engine::Promise< T > Class Template Reference

Table of Contents

• Detailed Description
• Example usage:
• Public Member Functions
• Constructor & Destructor Documentation
• Promise()
• ~Promise()
• Member Function Documentation
• get_future()
• operator=()
• set_exception()
• set_value() [1/2]
• set_value() [2/2]

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 122 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 235 of file future.hpp.

~Promise()

template<typename T >

engine::Promise< T >::~Promise

(

)

Definition at line 246 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 258 of file future.hpp.

operator=()

template<typename T >

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

noexcept

Definition at line 238 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 273 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 263 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 268 of file future.hpp.

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

• userver/engine/future.hpp