CreateFont、DeleteObject 和 GDI 句柄/内存泄漏

Question

我正在查看 https://docs.microsoft.com/en-us/windows/win32/api/wingdi/nf-wingdi-createfonta 的 CreateFont API 示例

它清楚地表明，在使用CreateFont 创建字体后，它应该被DeleteObject 调用销毁。 DeleteObject(hFont); 只被调用一次。 CreateFont 被调用了 3 次。这是 MS 文档中的错误吗？不应该通过SelectObject检索旧字体来存储旧字体并在使用新字体后重新设置吗？

Answer 1

是的，有两个创建的字体对象泄露了。

请注意，MS 示例代码在错误处理和对象清理方面通常很差（它们通常专注于演示示例的核心内容 - 这里是 CreateFont 调用 - 而忽略或最小化这些问题）。

Answer 2

文档中的示例确实导致了字体对象的泄漏

我构建了一个示例如下：

#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;
}

运行示例后启动任务管理器，可以详细查看：

然后触发WM_PAINT消息：

我们可以发现它的GDI Objects增加了2，每次触发都会增加，所以这个例子会造成对象泄漏。

当我们每次使用后调用DeleteObject(hFont);（在我的示例的第75行和第88行），并重复上述步骤，我们会发现GDI Objects不会增加，从而解决了对象泄漏的问题。