std::promise

 

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Technical Specifications

 

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std::promise

Member functions

promise::promise

promise::~promise

promise::operator=

promise::swap

Getting the result

promise::get_future

Setting the result

promise::set_value

promise::set_value_at_thread_exit

promise::set_exception

promise::set_exception_at_thread_exit

 

Defined in header <future>
template< class R > class promise;	(1)	(since C++11)
template< class R > class promise<R&>;	(2)	(since C++11)
template<>          class promise<void>;	(3)	(since C++11)

1) base template

2) non-void specialization, used to communicate objects between threads

3) void specialization, used to communicate stateless events

The class template std::promise provides a facility to store a value or an exception that is later acquired asynchronously via a std::future object created by the std::promise object.

Each promise is associated with a shared state, which contains some state information and a result which may be not yet evaluated, evaluated to a value (possibly void) or evaluated to an exception. A promise may do three things with the shared state:

• make ready: the promise stores the result or the exception in the shared state. Marks the state ready and unblocks any thread waiting on a future associated with the shared state.
• release: the promise gives up its reference to the shared state. If this was the last such reference, the shared state is destroyed. Unless this was a shared state created by std::async which is not yet ready, this operation does not block.
• abandon: the promise stores the exception of type std::future_error with error code std::future_errc::broken_promise, makes the shared state ready, and then releases it.

The promise is the "push" end of the promise-future communication channel: the operation that stores a value in the shared state synchronizes-with (as defined in std::memory_order) the successful return from any function that is waiting on the shared state (such as std::future::get). Concurrent access to the same shared state may conflict otherwise: for example multiple callers of std::shared_future::get must either all be read-only or provide external synchronization.

Member functions

(constructor)	constructs the promise object (public member function)
(destructor)	destructs the promise object (public member function)
operator=	assigns the shared state (public member function)
swap	swaps two promise objects (public member function)
Getting the result
get_future	returns a future associated with the promised result (public member function)
Setting the result
set_value	sets the result to specific value (public member function)
set_value_at_thread_exit	sets the result to specific value while delivering the notification only at thread exit (public member function)
set_exception	sets the result to indicate an exception (public member function)
set_exception_at_thread_exit	sets the result to indicate an exception while delivering the notification only at thread exit (public member function)

Non-member functions

std::swap(std::promise) (C++11)

specializes the std::swap algorithm (function template)

Helper classes

std::uses_allocator<std::promise> (C++11)

specializes the std::uses_allocator type trait (class template specialization)