userver: engine::Future< T > Class Template Reference
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Public Member Functions | List of all members
engine::Future< T > Class Template Referencefinal
Concurrency

#include <userver/engine/future.hpp>

Detailed Description

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

std::future replacement for asynchronous tasks that works in pair with engine::Promise

engine::Future can only be used from 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 51 of file future.hpp.

Public Member Functions

 Future ()=default
 Creates an Future without a valid state.
 
 Future (const Future &)=delete
 
 Future (Future &&) noexcept=default
 
Future & operator= (const Future &)=delete
 
Future & operator= (Future &&) noexcept=default
 
bool valid () const noexcept
 Returns whether this Future holds a valid state.
 
T get ()
 Waits for value availability and retrieves it.
 
FutureStatus wait () const
 Waits for value availability.
 
template<typename Rep , typename Period >
FutureStatus wait_for (std::chrono::duration< Rep, Period > timeout) const
 Waits for value availability until the timeout expires or until the task is cancelled.
 
template<typename Clock , typename Duration >
FutureStatus wait_until (std::chrono::time_point< Clock, Duration > until) const
 Waits for value availability until the deadline is reached or until the task is cancelled.
 
FutureStatus wait_until (Deadline deadline) const
 Waits for value availability until the deadline is reached or until the task is cancelled.
 

Member Function Documentation

◆ get()

template<typename T >
T engine::Future< T >::get ( )

Waits for value availability and retrieves it.

Exceptions
WaitInterruptedExceptionif the current task has been cancelled in the process.
std::future_errorif Future holds no state, if the Promise has been destroyed without setting a value or if the value has already been retrieved.

Definition at line 189 of file future.hpp.

◆ valid()

template<typename T >
bool engine::Future< T >::valid ( ) const
noexcept

Returns whether this Future holds a valid state.

Definition at line 184 of file future.hpp.

◆ wait()

template<typename T >
FutureStatus engine::Future< T >::wait ( ) const

Waits for value availability.

Returns
FutureStatus::kReady if the value is available.
FutureStatus::kCancelled if current task is being cancelled.
Exceptions
std::future_errorif Future holds no state.

Definition at line 195 of file future.hpp.

◆ wait_for()

template<typename T >
template<typename Rep , typename Period >
FutureStatus engine::Future< T >::wait_for ( std::chrono::duration< Rep, Period > timeout) const

Waits for value availability until the timeout expires or until the task is cancelled.

Returns
FutureStatus::kReady if the value is available.
FutureStatus::kTimeout if timeout has expired.
FutureStatus::kCancelled if current task is being cancelled.
Exceptions
std::future_errorif Future holds no state.

Definition at line 202 of file future.hpp.

◆ wait_until() [1/2]

template<typename T >
FutureStatus engine::Future< T >::wait_until ( Deadline deadline) const

Waits for value availability until the deadline is reached or until the task is cancelled.

Returns
FutureStatus::kReady if the value is available.
FutureStatus::kTimeout if deadline was reached.
FutureStatus::kCancelled if current task is being cancelled.
Exceptions
std::future_errorif Future holds no state.

Definition at line 215 of file future.hpp.

◆ wait_until() [2/2]

template<typename T >
template<typename Clock , typename Duration >
FutureStatus engine::Future< T >::wait_until ( std::chrono::time_point< Clock, Duration > until) const

Waits for value availability until the deadline is reached or until the task is cancelled.

Returns
FutureStatus::kReady if the value is available.
FutureStatus::kTimeout if until time point was reached.
FutureStatus::kCancelled if current task is being cancelled.
Exceptions
std::future_errorif Future holds no state.

Definition at line 209 of file future.hpp.

Friends And Related Symbol Documentation

◆ Promise< T >

template<typename T >
friend class Promise< T >
friend

Definition at line 102 of file future.hpp.

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