Thread: Deleting HDC w/ bitmap selected

Ok, this is some code that was included in a DirectX utility file thinger:

Code:

HDC                     hdcImage;
//some stuff not involving hdcImage
hdcImage = CreateCompatibleDC(NULL);
//if hdcImage is null, do something
SelectObject(hdcImage, hbm); //hbm is a HBITMAP passed to the function
//some stuff, only involving hdcImage for a StretchBlt()
DeleteDC(hdcImage);

If you select an object into an HDC, aren't you supposed to store the returned object in a temp variable and then select it back in before you delete the HDC? Or does it not matter because hdcImage was created within the function?

Yes, it is proper to do so. It's also proper to DeleteObject on gdi resources when finished with them too. However, windows ensures that all such resources are killed when the process/app that created them is killed.

I think that when you create a dc, the default bitmap is a 1x1 monochrome - but don't quote me on that.

Yes, it is proper to do so

Ok, thanks. But is it absolutely necessary? (i.e. otherwise will I get some of that very reliable "undefined behaviour"?)

(...) DeleteObject on gdi resources (...)

I would, but it's passed as a parameter, and it wouldn't be nice to kill somebody else's pet HBITMAP, would it?

So if you want to use the DC as a back buffer or something just create a compatible bitmap with correct size and select it into the dc.

I already knew that (I think), but thanks anyways

lol ok, I sort of get the point, but I'm still getting confused This is the psuedo-code:

Code:

create an HDC called hdcImage, compatible with the screen
select a bitmap (that is passed as an argument) into it
StretchBlt() it onto another HDC
delete it

My question is, should it be this instead:

Code:

create an HDC called hdcImage, compatible with the screen

select a bitmap (that is passed as an argument) into it,
--> and store the returned bitmap in "temp"

StretchBlt() it onto another HDC

--> select "temp" back in

delete hdcImage

Are the pointed-to lines needed? (i.e. otherwise would the original bitmap be deleted with it or something similarly sinister?)

Are the pointed-to lines needed? (i.e. otherwise would the original bitmap be deleted with it or something similarly sinister?)

this is the proper manor to this yes, else when you get rid of the dc the original bitmap isn't destroyed and thus causing memory leaks...
one should always restore GDI-objects initial data before deleting them..

/btq

Alright, thanks. But...

else when you get rid of the dc the original bitmap isn't destroyed and thus causing memory leaks...

Then does that mean that the currently selected bitmap is destroyed?

No, AFAIK.

An 'created' object will not be destroyed if currently selected into a DC. Either by a call to DestroyObject() ect on the object or the DC.

The resource leak from you code would be minor. Probably not mattering unless the app has to run on low end machines or for extended periods.

I would use your second example as a standard.

Remember GDI memory <<< available RAM

Then does that mean that the currently selected bitmap is destroyed?

no, it doesn't...
and those resource leaks can be a problem...I once did an app
that blited many many times a sec between several DC's and the
memory leaks were horrible when not selecting the original
bitmap into the DC before destroying it. could just sit and watch
the app eat up all the ram


/btq

Ok, I appreciate all the replies (and I guess I'll stick to method #2 ), but one thing is still confusing me -

-if I don't select the original bitmap back in before deleting the HDC, it won't be deleted...

-but the currently selected bitmap won't be deleted either

-Then the bitmap currently selected into the HDC isn't deleted when the HDC is

-so even if the original bitmap is selected back in, it won't be deleted, will it??

Or is there some special consideration for the original bitmap?

You don't delete the saved bitmap. Your sole responsibilty is to reselect it and the destroy the one you created.

DeleteObject(SelectObject(hdc, saved));

That applies to any and all GDI objects...except(!) for stock and system objects (HBITMAPS never come in those two flavors, but brushes, pens, fonts, etc. can). In those cases, just reselect the saved GDI object and leave the on you created alone...

Ok, I guess I get it... basically, I mind my own business and let Windows mind its own too, right? I delete my own objects, and let Windows delete its own in its own way, and how it goes about doing that (whether it deletes it when the HDC is deleted or not) is none of my beeswax, right?

Well put.