore, it is flagged as an error when the program is compiled.

In line 13, Meow() is not declared const. Although this is not an error, it is bad programming practice. A better design takes into account that this method doesn't change the member variables of Cat. Therefore, Meow() should be const.

Line 58 shows the definition of a Cat object, Frisky. Cats now have a constructor, which takes an integer as a parameter. This means that you must pass in a parameter. Because there is no parameter in line 58, it is flagged as an error.

Line 60 shows a call to a class method, Bark(). Bark() was never declared. Therefore, it is illegal.

Line 61 shows itsAge being assigned the value 7. Because itsAge is a private data member, it is flagged as an error when the program is compiled.

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