Simplest C++ Callback, from SumatraPDF(blog.kowalczyk.info)

    struct Foo {
        void bar(int a, int b, int c);
    };

    Foo foo;

    using Callback = std::function<void(int, int, int)>;

    // with std::bind (ugh):
    using namespace std::placeholders;
    Callback cb1(std::bind(&Foo::bar, &foo, _1, _2, _3));

    // lambda (without perfect forwarding):
    Callback cb2([&foo](auto&&... args) { foo.bar(args...); });

    // lambda (with perfect forwarding):
    Callback cb3([&foo](auto&&... args) { foo.bar(std::forward<decltype(args)>(args)...); });

    // std::bind_front
    Callback cb4(std::bind_front(&Foo::bar, &foo));

    template<class R, class... Args>
    struct FnBase {
       virtual R operator()(Args...) = 0;
    };

    class MyThread : FnBase<void> { ... };

    void Call(T arg) const {
        if (fn) {
            fn(userData, arg);
        }
    }

    void Call(T&& arg) const {
        if (fn) {
            fn(userData, std::forward<T>(arg));
        }
    } 

    std::vector<double> foo();
    void bar(Func1<std::vector<double>> f) {
        auto v = foo();
        f(std::move(v));
    }

    std::function<void()> cb([widget]() { widget->onButtonClicked(); });

    std::function<void()> cb(std::bind(&Widget::onButtonClicked, widget));