Visual C++ Programming: Concepts and Projects Chapter 6A: Methods (Concepts)

Visual C++ Programming: Concepts and Projects

Chapter 6A: Methods (Concepts)

Objectives

In this chapter, you will:• Learn how to use system-defined class

methods• Create programmer-defined methods• Pass data into a method by value• Pass data into a method by reference

Programming with Visual C++ 2

Objectives (continued)

• Use methods with return types• Explore the use of System::Drawing

objects and their methods• Use the PictureBox and Timer controls


Methods

• Methods are named, self-contained sections of code

• System-defined methods– Class methods

• Belong to system-defined classes• Example, the Math class

– System::Math::Sqrt()

– Instance methods• Belong to objects derived from system-defined classes• Example, an object of the TextBox class

– textBox1->Focus();


Methods (continued)


Methods (continued)

• Application methods– Event handlers• Example: btnCalc_Click()

– Programmer-defined methods• Created and named by the programmer


Methods (continued)


System-Defined Class Methods

• Part of System namespace• Scope resolution operator :: identifies the name

of the method and the System class it belongs to• Example:• System::Int32::TryParse()• System::Convert::ToDouble()

• Use of the System:: designator is optional• Int32::TryParse()• Convert::ToDouble()


The System::Math Class Library

• The Math class contains constants and methods for mathematical operations• See MSDN Web site for a complete list• Constant

• Math::PI

• Commonly used ones• Math::Sqrt() – square root• Math::Pow() – exponentiation• Math::Sin() – sine• Math::Cos() – cosine• Math::Tan() – tangent


The System::Math Class Library (continued)

• Some methods require parameters• Parameters are data values provided to the method when it

is called• Parameters are put in parameter lists• Example• Math::Sqrt(25) – the parameter is 25

• Math::Pow(2,3) • The first parameter is the mantissa (2)• The second parameter is the exponent (3)



• Methods requiring angles• Angles must be measured in radians, not degrees

• Include Math::Sin(), Math::Cos(), Math::Tan() , and others




• Consider a program in which: – The user enters an angle in degrees – The program computes the radians and then

determines the sine, cosine, and tangent of that angle



System-Defined Instance Methods

• An object is said to be an “instance” of the class definition used to create it

• An instance method is a method, defined within a class, that will belong to every object constructed from that class

• Example:– Every Random object has its own Next()

method (an instance method)– randomNumberGenerator->Next(0,100);


Application Methods

• Your programs are often called applications• Applications (like Form1) can have methods• Types of application methods– Event handlers

• The first line (signature line) and body { } are automatically generated

– Programmer-defined methods• Written by the programmer• Become instance methods belonging to the Form1 class


Application Methods (continued)• Formal method definitions• The first line is called the signature line

• Comes before the method body• Method body

• What the method does• Enclosed in { }

• Syntax diagram


Application Methods (continued)

• Signature line components• Access mode• Return type• Name• Parameter list


Application Methods (continued)


Programmer-Defined Methods• Programmer-defined methods• Programmer defines the signature line and body• Must be defined outside of all other methods

• Uses for programmer-defined methods• Used to reduce code redundancy• Used to isolate specific tasks

• Types of programmer-defined methods• Methods without parameters or a return type• Methods with a return type• Methods with value parameters• Methods with reference parameters


Programmer-Defined Methods (continued)


Methods without Parameters or a Return Type

• Return type is void (System::Void)• No parameters• Example– Reduce code redundancy with a Reset() method– Can be called from other methods– Called by its name and an empty set of

parentheses• Reset();


Methods without Parameters or a Return Type (continued)


• This program contains duplicate code in Form1_Load() and btnClear_Click()



• To remedy the duplicate code problem, write a programmer-defined method– reset()– Have Form1_Load() and btnClear_Click() each call reset()




Methods with Value Parameters

• Consider this program example:

• The last two instructions process the data that was read and display the results


Methods with Value Parameters (continued)




• The program has been modified to call a programmer-defined method called ProcessAndDisplay()

• Actual arguments• Specify the values sent into the method through its parameters• The actual arguments for ProcessAndDisplay() are width and height

• Copies of the values in width and height are passed into the method (pass-by-value)



• Here is the code for the ProcessAndDisplay() method• The parameters in the method definition signature line are called

formal parameters• They define what type of values are expected when the method is

called (value parameters)• Create local variables for those values• The formal parameters here are rect_width and rect_height







• Relationship between actual arguments and formal parameters– There must be the same number of each– The order must be the same– The data types must match– The names DO NOT have to be the same• This is because the formal parameters are local

variables.; they store copies that the data passed into them from the actual arguments

Methods with Reference Parameters

• Reference parameters are used when you wish to make a change in a variable named as an actual argument

• Reference parameters are not like value parameters– Value parameters store copies of data coming from actual

arguments– Reference parameters “refer back” to the memory cell

locations represented by the actual arguments– Pass-by-reference

• Reference parameters use the & symbol to indicate the relationship


Methods with Reference Parameters (continued)

• In this example, a programmer-defined method called ReadData() is called

• Its purpose is to read data into the actual arguments width and height




• The formal parameters for ReadData() are reference parameters (notice the & symbol)

• Any change made to rect_width and rect_height will be communicated back to the actual arguments (width and height)





Methods with Return Values• Methods can be assigned a return data type

– When the method is over, data of this type is returned to the calling program

• For example, TryParse() does more than read data into variables; it also returns a Boolean value– true if the parsing operation was successful– false if the parsing operation was not

• This return value can be stored in a Boolean variable


Methods with Return Values (continued)


• Consider a programmer-defined method called ConvertToRadians() that returns the value of an angle after conversion to radians

• It could be called like this:

Methods with Return Values (continued)


• Code for ConvertToRadians()• Notice the return statement

Graphics Class Objects and Methods

• Graphics objects are used to “draw” on other objects (like Form1)

• Graphics objects include brushes, pens and other System::Drawing objects

• A common Graphics object name is simply g• Declare Graphics objects as instance variables– Graphics^ g;

• Instantiate the Graphics object– Graphics^ g = Form1->CreateGraphics();


Graphics Class Objects and Methods (continued)

• The System::Drawing class can create objects like brushes, pens

• Declaration example:• Drawing::Brush^ yellowBrush;

• Instantiation:• yellowBrush = gcnew Drawing::SolidBrush(Color::Yellow);








• Graphics and Drawing objects are initially declared as instance variables

• The ^ operator is called a “handle”• A handle may be connected to an object at a

later time in the program



• To construct a new object and connect it to a handle, the gcnew operator is used


• Drawing takes place within Rectangles• Rectangle declarations have four parameters• X and y coordinates of the upper-left corner• For example: x = 38, y = 25

• Rectangle width and height (in pixels)• For example: width = 25, height = 25



• Both x and y coordinates are measured from the upper-left corner of the object that is being drawn in• X coordinates start at 0 and increase as you move

right• Y coordinates start with 0 and increase as you

move down






• To draw a circle, use the FillEllipse() method• Parameters are the Brush you wish to use and

the Rectangle object that the ellipse must be created within



The Use of Constants

• Constants are declared (similar to variables)• Unlike variables– They cannot be changed when the program runs

• Example– const int WIDTH = 150;

• Constants can be shared by all objects derived from a class (static constants)– static const int WIDTH = 150;



Summary• Methods may belong to: • System-defined classes• System-defined objects• Applications

• Event handlers• Programmer-defined methods

• Instance methods• Methods without a return type or parameters• Methods with value parameters• Methods with reference parameters• Methods with a return type


Summary (continued)

• Graphics objects• Belong to the object to be drawn on• Use brushes, pens, and other objects to draw

• Drawing objects• Brush, Pen, Rectangle

• Constants• Cannot be changed during execution• static constants are shared by all instances of a

class


Summary (continued)

• The drawing process• Create object to be drawn on (i.e., PictureBox)• Construct Graphics object assigned to it

• Graphics^ g = pictureBox1->CreateGraphics();

• Construct Drawing objects• Example: yellowBrush

• Construct Rectangle object• Example: circleRect

• Use the instance methods belonging to the Graphics object to draw• Parameters are drawing object and Rectangle


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Visual C++ Programming: Concepts and Projects Chapter 6A: Methods (Concepts)

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