HDC memory leak - c++

Question

Not sure why but im using Deleaker plugin to detect memory leaks.
in my code in debug build its saying i have amemory leak at GetDC
Then in release build it is saying i have a leak at CreateCompatibleDC
are these real leaks or false?
my delete objects are called when my class closes.
HDC hdc = GetDC(_hWnd);
_hdcMem = CreateCompatibleDC(hdc);
HBITMAP hbmOld = (HBITMAP)SelectObject(_hdcMem, _hBitmap);
while (_execute.load(std::memory_order_acquire))
{
func();
BitBlt(hdc, 0, 0, _Width, _Height, _hdcMem, 0, 0, SRCCOPY);
}
SelectObject(_hdcMem, hbmOld);
DeleteDC(_hdcMem);
DeleteObject(hbmOld);
DeleteObject(_hBitmap);
DeleteDC(hdc);

Answer 1

After GetDC, you must call ReleaseDC—not DeleteDC. DeleteDC is only used with CreateCompatibleDC. This is all spelled out in the documentation in case you forget.
Also, you are incorrectly cleaning up your device context. The reason you save the handles to the old objects is so that you can reselect them into the DC. You cannot delete them! Objects that are selected into a device context cannot be deleted—they are in use. (If you were checking the return values of these API functions, you would know that, because they would have returned an error.)
The code should look like this:
HDC hdc = GetDC(_hWnd);
_hdcMem = CreateCompatibleDC(hdc);
HBITMAP hbmOld = (HBITMAP)SelectObject(_hdcMem, _hBitmap);
while (_execute.load(std::memory_order_acquire))
{
func();
BitBlt(hdc, 0, 0, _Width, _Height, _hdcMem, 0, 0, SRCCOPY);
}
SelectObject(_hdcMem, hbmOld);
DeleteObject(_hBitmap);
DeleteDC(_hdcMem);
ReleaseDC(_hWnd, hdc);
It is unclear to me why you are using global variables for _hdcMem and _hBitmap when their scope is limited to this single bit of code. You create them at the top, and destroy them at the bottom, so they are useless outside of this code. You should limit their scope to just this section of the code, too. Being able to reason about the lifetime of objects/variables is key to stomping out memory leaks.
Note that using a library that wraps these native resources up in an RAII fashion (constructor acquires; destructor releases) would be a wonderful idea. It not only frees you from having to remember the details of how to clean up each time, but it also ensures that your code is exception-safe. If func() were to throw here, you would certainly have a memory leak. That is something a really good static analyzer would tell you as well.

Answer 2

You must use ReleaseDC() to free the HDC returned by GetDC().
Do not delete the hbmOld that SelectObject() returned. Just select it back into the HDC and let ReleaseDC() handle its deletion.