defer (2 of 3 overloads)

Submits a completion token or function object for execution.

template<
    typename Executor,
    typename NullaryToken = DEFAULT>
DEDUCED defer(
    const Executor & ex,
    NullaryToken && token = DEFAULT,
    typename constraint< execution::is_executor< Executor >::value||is_executor< Executor >::value >::type  = 0);

This function submits an object for execution using the specified executor. The function object is queued for execution, and is never called from the current thread prior to returning from defer().

The use of defer(), rather than post, indicates the caller's preference that the executor defer the queueing of the function object. This may allow the executor to optimise queueing for cases when the function object represents a continuation of the current call context.

Parameters

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), 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) 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(handler, ex);
```
Obtains the handler's associated allocator object alloc by performing
```
auto alloc = get_associated_allocator(handler);
```
If execution::is_executor<Ex1>::value is true, constructs a function object f with a member executor_ that is initialised with prefer(ex1, execution::outstanding_work.tracked), a member handler_ that is a decay-copy of completion_handler, and a function call operator that performs:
```
auto a = get_associated_allocator(handler_);
execution::execute(
    prefer(executor_,
      execution::blocking.possibly,
      execution::allocator(a)),
    std::move(handler_));
```
If execution::is_executor<Ex1>::value is false, constructs a function object f with a member work_ that is initialised with make_work_guard(ex1), a member handler_ that is a decay-copy of completion_handler, and a function call operator that performs:
```
auto a = get_associated_allocator(handler_);
work_.get_executor().dispatch(std::move(handler_), a);
work_.reset();
```

If execution::is_executor<Ex>::value is true, performs

execution::execute(
    prefer(
      require(ex, execution::blocking.never),
      execution::relationship.continuation,
      execution::allocator(alloc)),
    std::move(f));

If execution::is_executor<Ex>::value is false, performs
```
ex.defer(std::move(f), alloc);
```

Completion Signature

void()