How to send a string via PostMessage?

Inside my app, I want to send a message to a dialog from a different thread. I want to pass an std::exception derived class reference to the dialog.

Something like this:

try {
       //do stuff
}
catch (MyException& the_exception) {
    PostMessage(MyhWnd, CWM_SOME_ERROR, 0, 0); //send the_exception or the_exception.error_string() here
}

I want to receive the message in my dialog and show the error that is in the_exception.error_string()

LPARAM CMyDlg::SomeError(WPARAM, LPARAM)
{
    show_error( ?????
    return 0;
}

passing the std::string the_exception.error_string() using PostMessage would also be ok, I guess.