QProgressDialog with separate worker thread. Note that there's a possible reentrancy problem since setValue calls QApplication::processEvents.

gistfile1.txt

QProgressDialog dialog("Parameter sweep ...", "Cancel", 0, (int) sweep_values.size(), this);
dialog.setWindowTitle(MainWindow::WINDOW_CAPTION);
dialog.setWindowModality(Qt::WindowModal);
dialog.setMinimumDuration(0);
dialog.setValue(0);
std::atomic<bool> worker_canceled(false), worker_stopped(false);
std::exception_ptr worker_exception;
std::thread worker_thread([&]() {
	try {
		for(size_t i = 0; i < sweep_values.size(); ++i) {
			if(worker_canceled) {
				throw std::runtime_error("Parameter sweep canceled by user.");
			}
			context.m_parameters[param_index].Value() = FloatScale(sweep_values[i]);
			tline_type.m_simulate(context);
			std::copy(context.m_results.begin(), context.m_results.end(), combined_results.begin() + TLINERESULT_COUNT * tline_type.m_modes.size() * i);
			bool res = QMetaObject::invokeMethod(&dialog, "setValue", Qt::QueuedConnection, Q_ARG(int, (int) i + 1));
		}
	} catch(...) {
		worker_exception = std::current_exception();
	}
	worker_stopped = true;
});
try {
	while(!worker_stopped) {
		QApplication::processEvents(QEventLoop::WaitForMoreEvents);
		if(dialog.wasCanceled()) {
			worker_canceled = true;
			break;
		}
	}
} catch(...) {
	worker_canceled = true;
	worker_thread.join();
	throw;
}
worker_thread.join();
if(worker_exception) {
	std::rethrow_exception(worker_exception);
}