Save application settings MFC CArchive way

I have several settings in app witch I want to be saved on changing it. there is check box:

void CL2HamsterDlg::OnBnClickedCheckAutoselect()
{
    CFile theFile( pFileName1,CFile::modeWrite );
    CArchive ar(&theFile, CArchive::store);
    if(c_Acheck.GetCheck()==BST_UNCHECKED)
        a = 0;
    else
        a = 1;
    ar << a;
    ar.Close();
    theFile.Close();
}

and combo box with save button (When pressing save saves combo content):

void CL2HamsterDlg::OnClickedButtonLoad()
{
    Value.GetWindowText(str);
    CFile theFile( pFileName1,CFile::modeWrite );
    CArchive ar(&theFile, CArchive::store);
    ar << Savename;
    ar.Close();
    theFile.Close();
}

on initialization app must load these settings and set it:

BOOL CL2HamsterDlg::OnInitDialog()
{
    CDialogEx::OnInitDialog();
    pFileName1 = "settings.dat";    
    theFile.Open( pFileName1,CFile::modeRead );
    CArchive ar(&theFile, CArchive::load);
    ar >> a;
    ar.Close();
    theFile.Close();
    if(a == 1)
        c_Acheck.SetCheck(BST_CHECKED);
    return TRUE;
}

but all the time when one or another setting changed the program overwrites file.

ps. I don't want use registry (because of cause portable)

my question is how I can write and read settings in serialization way?

what available alternatively ways to save program settings state? (please with example)

You need to write all settings to the archive every time, when something changed. Accordingly, load all settings from the archive file, when the program starts.