CODEWITHSUNDEEP
c++mfc
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Reputation: 15

CAboutDlg aboutDlg and local variable

I have a question seems simple but confuse me. I didn't get answer from Google. In Visual Studio MFC project, I test this sample code:

    void CEmptySingleDocApp::OnAppAbout()
    {
        TmpVarAboutDlg();
    }

    void TmpVarAboutDlg() {
        CAboutDlg aboutDlg;
        aboutDlg.DoModal();
    }

The aboutDlg in the sample code is a local variable, but why the code runs well? My blind guess is that when create a CAboutDlg, you are using a resource, which is in this line:

    CAboutDlg::CAboutDlg() noexcept : CDialogEx(IDD_ABOUTBOX)

Do I guess right?

Upvotes: 0

Views: 430

Answers (1)

IInspectable
IInspectable

Reputation: 51496

aboutDlg is indeed a local variable that is destroyed when it goes out of scope at the end of TmpVarAboutDlg. This is not a problem as the code calls DoModal. DoModal only returns after the dialog has been dismissed, and its corresponding C++ object is no longer needed.

While the code does use an application-provided resource to construct the dialog, the same principles hold if the dialog were created from an in-memory dialog template, or dynamically through code.

Note that while a program is blocked on a modal dialog, the system still dispatches messages for other windows. For example, if you move the dialog, the system will generate WM_PAINT messages for windows underneath it as appropriate.

Upvotes: 2

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