Question

The code below is based on Herb Sutter’s ideas of an implementation of a .then() type continuation.

  template<typename Fut, typename Work>
auto then(Fut f, Work w)->std::future<decltype(w(f.get()))>
  { return std::async([=] { w(f.get()); }); }

This would be used like auto next = then(f, [](int r) { go_and_use(r); }); or similar.

This is a neat idea, but as it stands will not work (futures are move only and not copyable). I do like the idea as it is likely to appear in upcoming versions of c++ as far as I can guess (although as .then() or even await.)

Before making the futures shared or similar I wonder what the stack overflow community would think of this implementation specifically with improvements and suggestions (even shared futures)?

Thanks in advance for any suggestions.

(I am aware this is a fix till a standards based mechanism exists as it will cost a thread (maybe))).

Answer 1

I find 3 problems with the above implemention:

• It will only work if you pass std::shared_future as Fut.
• The continuation might want a chance to handle exceptions.
• It will not always behave as expected, since if you do not specify std::launch::async it might be deferred, thus the continuation is not invoked as one would expect.

I’ve tried to address these:

template<typename F, typename W, typename R>
struct helper
{
    F f;
    W w;

    helper(F f, W w)
        : f(std::move(f))
        , w(std::move(w))
    {
    }

    helper(const helper& other)
        : f(other.f)
        , w(other.w)
    {
    }

    helper(helper&& other)
        : f(std::move(other.f))
        , w(std::move(other.w))
    {
    }

    helper& operator=(helper other)
    {
        f = std::move(other.f);
        w = std::move(other.w);
        return *this;
    }

    R operator()()
    {
        f.wait();
        return w(std::move(f)); 
    }
};

}

template<typename F, typename W>
auto then(F f, W w) -> std::future<decltype(w(F))>
{ 
    return std::async(std::launch::async, detail::helper<F, W, decltype(w(f))>(std::move(f), std::move(w))); 
}

Used like this:

std::future<int> f = foo();

auto f2 = then(std::move(f), [](std::future<int> f)
{
    return f.get() * 2; 
});