engine::Promise< T > Class Template Reference

Detailed Description

template<typename T>
class engine::Promise< T >

std::promise replacement for asynchronous tasks that works in pair with engine::Future

engine::Promise can be used both from coroutines and from non-coroutine threads.

Example usage:

    engine::Promise<int> int_promise;
    engine::Promise<std::string> string_promise;
    auto deadline = GetDeadline();
 
    constexpr auto kBadValue = -1;
    constexpr auto kFallbackString = "Bad string";
 
    constexpr auto kTestValue = 777;
    constexpr auto kTestString = "Test string value";
 
    auto int_future = int_promise.get_future();
    auto string_future = string_promise.get_future();
 
    auto calc_task = utils::Async(
        "calc",
        [int_promise = std::move(int_promise), string_promise = std::move(string_promise), kTestValue]() mutable {
            // Emulating long calculation of x and s...
            engine::SleepFor(100ms);
            int_promise.set_value(kTestValue);
 
            engine::SleepFor(100ms);
            string_promise.set_value(kTestString);
            // Other calculations.
        }
    );
 
    auto int_consumer =
        utils::Async("int_consumer", [deadline, int_future = std::move(int_future), kTestValue]() mutable {
            auto status = int_future.wait_until(deadline);
            auto x = kBadValue;
            switch (status) {
                case engine::FutureStatus::kReady:
                    x = int_future.get();
                    ASSERT_EQ(x, kTestValue);
                    // ...
                    break;
                case engine::FutureStatus::kTimeout:
                    // Timeout Handling
                    break;
                case engine::FutureStatus::kCancelled:
                    // Handling cancellation of calculations
                    // (example, return to queue)
                    return;
            }
        });
 
    auto string_consumer =
        utils::Async("string_consumer", [string_future = std::move(string_future), kTestString]() mutable {
            std::string s;
            try {
                s = string_future.get();
                ASSERT_EQ(s, kTestString);
            } catch (const std::exception& ex) {
                // Exception Handling
                s = kFallbackString;
            }
            // ...
        });
 
    calc_task.Get();
    int_consumer.Get();
    string_consumer.Get();

See also

Synchronization Primitives

Definition at line 129 of file future.hpp.

Public Member Functions
	Promise ()
	Creates a new asynchronous value store.
	Promise (const Promise &)=delete
	Promise (Promise &&) noexcept=default
Promise &	operator= (const Promise &)=delete
Promise &	operator= (Promise &&) noexcept
Future< T >	get_future ()
void	set_value (const T &)
void	set_value (T &&)
void	set_exception (std::exception_ptr ex)

Constructor & Destructor Documentation

Promise()

template<typename T >

engine::Promise< T >::Promise

(

)

Creates a new asynchronous value store.

Definition at line 245 of file future.hpp.

~Promise()

template<typename T >

engine::Promise< T >::~Promise

(

)

Definition at line 256 of file future.hpp.

Member Function Documentation

get_future()

template<typename T >

Future< T > engine::Promise< T >::get_future

(

)

Retrieves the Future associated with this value store.

Exceptions

std::future_error

if the Future has already been retrieved.

Definition at line 267 of file future.hpp.

operator=()

template<typename T >

Promise< T > & engine::Promise< T >::operator= ( Promise< T > && other )

noexcept

Definition at line 248 of file future.hpp.

set_exception()

template<typename T >

void engine::Promise< T >::set_exception

(

std::exception_ptr

ex

)

Stores an exception to be thrown on retrieval.

Exceptions

std::future_error

if a value or an exception has already been set.

Definition at line 282 of file future.hpp.

set_value() [1/2]

template<typename T >

void engine::Promise< T >::set_value

(

const T &

value

)

Stores a value for retrieval.

Exceptions

std::future_error

if a value or an exception has already been set.

Definition at line 272 of file future.hpp.

set_value() [2/2]

template<typename T >

void engine::Promise< T >::set_value

(

T &&

value

)

Stores a value for retrieval.

Exceptions

std::future_error

if a value or an exception has already been set.

Definition at line 277 of file future.hpp.

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The documentation for this class was generated from the following file:

• userver/engine/future.hpp