compile time error in example program

**dragonlady** · 12-31-2005

Hello,

I was trying to compile an example Windows program from the main web site (code duplicated below) in my Bloodshed Dev-C++ compiler, and I got the following error:

Code:

8 C:\Dev-Cpp\winDemo.cpp [Warning] passing NULL used for non-pointer converting 4 of `int MessageBoxA(HWND__*, const CHAR*, const CHAR*, UINT)'

Here's the example program:

Code:

#include <windows.h>
int APIENTRY WinMain(HINSTANCE hInstance,
                     HINSTANCE hPrevInstance,
                     LPSTR     lpCmdLine,
                     int       nCmdShow)
{
 	
	MessageBox(NULL, "\tHello World!", "My first windows app", NULL);
	return 0;
}

The debugger found the error in the line starting with MessageBox. Does anyone have any ideas? Thanks

**Cactus_Hugger** · 12-31-2005

That last argument to MessageBox is a integer, not a pointer, so you'd pass 0 instead of NULL.

At any rate, passing 0 might be a bad idea. The last argument sets the icon and buttons of the message box, and I can't remember if 0 puts no buttons or if it defaults to something. I usually always pass something, I'd choose MB_OK in your case.

Same for the first argument... I usually pass something there too, though NULL is ok. Instead of NULL, I usually give HWND_DESKTOP. (Equivalent in the long run, I think)

Code:

MessageBox(HWND_DESKTOP, "This text appears in the box.", "This is the title or caption text.", MB_OK);

See MessageBox.

**Niara** · 01-01-2006

I think that there's no problem with the window handler, insetad of that (Cactus_Hugger has posted the corrected code) the problem is on the last parameter, you have to use a valid value or you should specify that 'NULL' is an UINT

Code:

MessageBox(NULL,"Hey","",(UINT)NULL);

niara

**Cactus_Hugger** · 01-01-2006

You should not pass NULL for the last argument of MessageBox. Perhaps I was not clear: The last argument of MessageBox() is UINT uType. Unsigned integer, a number. So if you don't want to pass anything, use 0, not NULL. NULL should be used for pointers, and the casting of NULL to UINT should tell you somethings up.

Code:

MessageBox(NULL, "Hey", "", 0);

Read the FAQ article on why NULL and 0 are not the same.

Edit: Looked it up. Turns out MB_OK is #defined as 0. So passing 0 and MB_OK are the same, but the latter is clearer in meaning. (Hence my original example.)

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