GDI: Creating a pen VS taking it as a stock object.

**limacat** · 03-30-2003

Hi there. I am studying some Visual C++ MFC programming just to understand some GUI programming in Windows.

Mind you that my environment is Visual Studio 6, and I'm following the book "Learn Visual C++ Programming in 21 Days".

(I am following too a book on C++ and one on Direct-X, the first just to make the transition from Java, and the latter because I am interested in Direct X).

Anyway, my problem (was) this:

In one of the examples wanted to trace a white line instead of a black one. First I came across (nice MSDN documentation) the DeviceContext.GetStockObject() function, and I tried to pass it to DeviceContext.SelectObject()...

I tried to do this:

Code:

dc.SelectObject((CPen *)(dc.SelectStockObject(WHITE_PEN))

This did not work, even if it looked as a legal function: the value returned was different than null (signal that the White Pen was loaded) and I think no exception was thrown since the program continued to go on without problems (mind you, I come from a Java background).

After some research I was convinced to create a white pen by myself and to use it:

Code:

CPen zPen(PS_SOLID, 1, RGB(255, 255, 255));
dc.SelectObject(&zPen);

This works.
Why?
Is the GetStockObject an obsolete function? Does it work in Windows 2000? Did I make a mess out of pointers? (I used some C a long long long time ago... that is 2 years ago, always hated all the pointer stuff

)

Thankyou at least for reading!

**novacain** · 03-30-2003

>>dc.SelectObject((CPen *)(dc.SelectStockObject(WHITE_PEN))

I think you are trying to select the pen twice

try either;

dc.SelectObject((CPen *)(dc.GetStockObject(WHITE_PEN))

or

dc.SelectStockObject(WHITE_PEN);

By the way, if you change the GDI objects in a DC you MUST return the original before you delete the DC.

If you create a GDI object then you must remove it from the DC then delete it (or release it if you used a Get function)

Stock objects do not have to be deleted but must be removed from the dc.

**limacat** · 03-31-2003

So you mean something like...

Code:

CPen tempPen = dc.GetCurrentPen();
dc.RemoveObject(tempPen);
dc.SelectStockObject(WHITE_PEN);

?

Edit:

Ok, RemoveObject doesn't exist (I wrote it without checking the manuals before).
What do I have to use in this case to remove the stuff? Haven't found anything, neither watching the CClientDC, nor the CDC member functions.

SelectStockObject instead works fine ^^;

Thankyou.

**novacain** · 03-31-2003

the return from SelectObject() is the GDI resource already contained in the DC. Catch it for later use.

hdc=GetDC(NULL);
hOldPen=(HPEN*)SelectObject(hdc,hNewPen);

//draw

SelectObject(hdc,hOldPen);
DeleteObject(hNewpen);
ReleaseDC(NULL,hdc);//if you created the dc then you must use deleteDC

**limacat** · 03-31-2003

Thank you very much novacain. I owe you a beer (If you like beers that is

).

**adrianxw** · 03-31-2003

>>> If you like beers that is

He's Australian...

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