Casting Down

**incognito** · 03-28-2002

Ok, so the book I used wasn't really clear on casting, but now the graphics programming book that I have talk about it....for example the following line......

Code:

bool Prog_Init()
{
	//create the new pen
	hpenNew=CreatePen(PS_SOLID,0,RGB(255,255,255));

	//borrow dc from main window
	HDC hdc=GetDC(hWndMain);

	//select new pen into dc
	hpenOld=(HPEN)SelectObject(hdc,hpenNew);//it says that the assignment to the hpenOld is a cast of something, 

can you shed some light into this? Why is this necessary?

	//release dc to system
	ReleaseDC(hWndMain,hdc);

	return(true);//return success
}

**golfinguy4** · 03-28-2002

Casting changes the type of data. For example:

char blah='d';
int blah2=(int) blah;//makes blah2 equal the ascii value of blah

OR

char blah='d';
int blah2=blah;//this is an implicit cast, not preferred

**incognito** · 03-28-2002

oh cool but this doesn't always work right or does it, and why do you think it's necessary on this piece of code I showed you, any ideas?

**Dual-Catfish** · 03-28-2002

Originally posted by elchulo2002
oh cool but this doesn't always work right or does it, and why do you think it's necessary on this piece of code I showed you, any ideas?

Sometimes, it doesn't work at all. Conversion operators have to be in place to convert from one type to another.

**incognito** · 03-28-2002

oh ok, but do you have any ideas why this is needed here?

**Sorensen** · 03-28-2002

The win32 'handles' are of all the same basic type. They refer to something in win32 land. Pretend that handle is a synonym for void*. If you had a generic function in C that you wanted to work with different types then you could pass and return void pointers.

However, to maintain type safety and prevent subtle bugs, void pointers should be cast to the relevant type before any assignment. This tells the compiler that you haven't forgotten what the types are, and if you attempt to assign the cast of a generic function returning a int* to a pointer to be used as a float* then the compiler can issue an error and you can fix your bug.

Substitute HANDLE for pointer, HPEN for int* and HBRUSH for float* in the above example and you'll get the idea.

**incognito** · 03-29-2002

Oh thanks so the compiler wants you to specify the return value of what you are going to assign........I understand cool. Thank.

**golfinguy4** · 03-29-2002

You don't always NEED to tell what type you are converting it to. However, it is preferred that you be explicit and tell what you are converting to.

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