Question

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.

Answer 1

You can’t pass the address of the string in PostMessage, since the string is probably thread-local on the stack. By the time the other thread picks it up, it could have been destroyed.

Instead, you should create a new string or exception object via new and pass its address to the other thread (via the WPARAM or LPARAM parameter in PostMessage.) The other thread then owns the object and is responsible for destroying it.

Here is some sample code that shows how this could be done:

try
{
    // do stuff
}
catch (const MyException& the_exception)
{
    PostMessage(myhWnd, CWM_SOME_ERROR, 0, new std::string(the_exception.error_string));
}


LPARAM CMyDlg::SomeError(WPARAM, LPARAM lParam)
{
    // Wrap in a unique_ptr so it is automatically destroyed.
    std::unique_ptr<std::string> msg = reinterpret_cast<std::string*>(lParam);

    // Do stuff with message

    return 0;
}