Extending classes and Override question

[wiebowParticipant]

Hello,

I have a question regarding extending classes and overriding methods. In BlitzMax, in a base Type, I was able create and for instance, call that empty method in the New() method. In the extended type, I would override that method. Creating a new extended would result in the method being called automatically because the the base type New() method would would call it.

Now in M2, I try the following:

Monkey

'using mojo to make stuff Namespace test #import "<std>" #import "<mojo>" Using std.. Using mojo.. Function Main:Void() New Test Print( "done" ) End Function Class Test Extends Game2d Method New() Print( "new in test" ) End Method Method Startup:Void() Override Print( "startup in test" ) End Method End Class Class Game2d Method New() Print( "new in game2d" ) Self.Startup() Start() End Method Method Start:Void() Print( "start in game2d" ) End Method #Rem monkeydoc Hook for user startup code. #End Method Startup() Virtual Print( "startup in game2d" ) End Method End Class

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'using mojo to make stuff   Namespace test   #import "<std>" #import "<mojo>"   Using std.. Using mojo..   Function Main:Void() New Test Print( "done" ) End Function   Class Test Extends Game2d Method New() Print( "new in test" ) End Method   Method Startup:Void() Override Print( "startup in test" ) End Method End Class   Class Game2d Method New() Print( "new in game2d" ) Self.Startup() Start() End Method   Method Start:Void() Print( "start in game2d" ) End Method   #Rem monkeydoc Hook for user startup code. #End Method Startup() Virtual Print( "startup in game2d" ) End Method End Class

I expected the extended version of Startup being called, but instead the base version is called. This is different than how it was in Max.. Is this intended behaviour? And if so, does anyone have an idea to emulate ‘hooks’ ?

I really liked this feature as it was really simple to call code automatically.

Thanks!

[therevillsParticipant]

This looks like a bug to me. In most languages the extended version of startup should be called…

Java:

Monkey

[crayon-5cb9bd63a1ac7946682640 inline="true" ] class Testing { public static void main(String[] args) { Testing t = new Testing(); } public Testing() { Test c = new Test(); } private class Test extends Game2d{ public Test() { System.out.println("Test - Ctor..."); } public void startup() { System.out.println("Test - startup"); } } private class Game2d { public Game2d() { System.out.println("Game2d - Ctor..."); this.startup(); start(); } public void start() { System.out.println("Game2d - start"); } public void startup() { System.out.println("Game2d - startup"); } } }

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[crayon-5cb9bd63a1ac7946682640 inline="true" ] class Testing { public static void main(String[] args) { Testing t = new Testing(); } public Testing() { Test c = new Test(); } private class Test extends Game2d{ public Test() { System.out.println("Test - Ctor..."); } public void startup() { System.out.println("Test - startup"); } } private class Game2d { public Game2d() { System.out.println("Game2d - Ctor..."); this.startup(); start(); } public void start() { System.out.println("Game2d - start"); } public void startup() { System.out.println("Game2d - startup"); } } }

[/crayon]

Outputs:

Monkey

[crayon-5cb9bd63a1acd248327588 inline="true" ]Game2d - Ctor... Test - startup Game2d - start Test - Ctor...

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[crayon-5cb9bd63a1acd248327588 inline="true" ]Game2d - Ctor... Test - startup Game2d - start Test - Ctor...

[/crayon]

MX1:

Monkey

[crayon-5cb9bd63a1ad2314878826 inline="true" ]Game2d - Ctor Test - startup Game2d - start Test - Ctor

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[crayon-5cb9bd63a1ad2314878826 inline="true" ]Game2d - Ctor Test - startup Game2d - start Test - Ctor

[/crayon]

MX2:

Monkey

[crayon-5cb9bd63a1ad7006662865 inline="true" ]Game2d - Ctor Game2d - startup Game2d - start Test - Ctor

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[crayon-5cb9bd63a1ad7006662865 inline="true" ]Game2d - Ctor Game2d - startup Game2d - start Test - Ctor

[/crayon]

[Mark SiblyKeymaster]

> In most languages the extended version of startup should be called…

Well, most languages except c++  (and I think Java gives an error).

A bit more on this here:

http://www.monkey-x.com/Community/posts.php?topic=10557

I actually haven’t decided 100% which way I’m going here either.

[therevillsParticipant]

[quote]Well, most languages except c++ (and I think Java gives an error).[/quote]

Well I always thought C++ was a bit odd

Java outputs:

Monkey

[crayon-5cb9bd63a8c6a341289525 inline="true" ]Game2d - Ctor... Test - startup Game2d - start Test - Ctor...

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[crayon-5cb9bd63a8c6a341289525 inline="true" ]Game2d - Ctor... Test - startup Game2d - start Test - Ctor...

[/crayon]

Run the code in the above post, pure Java and no errors…

[therevillsParticipant]

Playing with C#, using override and virtual:

Monkey

[crayon-5cb9bd63ab14a739441415 inline="true" ]using System; namespace test { class Program { public static void Main(string[] args) { new Test(); Console.ReadKey(true); } } class Test:Game2d { public Test() { Console.WriteLine("Test ctor"); } public override void startup() { Console.WriteLine("Test startup"); } } class Game2d { public Game2d() { Console.WriteLine("Game2d ctor"); this.startup(); } public virtual void startup() { Console.WriteLine("Game2d startup"); } } }

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[crayon-5cb9bd63ab14a739441415 inline="true" ]using System;   namespace test { class Program { public static void Main(string[] args) { new Test(); Console.ReadKey(true); } } class Test:Game2d { public Test() { Console.WriteLine("Test ctor"); } public override void startup() { Console.WriteLine("Test startup"); } }   class Game2d { public Game2d() { Console.WriteLine("Game2d ctor"); this.startup(); } public virtual void startup() { Console.WriteLine("Game2d startup"); } }   }

[/crayon]

Outputs:

Monkey

[crayon-5cb9bd63ab151020078239 inline="true" ]Game2d ctor Test startup Test ctor

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[crayon-5cb9bd63ab151020078239 inline="true" ]Game2d ctor Test startup Test ctor

[/crayon]

Not using override and virtual keywords:

Monkey

[crayon-5cb9bd63ab156281209706 inline="true" ]using System; namespace test { class Program { public static void Main(string[] args) { new Test(); Console.ReadKey(true); } } class Test:Game2d { public Test() { Console.WriteLine("Test ctor"); } public void startup() { Console.WriteLine("Test startup"); } } class Game2d { public Game2d() { Console.WriteLine("Game2d ctor"); this.startup(); } public void startup() { Console.WriteLine("Game2d startup"); } } }

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[crayon-5cb9bd63ab156281209706 inline="true" ]using System;   namespace test { class Program { public static void Main(string[] args) { new Test(); Console.ReadKey(true); } } class Test:Game2d { public Test() { Console.WriteLine("Test ctor"); } public void startup() { Console.WriteLine("Test startup"); } }   class Game2d { public Game2d() { Console.WriteLine("Game2d ctor"); this.startup(); } public void startup() { Console.WriteLine("Game2d startup"); } }   }

[/crayon]

Outputs:

Monkey

[crayon-5cb9bd63ab15a830591905 inline="true" ]Game2d ctor Game2d startup Test ctor

1 2 3

[crayon-5cb9bd63ab15a830591905 inline="true" ]Game2d ctor Game2d startup Test ctor

[/crayon]

[Mark SiblyKeymaster]

Ok, looks like c++ enforces it, but other languages only warn about it:

http://stackoverflow.com/questions/3404301/whats-wrong-with-overridable-method-calls-in-constructors

https://msdn.microsoft.com/en-us/library/ms182331.aspx

I think it’s generally a good rule – if a ctor calls a method that is overridden by a subclass, that method can no longer depend on it’s class fields being properly initialized because it’s ctor hasn’t been executed yet. Or just as bad, the override could be called, but then anything it did gets undone by the ctor being called later, eg:

http://stackoverflow.com/questions/12100449/java-calling-superclass-constructor-which-calls-overridden-method-which-sets-a

But I’ll likely end up sticking with what’s most practical/flexible on the translator side of things.

[therevillsParticipant]

Just fired up Code::Blocks (last time I used that when I was testing the BMX 3D stuff!):

Monkey

[crayon-5cb9bd63b43f6247575185 inline="true" ]#include <stdio.h> class Game2d { public: Game2d() { printf("Game2d ctor\n"); startup(); start(); } void start() { printf("Game2d start\n"); } virtual void startup() { printf("Game2d startup\n"); } }; class Test: public Game2d { public: Test() { printf("Test ctor\n"); } virtual void startup() { printf("Test startup\n"); } }; int main() { Test g; return 0; }

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[crayon-5cb9bd63b43f6247575185 inline="true" ]#include <stdio.h>   class Game2d { public: Game2d() { printf("Game2d ctor\n"); startup(); start(); }   void start() {     printf("Game2d start\n"); }   virtual void startup() { printf("Game2d startup\n"); } };   class Test: public Game2d { public: Test() { printf("Test ctor\n"); }   virtual void startup() { printf("Test startup\n"); } };   int main() { Test g; return 0; }

[/crayon]

Outputs:

Monkey

[crayon-5cb9bd63b440a464786154 inline="true" ]Game2d ctor Game2d startup Game2d start Test ctor

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[crayon-5cb9bd63b440a464786154 inline="true" ]Game2d ctor Game2d startup Game2d start Test ctor

[/crayon]

So as long as what MX2 is going to do is well defined and documented I dont think we can complain too much

[wiebowParticipant]

Nope, thanks for investigating this! It’s good to know and understand that some of the ways of working in Max are not valid anymore.

[JesseParticipant]

so how do you access the method in the extended class are we still using the extended Class?

[wiebowParticipant]

You can access it, but only from the extended class, not from the base class.

[Mark SiblyKeymaster]

Just to clarify, we’re talking about what happens when a constructor (and only a constructor) calls an overridden (ie: virtual) method.

The mx2 behaviour might yet change before V1.0, but in general it’s not a good idea for a constrcutor to call an overridden method. I think this sums it up best:

https://msdn.microsoft.com/en-us/library/ms182331.aspx

[wiebowParticipant]

Looking back at it now, I’m all for it. I see the possible negative consequences of the old way of doing it.

[Author]

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