dispatch (4 of 7 overloads)

Submits a function to be run on a specified target executor, and after completion submits the completion handler.

template<
    typename Function,
    typename Executor,
    typename NullaryToken = default_completion_token_t<Executor>>
auto dispatch(
    Function && function,
    const Executor & ex,
    NullaryToken && token = default_completion_token_t< Executor >(),
    constraint_t< is_void< result_of_t< decay_t< Function >()> >::value >  = 0,
    constraint_t<(execution::is_executor< Executor >::value &&can_require< Executor, execution::blocking_t::never_t >::value)||is_executor< Executor >::value >  = 0);

This function submits a function object for execution using the specified executor. The function object may be called from the current thread prior to returning from dispatch(). Otherwise, it is queued for execution. After the submitted function completes, the completion handler is dispatched to run on its associated executor.

Parameters

function

A nullary function to be executed on the target executor.

ex

The target executor.

token

The completion token that will be used to produce a completion handler. The function signature of the completion handler must be:

void handler();

Return Value

This function returns async_initiate<NullaryToken, void()>(Init{ex}, token, forward<Function>(function)), where Init is a function object type defined as:

class Init
{
public:
  using executor_type = Executor;
  explicit Init(const Executor& ex) : ex_(ex) {}
  executor_type get_executor() const noexcept { return ex_; }
  template <typename CompletionHandler>
    void operator()(CompletionHandler&& completion_handler,
      Function&& function) const;
private:
  Executor ex_; // exposition only
};

The function call operator of Init:

Obtains the handler's associated executor object ex1 of type Ex1 by performing

auto ex1 = get_associated_executor(completion_handler, ex);
Obtains the handler's associated allocator object alloc by performing

auto alloc = get_associated_allocator(completion_handler);
If execution::is_executor<Ex1>::value is true, constructs a function object wrapper f with a member executor_ that is initialised with prefer(ex1, execution::outstanding_work.tracked), a member function_ that is a decay-copy of function, a member handler_ that is a decay-copy of completion_handler, and a function call operator that performs:

std::move(function_)(); auto a = get_associated_allocator(handler_); prefer(executor_, execution::allocator(a)).execute(std::move(handler_));
If execution::is_executor<Ex1>::value is false, constructs a function object wrapper f with a member work_ that is initialised with make_work_guard(ex1), a member function_ that is a decay-copy of function, a member handler_ that is a decay-copy of completion_handler, and a function call operator that performs:

std::move(function_)(); auto a = get_associated_allocator(handler_); work_.get_executor().dispatch(std::move(handler_), a); work_.reset();
If execution::is_executor<Executor>::value is true, performs

prefer( require(ex, execution::blocking.never), execution::relationship.fork, execution::allocator(alloc) ).execute(std::move(f));
If execution::is_executor<Executor>::value is false, performs

ex.dispatch(std::move(f), alloc);

Remarks

If the function object throws an exception, that exception is allowed to propagate to the target executor. The behaviour in this case is dependent on the executor. For example, io_context will allow the exception to propagate to the caller that runs the io_context, whereas thread_pool will call std::terminate.

Completion Signature

void()