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Appendix E: Solutions to Common Errors

Error LNK2001: unresolved external symbol _main

An unresolved external occurs when some code has a call to a function in another module and the linker can't find that function in any of the modules or libraries that you are currently linking to.

In this specific case, it means one of two things. Either you are trying to write a Win32 GUI application (or non-console application) and accidently compiled it as a Console application... or you really are trying to compile a console application and didn't write or properly compile in a main() function.

Generally the first is the most common, if you specify Win32 Console as the project type in VC++ when you create your project you will get this error. You will also likely get it if you try to compile from the command line using BC++ but you neglect to specify the correct parameters to tell it to make a Win32 GUI application instead of a console app which is the default.

Fixing

If you're using VC++ re-create your project and select the Win32 Application project type (NOT "Console").

If you're using BC++ command line compiler, use -tW to specify a windows application.

Error C2440: cannot convert from 'void*' to 'HICON__ *' (or similar)

If you're compiling the code from this tutorial, it means that you are trying to compile it as C++ code. The code is written for the bcc32 and VC++ C compilers, and as such may not compile exactly the same under C++ since C++ has much stricter rules about converting types. C will just let it happen, C++ wants to you to make it explicit.

If you're compiling code not from this tutorial, I can't guarantee that it's correct and therefor it may actually be an error that needs resolving. You'll have to use your own judgement to determine if it's safe to cast the value and remove the error, or if you are actually trying to make a variable be something it's not.

Fixing

Cast. You DO know C right, it's a prerequisite for this tutorial :)

For example, in C this will work: 

	HBITMAP hbmOldBuffer = SelectObject(hdcBuffer, hbmBuffer);

But in C++ requires a cast: 

	HBITMAP hbmOldBuffer = (HBITMAP)SelectObject(hdcBuffer, hbmBuffer);

Fatal error RC1015: cannot open include file 'afxres.h'.

Oddly enough, VC++ adds afxres.h to resource files even when you aren't using an MFC project, and yet the file may only be installed if you install MFC. This perticular file isn't actually required, so to fix the error you can edit the .rc file in notepad and replace both occurances of "afxres.h" with "winres.h" (note that there should be two of them, and you need to change both).

Error LNK2001: unresolved external symbol InitCommonControls

You aren't linking to comctl32.lib which this API is defined in. This library is not included by default so you will either need to add it to the libraries on your command line, or add it in your VC++ project settings on the Link tab.

Dialog does not display when certain controls are added

Controls such as the ListView, TreeView, Hotkey, Progress Bar, and others are classified as Common Controls, as they were added to windows in comctl32.dll and were not available prior to Windows 95. Controls such as BUTTON, EDIT, LISTBOX, etc... while no doubt being common, are not "Common Controls" and I generally refer to them as "Standard Controls".

If you add a Common Control to a dialog and it fails to display, you most likely failed to call InitCommonControls() before running your dialog, or perhaps at all. The best place to call it is first thing in WinMain(). Calling it in WM_INITDIALOG is too late, since the dialog will fail before it reaches this point and it will never get called.

Some people and documentation may tell you that InitCommonControls() is deprecated and you should use InitCommonControlsEx(). Feel free to do this if you want, InitCommonControls() is just simpler and there's nothing wrong with using it.

Copyright © 1998-2002, Brook Miles (theForger). All rights reserved.