ERROR: terminate called without an active exception on QT application

[xkungfu]

main.cpp

#include "mainwindow.h"
#include <iostream>
#include <QApplication>
#include "async++.h"

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    auto task1 = async::spawn([] {
        std::cout << "Task 1 executes asynchronously" << std::endl;
    });



    auto task2 = async::spawn([]() -> int {
        std::cout << "Task 2 executes in parallel with task 1" << std::endl;
        return 42;
    });
    auto task3 = task2.then([](int value) -> int {
        std::cout << "Task 3 executes after task 2, which returned "
                  << value << std::endl;
        return value * 3;
    });
    auto task4 = async::when_all(task1, task3);
    auto task5 = task4.then([](std::tuple<async::task<void>,
                                          async::task<int>> results) {
        std::cout << "Task 5 executes after tasks 1 and 3. Task 3 returned "
                  << std::get<1>(results).get() << std::endl;
    });

    task5.get();
    std::cout << "Task 5 has completed" << std::endl;


    async::parallel_invoke([] {
        std::cout << "This is executed in parallel..." << std::endl;
    }, [] {
        std::cout << "with this" << std::endl;
    });

    async::parallel_for(async::irange(0, 5), [](int x) {
        std::cout << x;
    });
    std::cout << std::endl;

    int r = async::parallel_reduce({1, 2, 3, 4}, 0, [](int x, int y) {
        return x + y;
    });
    std::cout << "The sum of {1, 2, 3, 4} is " << r << std::endl;

    MainWindow w;
    w.show();

    return a.exec();
}

the window wouldn't show, and error message are:

terminate called without an active exception
terminate called recursively
terminate called recursively

how to run some tasks behind Window UI?