Classes tutorial

Most programming languages have classes. A class is a user-defined type that also has functions. To understand user-defined classes, you have to be familiar with variables, user-defined types and user-defined functions.

Creating and using a class

Note: To create new class you can use menu File -> New -> New Class. This tutorial does not use it.

Let's create a simple rectangle class. Class definition includes class name and member variables.

class CRect
	double'm_width
	double'm_height

Put it in some macro and compile or run the macro. It lets QM know about the new class. To make it always available, put it in a function that runs at startup. For example, in init2 (create it if does not exist).

Now create 3 member functions. To add member functions use menu File -> New -> New Member Function. The item name consists of class name, . and function name.

Member function CRect.Init

function double'width double'height

 Initializes the object.


m_width=width
m_height=height

Member function CRect.Area

function'double

 Calculates rectangle area.


ret m_width*m_height

Member function CRect.Hypotenuse

function'double

 Calculates rectangle hypotenuse.
 It is distance between two opposite corners.


ret _hypot(m_width m_height)

Now the class is created and you can use it anywhere. Declare variables of CRect type and call functions using syntax variable.Function(arguments). Example:

CRect r.Init(10 20)
out r.Area
out r.Hypotenuse

CRect r2.Init(11 19)
if(r2.Area>r.Area) out "r2 is bigger"
else out "r is bigger"

Class inheritance

You can create new classes that inherit member variableas and member functions of existing classes. Let's create a class that inherits from CRect.

class CColorRect :CRect
	m_color

Member function CColorRect.Init

function double'width double'height color

this.CRect.Init(width height)
m_color=color

Member function CColorRect.GetColor

function#

ret m_color

Example:

CColorRect r3.Init(10 20 0xffff)
out "0x%X" r3.GetColor ;;call function of CColorRect
out r3.Area ;;call function inherited from CRect

Class member access control

By default, member variables and functions are public. They can be accessed like

CRect r
r.m_width=15

To protect member variables from accessing from outside of the class, in class definition add one or two hyphens before these members:

class CRect
	-double'm_width ;;m_width is protected. It can be used only in functions of CRect and inherited classes (eg CColorRect).
	--double'm_height ;;m_height is private. It can be used only in functions of CRect class.

Now r.m_width=15 would generate error.

You also can protect some member functions from calling from outside the class. You can do it in function's Properties dialog.

To hide member variables and functions without protecting, let the variable/function name begin with __. Examples: __m_hidden, CRect.__Hidden. Or place the functions in a folder that has 'private functions' checked in Folder Properties dialog.

Accessing the variable from a member function

Member functions always are called with a variable of that type. Example:

CRect r1 r2
 ...
r1.Func
r2.Func

If Func wants to access the variable (r1, r2 or other), it can use this. It is a reference to the variable for which the function called. Example:

Member function CRect.Func

out this.m_width ;;same as out m_width
out this.Area ;;same as out Area
out &this ;;address of the variable

Special functions

A class can optionally have constructor, destructor and operator=. To add them, use item names like in the examples.

Constructor

Member function CRect (the name is the same as the class name)

out "This function is called when the variable is created."

m_width=1
m_height=1

Destructor

Member function CRect. (the name is the same as the class name, with . at the end)

out "This function is called before destroying the variable."

Operator =

Member function CRect= (the name is the same as the class name, with = at the end)

function CRect&source

out "This function is called when you assigh one variable to another variable, both of CRect type."

this.m_width=source.m_width
this.m_height=source.m_height

Test:

CRect r1 r2
r1.Init(5 5)
out r1.Area
out r2.Area
r2=r1
out r1.Area
out r2.Area