Abstract Classes (Types)
BlitzMax Forums/BlitzMax Programming/Abstract Classes (Types)
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| What is the purpose of abstract classes? Since you can not create instances of those classes, your goal would be to write less code? Since in the abstract class you define certain variable fields that will have their child classes. For example :
Type TAnimal Abstract
field legs:int
field alive:int
End Type
Type TCat extends TAnimal
function Init_TCat:TCat()
Local cat:TCat = New Tcat
self.alive:int = true
self.legs:int = 4
Return cat:TCat
End Type
End Type extends TAnimal
Type Tbird Extends TAnimal
function Init_TBird:TBird()
Local bird:TBird = New TBird
self.alive:int = true
self.legs:int = 2
Return bird:TBird
End Type
End Type
local catWhite:TCat = TCat.Init_TCat()
local birdBlue:TBird = TBird.Init_TBird()
I am right? If this is how you can implement abstract methods in the abstract class then instantiate it in the child class. |
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| Seems like I've answered this one before. http://www.blitzbasic.com/Community/post.php?topic=105198&post=1280082 |
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See if this helps with your own code:
Type TAnimal Abstract
Field legs:Int
Field alive:Int
Method Draw() Abstract *****************
End Type
Type TCat Extends TAnimal
Function Init:TCat()
Local cat:TCat = New Tcat
cat.alive:Int = True
cat.legs:Int = 4
Return cat
End Function
Method Draw() '****** Must match the abstract method in TAnimal
Print "cat Legs: "+ legs
End Method
End Type
Type Tbird Extends TAnimal
Function Init:TBird()
Local bird:TBird = New TBird
bird.alive:Int = True
bird.legs:Int = 2
Return bird
End Function
Method Draw() ' ****** Must match the abstract method in TAnimal
Print "bird Legs: "+legs
End Method
End Type
Local list:TList = New TList
list.AddLast(Tbird.Init())
list.AddLast(Tcat.Init())
For Local animal:Tanimal = EachIn list '***** declared as abstract animal
animal.Draw()
Next
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| @Jesse Thanks You, Simple examples help me understand all this. Now, one more question, what does Super ?, What does Final? |
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Super :
Type TBase
Method MyName:string()
return "Base"
End Method
End Type
Type TExt extends TBase
Method MyName:string()
'return Super.MyName()
return "TExt"
End Method
End Type
local ext:Text = new TExt
print ext.MyName()
Super refers to the parent of the type. So the one it extends from. Final: does not allow extensions bye Ron |
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| Final used to prevent a type from being extended. Super:
Type TAnimal Abstract ' this is not required if there is an abstract method in the Type.
Field legs:Int
Field alive:Int
Field name:String '*******************
Method Draw()
Print name+" Legs: "+legs
End Method
End Type
Type TCat Extends TAnimal
Function Init:TCat()
Local cat:TCat = New Tcat
cat.alive:Int = True
cat.legs:Int = 4
cat.name = "cat" '*************
Return cat
End Function
Method Draw()
Super.Draw() 'use the base Draw()
End Method
End Type
Type Tbird Extends TAnimal
Function Init:TBird()
Local bird:TBird = New TBird
bird.alive:Int = True
bird.legs:Int = 2
bird.name = "bird" '***********
Return bird
End Function
Method Draw()
Super.Draw() '******************* use the base Draw() ************************
End Method
End Type
Local list:TList = New TList
list.AddLast(Tbird.Init())
list.AddLast(Tcat.Init())
For Local animal:Tanimal = EachIn list
animal.Draw()
Next
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| Thank you all. Now to finish my questionnaire. I have two more questions: What do you do privately ?, What do you do public ?. Yasha a while ago I try to explain it, but I can not fully grasp it. When I see those two words, I immediately think of C #, but I can not fully understand BlitzMax. Is it used to encapsulate data within Types? Private Public |
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| Private keeps a programmer from accessing certain fields and methods in a Type. This is so that if the developer of a module should change the implementation of a Method, it wont break existing code. For example, suppose you have a type in your module like so Now someone can create a rectangle by defining its corners, and call Draw to draw it Local Rect:TRect = TRect.Create(10,10,500,300) 'Draws a rectangle from 10,10 to 500,300 Rect.Draw 'Draws the rectangle but there is a bug. if x2 or y2 is smaller than x1 or y1 then you have unpredictable results. So you decide to have the coordinates checked when creating the rectangle and modify the code like so. That fixes the order bug, and it wont interfere with existing code as the function calls have not changed. But suppose that the programmer decides to access the fields directly 'Case 1 Local Rect:TRect = New TRect Rect.x1 = 10 Rect.x2 = 500 Rect.y1 = 300 Rect.y2 = 10 Rect.Draw 'Case 2 Local Rect:Trect = TRect.Create(500,300,10,10) Local x1:Int = Rect.x1 Local y1:int = Rect.y1 In case 1, the checks were bypassed completely. In case 2, the values you retrieve might be different from what you entered (500 was entered in the x1 parameter of TRect.Create function, but was assigned to field x2) and they will blame you for their program not working. By using Private on those fields you don't want accessed, you prevent such a thing from happening, forcing the programmer to only use the interfaces you want them to use. Like so |
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| I should add that TomToads example only applies to BlitzMaxNG, as the classic version doesn't support Private in Types. I personally think it adds a lot to the language though. |
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| @TomToad Thanks You. Error here:  |
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Read Brucey's postType TRect Private Field x1:Int, y1:Int, x2:Int, y2:Int Public Function Create:TRect(x1:Int,y1:Int,x2:Int,y2:Int) Local Rect:Trect = New TRect If x2 < x1 Rect.x1 = x2 Rect.x2 = x1 Else Rect.x1 = x1 rect.x2 = x2 End If If y2 < y1 Rect.y1 = y2 Rect.y2 = y1 Else Rect.y1 = y1 Rect.y2 = y2 End If Return Rect End Function Method Draw() DrawRect x1,y1,x2-x1,y2-y1 End Method End Type |
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| @Jesse No work. Edit: oppps!, Skipped the Brucy Post, it only works in BlitzMax NG, I need to know how to use it in Classic BlitzMax. |
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| I should add that TomToads example only applies to BlitzMaxNG, as the classic version doesn't support Private in Types. Would've sworn it could also be used in Types. Guess I was wrong. Seems it can only be used in modules and Includes outside of types which really limits its usefulness. |
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| Now, include vs import. What dramatic difference is there? |
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| Now, include vs import. What dramatic difference is there? |
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| Include vs Import: http://www.blitzbasic.com/Community/posts.php?topic=103748#1251952 |
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