CreateFont, DeleteObject and GDI handle/memory leak - c++

Question

I am looking at this sample https://learn.microsoft.com/en-us/windows/win32/api/wingdi/nf-wingdi-createfonta of CreateFont API
It clearly says that after the font been created with CreateFont it should be destroyed by DeleteObject call. DeleteObject(hFont); is only called once. CreateFont is called 3 times. Is this a bug in the MS docs? Shouldn't the old font be stored by retrieving it with SelectObject and set back after new fonts been used?

Answer 1

Yes, two of the created font objects are leaked.
Note that MS sample code is generally poor about error handling and object cleanup (they generally focus on demonstrating whatever is central to the sample - here the CreateFont call - while ignoring or minimizing those issues).

Answer 2

The example in the documentation did cause the leak of the font object
I built a sample as follows:
#include <Windows.h>
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
int WINAPI WinMain(_In_ HINSTANCE hInstance, _In_opt_ HINSTANCE hPrevInstance, _In_ LPSTR szCmdLine, _In_ int iCmdShow)
{
static TCHAR szAppName[] = TEXT("hello windows");
HWND hwnd;
MSG msg;
WNDCLASS wndclass;
wndclass.style = CS_HREDRAW | CS_VREDRAW;
wndclass.lpfnWndProc = WndProc;
wndclass.cbClsExtra = 0;
wndclass.cbWndExtra = 0;
wndclass.hInstance = hInstance;
wndclass.hIcon = LoadIcon(NULL, IDI_APPLICATION);
wndclass.hCursor = LoadCursor(NULL, IDC_ARROW);
wndclass.hbrBackground = (HBRUSH)GetStockObject(WHITE_BRUSH);
wndclass.lpszMenuName = NULL;
wndclass.lpszClassName = szAppName;
if (!RegisterClass(&wndclass))
{
MessageBox(NULL, TEXT("This program requires Windows NT!"), szAppName, MB_ICONERROR);
}
hwnd = CreateWindow(szAppName,
TEXT("the hello program"),
WS_OVERLAPPEDWINDOW,
CW_USEDEFAULT,
CW_USEDEFAULT,
CW_USEDEFAULT,
CW_USEDEFAULT,
NULL,
NULL,
hInstance,
NULL);
ShowWindow(hwnd, iCmdShow);
UpdateWindow(hwnd);
while (GetMessageW(&msg, NULL, 0, 0))
{
TranslateMessage(&msg);
DispatchMessageW(&msg);
}
return msg.wParam;
}
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
int wmId, wmEvent;
PAINTSTRUCT ps;
HDC hdc;
switch (message)
{
case WM_PAINT:
{
RECT rect;
HBRUSH hBrush;
HFONT hFont;
hdc = BeginPaint(hWnd, &ps);
//Logical units are device dependent pixels, so this will create a handle to a logical font that is 48 pixels in height.
//The width, when set to 0, will cause the font mapper to choose the closest matching value.
//The font face name will be Impact.
hFont = CreateFont(48, 0, 0, 0, FW_DONTCARE, FALSE, TRUE, FALSE, DEFAULT_CHARSET, OUT_OUTLINE_PRECIS,
CLIP_DEFAULT_PRECIS, CLEARTYPE_QUALITY, VARIABLE_PITCH, TEXT("Impact"));
SelectObject(hdc, hFont);
//Sets the coordinates for the rectangle in which the text is to be formatted.
SetRect(&rect, 100, 100, 700, 200);
SetTextColor(hdc, RGB(255, 0, 0));
DrawText(hdc, TEXT("Drawing Text with Impact"), -1, &rect, DT_NOCLIP);
//DeleteObject(hFont);
//Logical units are device dependent pixels, so this will create a handle to a logical font that is 36 pixels in height.
//The width, when set to 20, will cause the font mapper to choose a font which, in this case, is stretched.
//The font face name will be Times New Roman. This time nEscapement is at -300 tenths of a degree (-30 degrees)
hFont = CreateFont(36, 20, -300, 0, FW_DONTCARE, FALSE, TRUE, FALSE, DEFAULT_CHARSET, OUT_OUTLINE_PRECIS,
CLIP_DEFAULT_PRECIS, CLEARTYPE_QUALITY, VARIABLE_PITCH, TEXT("Times New Roman"));
SelectObject(hdc, hFont);
//Sets the coordinates for the rectangle in which the text is to be formatted.
SetRect(&rect, 100, 200, 900, 800);
SetTextColor(hdc, RGB(0, 128, 0));
DrawText(hdc, TEXT("Drawing Text with Times New Roman"), -1, &rect, DT_NOCLIP);
//DeleteObject(hFont);
//Logical units are device dependent pixels, so this will create a handle to a logical font that is 36 pixels in height.
//The width, when set to 10, will cause the font mapper to choose a font which, in this case, is compressed.
//The font face name will be Arial. This time nEscapement is at 250 tenths of a degree (25 degrees)
hFont = CreateFont(36, 10, 250, 0, FW_DONTCARE, FALSE, TRUE, FALSE, DEFAULT_CHARSET, OUT_OUTLINE_PRECIS,
CLIP_DEFAULT_PRECIS, ANTIALIASED_QUALITY, VARIABLE_PITCH, TEXT("Arial"));
SelectObject(hdc, hFont);
//Sets the coordinates for the rectangle in which the text is to be formatted.
SetRect(&rect, 500, 200, 1400, 600);
SetTextColor(hdc, RGB(0, 0, 255));
DrawText(hdc, TEXT("Drawing Text with Arial"), -1, &rect, DT_NOCLIP);
DeleteObject(hFont);
EndPaint(hWnd, &ps);
break;
}
case WM_DESTROY:
PostQuitMessage(0);
break;
default:
return DefWindowProc(hWnd, message, wParam, lParam);
}
return 0;
}
Start the task manager after running the sample, you can see it in details:
Then trigger a WM_PAINT message:
We can find that its GDI Objects has increased by 2, and it will increase every time it is triggered, so this example will cause object leakage.
When we call DeleteObject(hFont); after each use(In line 75 and line 88 of my sample), and repeat the above steps, we will find that GDI Objects will not increase, thus solving the problem of object leakage.