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8 .1 8.2 8.3 8.4 A pub1 i c member is accessible from outside the class; a private member is not. The class interface consists of the member data and the member function prototypes (i.e. just the function declarations). The class implementation contains the definitions of the member functions. A class member function is part of the class, so it has access to the class s private parts. An applicaparts. tion function is declared outside the class, and so it does not have access to the class s private A constructor is a class member function that executes automatically whenever an object of that class is instantiated (i.e., constructed). A destructor is a class member function that executes automatically whenever the scope of that object terminates (i.e., is destructed). The default constructor is the unique constructor that has no parameters (or the one whose parameters all have default values). A class s copy constructor executes whenever an object of that class is copied by any mechanism except direct assignment. This includes initialization, passing a parameter by value, and returning by value. An access function is a pub1 i c class member function that returns the value of one of the class s data members. A utility function is a private class member function that is used only within the class to perform technical tasks. A c las s a n d a s t rut t in C++ are essentially the same. The only significant difference is that the default access level for a class of private, while that for a struct is pub1 ic. Every class constructor must h a v e the same name as the class itself.

8.5 8.6 8.7

(4.11)

To illustrate the use of timed events, let us use the timer control to create a metronome. As you are probably aware, a metronome is an instrument used by musicians to maintain a specified tempo (i.e., a specified interval between beats). Commercial metronomes produce a distinct sound (a beep ), sometimes accompanied by a flashing light, to represent each beat. In this example we will use two flashing circles to represent the beat, because of the lengthy (and uncontrollable) sound produced by the Visual Basic Beep command. One circle will always be highlighted (shown in bright red) while the other will be shown in the gray background color. The red-gray combination will alternate at the specified tempo. We will restrict the tempo to values falling within the interval 40 220 beats per minute, as most music falls within this interval. We begin with the Form Design Window shown in Fig. 4.38. Note that clock enclosed by the square at the center of the Form Design Window. This is the timer control, whose name is Timer1. The left rectangle is Shape1, and the right rectangle is Shape2. These rectangles will be converted to circles, which will display the tempo by alternating in color. The command buttons will start and stop the metronome, and end the computation. The text box will specify the tempo.

8.8 8.9

Fig. 4-69

8.10 Every class destructor must have the same name as the class itself, prefixed with a tilde (-). 8.11 There is no limit to the number of constructors that a class may have. But since multiple constructors are function overloads, they all must be distinguishable by their parameter lists. 8.12 A class can only one destructor. 8.13 The scope resolution operator : : used in general to resolve external references. It is used in a class definition whenever the definition of a member function is given outside the scope of the class definition. 8.14 There are four class member functions that are created automatically by the compiler if they are not included (by the programmer) in the class definition: the default constructor, the copy constructor, the destructor, and the overloaded assignment operator.

Find x and y in each part of Fig. 4-70.

The variable p is called a pointer because its value points to the location of another value. It is an int pointer because the value to which it points is an int. The value of a pointer is an address. That address depends upon the state of the individual computer on which the program is running. In most cases, the actual value of that address (e.g., 0x3 f f f dl4) is not relevant to the issues that concern the programmer. So diagrams like the one above are usually drawn something like this:

(4.12)

This captures the essential features of p and n: p is a pointer to n, and n has the value 33. A pointer can be thought of as a locator : it tells where to locate another value. Often we will need to use the pointer p alone to obtain the value to which it points. This is called dereferencing the pointer, and is accomplished simply by applying the star * (the asterisk) symbol as an operator to the pointer: .

Here p points to the integer named n, so *p and n are the same value: main0 int n = 33; int* p = &n; // p points to n tout CC "*p = H CC *p C-C endl; p = ,33. : This shows that *p is an alias for n. .. _ : x ,

Fig. 4-70

The address operator SC and the dereference operator * are inverses of each other: n = = *p whenever p == &n. This can also be expressed as n = = *6cn and p == sc*p.

CHAP. 4]