HALCON/COM User's Manual

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14 CHAPTER 2. THE HALCON / COM INTERFACE There are two important things here. First, the ËØ keyword: it replaces the ÐÐ keyword when the called method returns another COM object (in this case a region). Furthermore, the second variable declaration omits the ÆÛ keyword. This is because the corresponding variable need not instantiate an object at declaration time. This is done within the ÌÖ×ÓÐ´µ method, which creates a new COM object itself and passes a reference to this object as its return value. HALCON/COM objects get destroyed as soon as no variable references them anymore. For local variables, this is the case when they go out of scope (e.g. a subroutine is left). For global variables, or if an explicit destruction is desired, this has to be done by the user: Ñ Ñ½ × ÆÛ ÀÁÑ Ñ ÖÓÒ½ × ÀÊÓÒ ÐÐ Ñ½ºÊÁÑ´³×ÓÑÐÒÑ³µ ËØ ÖÓÒ½ Ñ½ºÌÖ×ÓÐ´½¾¸ ¾µ ËØ Ñ½ ÆÓØÒ ËØ ÖÓÒ½ ÆÓØÒ Here, both variables are assigned the special Visual Basic keyword ’ÆÓØÒ’ which denotes that they do not reference their related COM objects anymore. These COM objects thus are not referenced at all which leads to their immediate destruction. There is, of course, a lot more to say about Visual Basic/HALCON programming. Some further aspects might become clear in the following example session. 2.8.2 Error Handling When using Visual Basic, errors can be trapped by an error handler. If no custom error handler is present, Visual Basic itself supplies a default one, which shows a message box containing the textual error description. Error handlers in Visual Basic can be set with the keyword ÇÒ ÖÖÓÖ. To trap an error in a portion of code, the apropriate construct could look like this: Ñ Ä×ØÖÖÓÖÓ × ÄÓÒ Ñ ËÝ×ÇØ × ÆÛ ÀËÝ×ØÑ ÇÒ ÖÖÓÖ ÓØÓ ÑÝÖÖÓÖÀÒÐÖ ×ÓÑ Ó ÑÝÖÖÓÖÀÒÐÖ ³ Ó ×ÓÑØÒ ÛØ Ø ÖÖÓÖ ÒÓÖÑØÓÒ¸ ÓÖ ÜÑÔÐ ÙºÈÖÒØ ÖÖÓÖ ÓÙÖÖ · ÖÖº×ÖÔØÓÒ Ä×ØÖÖÓÖÓ ÖÖºÆÙÑÖ ¹ ËÝ×ÇØºÖÖÓÖ×ÀÐÓÒ Ê×ÙÑ ÆÜØ If an error occurs in ×ÓÑ Ó, an immediate jump to the label ÑÝÖÖÓÖÀÒÐÖ is made, where an arbitrary error processing mechanism can be placed. The scheme used in the example tries to model a traditional, ’procedural’ error recovery strategy, where every function call returns an error code, which has to be checked before program execution can continue. When an error occurs, the error handling code at the corresponding label takes over, writes a status HALCON/ COM / 2000-11-16
2.8. HALCON/COM AND VISUAL BASIC 15 message (the textual error representation) to the Visual Basic’Immediate’-window and stores the error code in a global integer variable. The global Visual Basic object ÖÖ is the source of information in this case. Afterwards, control is put back to the line following the statement which produced the error. This is done by the Visual Basic command Ê×ÙÑ ÆÜØ. The next line of code then would be responsible for checking the error code stored in Ä×ØÖÖÓÖÓ. Of course, this method is a little long winded and not quite up-to-date — an exception handling mechanism in general is much more flexible and robust — but it’s quite easy to understand. We have seen, that there are two types of errors: HALCON-related errors and COM-interfacerelated ones. Since the COM-interface errors have smaller error numbers than the HALCON error codes, the above mechanism would lead to negative numbers. In that case, the produced error code would have to be subtracted from ËÝ×ÇØºÖÖÓÖ×ÇÅ to get the correct (then interface related) error code. HALCON 6.0
Page 1 and 2: HALCON Version 6.0 MVTec Software G
Page 3 and 4: About This Manual This manual gives
Page 5 and 6: Contents 1 Introduction 1 1.1 Why C
Page 7 and 8: Chapter 1 Introduction This chapter
Page 9 and 10: 1.2. HALCON AND COM 3 re-defined in
Page 11 and 12: Chapter 2 The HALCON COM Interface
Page 13 and 14: 2.2. MORE ABOUT CLASSES 7 instantia
Page 15 and 16: 2.3. OBJECT CONSTRUCTION AND DESTRU
Page 17 and 18: 2.4. INTERFACES AND INHERITANCE 11
Page 19: 2.8. HALCON/COM AND VISUAL BASIC 13
Page 23 and 24: Chapter 3 Example Visual Basic Sess
Page 25 and 26: 3.1. FIRST STEP: THE GUI 19 Figure
Page 27 and 28: 3.3. FINAL STEP: MORE FUNCTIONALITY
Page 29 and 30: 3.4. OTHER EXAMPLES 23 2. ±ÀÄÇ
Page 31 and 32: Bibliography [Arm98] Tom Armstrong.
Page 33 and 34: Index abstract class, 5-8 ActiveX,
Page 35: Index 29 HALCON 6.0

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