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// <future> -*- C++ -*-
// Copyright (C) 2009-2023 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library. This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.
// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
// <http://www.gnu.org/licenses/>.
/** @file include/future
* This is a Standard C++ Library header.
*/
#ifndef _GLIBCXX_FUTURE
#define _GLIBCXX_FUTURE 1
#pragma GCC system_header
#include <bits/requires_hosted.h> // concurrency
#if __cplusplus < 201103L
# include <bits/c++0x_warning.h>
#else
#include <mutex> // call_once
#include <condition_variable> // __at_thread_exit_elt
#include <system_error>
#include <bits/atomic_base.h> // atomic_flag
#include <bits/allocated_ptr.h>
#include <bits/atomic_futex.h>
#include <bits/exception_defines.h>
#include <bits/invoke.h>
#include <bits/unique_ptr.h>
#include <bits/shared_ptr.h>
#include <bits/std_function.h>
#include <bits/std_thread.h>
#include <bits/uses_allocator.h>
#include <ext/aligned_buffer.h>
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
/**
* @defgroup futures Futures
* @ingroup concurrency
*
* Futures and promises provide support for retrieving the result from
* an asynchronous function, e.g. one that is running in another thread.
* A `std::future` represents an asynchronous result that will become
* ready at some later time. A consumer can wait on a future until the
* result is ready to be accessed.
*
* @since C++11
* @{
*/
/// Error code for futures
enum class future_errc
{
future_already_retrieved = 1,
promise_already_satisfied,
no_state,
broken_promise
};
/// Specialization that allows `future_errc` to convert to `error_code`.
template<>
struct is_error_code_enum<future_errc> : public true_type { };
/// Points to a statically-allocated object derived from error_category.
[[__nodiscard__, __gnu__::__const__]]
const error_category&
future_category() noexcept;
/// Overload of make_error_code for `future_errc`.
[[__nodiscard__]]
inline error_code
make_error_code(future_errc __errc) noexcept
{ return error_code(static_cast<int>(__errc), future_category()); }
/// Overload of make_error_condition for `future_errc`.
[[__nodiscard__]]
inline error_condition
make_error_condition(future_errc __errc) noexcept
{ return error_condition(static_cast<int>(__errc), future_category()); }
/**
* @brief Exception type thrown by futures.
* @ingroup exceptions
* @since C++11
*/
class future_error : public logic_error
{
public:
explicit
future_error(future_errc __errc)
: future_error(std::make_error_code(__errc))
{ }
virtual ~future_error() noexcept;
virtual const char*
what() const noexcept;
const error_code&
code() const noexcept { return _M_code; }
private:
explicit
future_error(error_code __ec)
: logic_error("std::future_error: " + __ec.message()), _M_code(__ec)
{ }
friend void __throw_future_error(int);
error_code _M_code;
};
// Forward declarations.
template<typename _Res>
class future;
template<typename _Res>
class shared_future;
template<typename _Signature>
class packaged_task;
template<typename _Res>
class promise;
/// Launch code for futures
enum class launch
{
async = 1,
deferred = 2
};
constexpr launch operator&(launch __x, launch __y) noexcept
{
return static_cast<launch>(
static_cast<int>(__x) & static_cast<int>(__y));
}
constexpr launch operator|(launch __x, launch __y) noexcept
{
return static_cast<launch>(
static_cast<int>(__x) | static_cast<int>(__y));
}
constexpr launch operator^(launch __x, launch __y) noexcept
{
return static_cast<launch>(
static_cast<int>(__x) ^ static_cast<int>(__y));
}
constexpr launch operator~(launch __x) noexcept
{ return static_cast<launch>(~static_cast<int>(__x)); }
inline launch& operator&=(launch& __x, launch __y) noexcept
{ return __x = __x & __y; }
inline launch& operator|=(launch& __x, launch __y) noexcept
{ return __x = __x | __y; }
inline launch& operator^=(launch& __x, launch __y) noexcept
{ return __x = __x ^ __y; }
/// Status code for futures
enum class future_status
{
ready,
timeout,
deferred
};
/// @cond undocumented
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// 2021. Further incorrect usages of result_of
template<typename _Fn, typename... _Args>
using __async_result_of = typename __invoke_result<
typename decay<_Fn>::type, typename decay<_Args>::type...>::type;
/// @endcond
template<typename _Fn, typename... _Args>
future<__async_result_of<_Fn, _Args...>>
async(launch __policy, _Fn&& __fn, _Args&&... __args);
template<typename _Fn, typename... _Args>
future<__async_result_of<_Fn, _Args...>>
async(_Fn&& __fn, _Args&&... __args);
#if defined(_GLIBCXX_HAS_GTHREADS)
/// @cond undocumented
/// Base class and enclosing scope.
struct __future_base
{
/// Base class for results.
struct _Result_base
{
exception_ptr _M_error;
_Result_base(const _Result_base&) = delete;
_Result_base& operator=(const _Result_base&) = delete;
// _M_destroy() allows derived classes to control deallocation
virtual void _M_destroy() = 0;
struct _Deleter
{
void operator()(_Result_base* __fr) const { __fr->_M_destroy(); }
};
protected:
_Result_base();
virtual ~_Result_base();
};
/// A unique_ptr for result objects.
template<typename _Res>
using _Ptr = unique_ptr<_Res, _Result_base::_Deleter>;
/// A result object that has storage for an object of type _Res.
template<typename _Res>
struct _Result : _Result_base
{
private:
__gnu_cxx::__aligned_buffer<_Res> _M_storage;
bool _M_initialized;
public:
typedef _Res result_type;
_Result() noexcept : _M_initialized() { }
~_Result()
{
if (_M_initialized)
_M_value().~_Res();
}
// Return lvalue, future will add const or rvalue-reference
_Res&
_M_value() noexcept { return *_M_storage._M_ptr(); }
void
_M_set(const _Res& __res)
{
::new (_M_storage._M_addr()) _Res(__res);
_M_initialized = true;
}
void
_M_set(_Res&& __res)
{
::new (_M_storage._M_addr()) _Res(std::move(__res));
_M_initialized = true;
}
private:
void _M_destroy() { delete this; }
};
/// A result object that uses an allocator.
template<typename _Res, typename _Alloc>
struct _Result_alloc final : _Result<_Res>, _Alloc
{
using __allocator_type = __alloc_rebind<_Alloc, _Result_alloc>;
explicit
_Result_alloc(const _Alloc& __a) : _Result<_Res>(), _Alloc(__a)
{ }
private:
void _M_destroy()
{
__allocator_type __a(*this);
__allocated_ptr<__allocator_type> __guard_ptr{ __a, this };
this->~_Result_alloc();
}
};
// Create a result object that uses an allocator.
template<typename _Res, typename _Allocator>
static _Ptr<_Result_alloc<_Res, _Allocator>>
_S_allocate_result(const _Allocator& __a)
{
using __result_type = _Result_alloc<_Res, _Allocator>;
typename __result_type::__allocator_type __a2(__a);
auto __guard = std::__allocate_guarded(__a2);
__result_type* __p = ::new((void*)__guard.get()) __result_type{__a};
__guard = nullptr;
return _Ptr<__result_type>(__p);
}
// Keep it simple for std::allocator.
template<typename _Res, typename _Tp>
static _Ptr<_Result<_Res>>
_S_allocate_result(const std::allocator<_Tp>& __a)
{
return _Ptr<_Result<_Res>>(new _Result<_Res>);
}
// Base class for various types of shared state created by an
// asynchronous provider (such as a std::promise) and shared with one
// or more associated futures.
class _State_baseV2
{
typedef _Ptr<_Result_base> _Ptr_type;
enum _Status : unsigned {
__not_ready,
__ready
};
_Ptr_type _M_result;
__atomic_futex_unsigned<> _M_status;
atomic_flag _M_retrieved = ATOMIC_FLAG_INIT;
once_flag _M_once;
public:
_State_baseV2() noexcept : _M_result(), _M_status(_Status::__not_ready)
{ }
_State_baseV2(const _State_baseV2&) = delete;
_State_baseV2& operator=(const _State_baseV2&) = delete;
virtual ~_State_baseV2() = default;
_Result_base&
wait()
{
// Run any deferred function or join any asynchronous thread:
_M_complete_async();
// Acquire MO makes sure this synchronizes with the thread that made
// the future ready.
_M_status._M_load_when_equal(_Status::__ready, memory_order_acquire);
return *_M_result;
}
template<typename _Rep, typename _Period>
future_status
wait_for(const chrono::duration<_Rep, _Period>& __rel)
{
// First, check if the future has been made ready. Use acquire MO
// to synchronize with the thread that made it ready.
if (_M_status._M_load(memory_order_acquire) == _Status::__ready)
return future_status::ready;
if (_M_is_deferred_future())
return future_status::deferred;
// Don't wait unless the relative time is greater than zero.
if (__rel > __rel.zero()
&& _M_status._M_load_when_equal_for(_Status::__ready,
memory_order_acquire,
__rel))
{
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// 2100. timed waiting functions must also join
// This call is a no-op by default except on an async future,
// in which case the async thread is joined. It's also not a
// no-op for a deferred future, but such a future will never
// reach this point because it returns future_status::deferred
// instead of waiting for the future to become ready (see
// above). Async futures synchronize in this call, so we need
// no further synchronization here.
_M_complete_async();
return future_status::ready;
}
return future_status::timeout;
}
template<typename _Clock, typename _Duration>
future_status
wait_until(const chrono::time_point<_Clock, _Duration>& __abs)
{
#if __cplusplus > 201703L
static_assert(chrono::is_clock_v<_Clock>);
#endif
// First, check if the future has been made ready. Use acquire MO
// to synchronize with the thread that made it ready.
if (_M_status._M_load(memory_order_acquire) == _Status::__ready)
return future_status::ready;
if (_M_is_deferred_future())
return future_status::deferred;
if (_M_status._M_load_when_equal_until(_Status::__ready,
memory_order_acquire,
__abs))
{
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// 2100. timed waiting functions must also join
// See wait_for(...) above.
_M_complete_async();
return future_status::ready;
}
return future_status::timeout;
}
// Provide a result to the shared state and make it ready.
// Calls at most once: _M_result = __res();
void
_M_set_result(function<_Ptr_type()> __res, bool __ignore_failure = false)
{
bool __did_set = false;
// all calls to this function are serialized,
// side-effects of invoking __res only happen once
call_once(_M_once, &_State_baseV2::_M_do_set, this,
std::__addressof(__res), std::__addressof(__did_set));
if (__did_set)
// Use release MO to synchronize with observers of the ready state.
_M_status._M_store_notify_all(_Status::__ready,
memory_order_release);
else if (!__ignore_failure)
__throw_future_error(int(future_errc::promise_already_satisfied));
}
// Provide a result to the shared state but delay making it ready
// until the calling thread exits.
// Calls at most once: _M_result = __res();
void
_M_set_delayed_result(function<_Ptr_type()> __res,
weak_ptr<_State_baseV2> __self)
{
bool __did_set = false;
unique_ptr<_Make_ready> __mr{new _Make_ready};
// all calls to this function are serialized,
// side-effects of invoking __res only happen once
call_once(_M_once, &_State_baseV2::_M_do_set, this,
std::__addressof(__res), std::__addressof(__did_set));
if (!__did_set)
__throw_future_error(int(future_errc::promise_already_satisfied));
__mr->_M_shared_state = std::move(__self);
__mr->_M_set();
__mr.release();
}
// Abandon this shared state.
void
_M_break_promise(_Ptr_type __res)
{
if (static_cast<bool>(__res))
{
__res->_M_error =
make_exception_ptr(future_error(future_errc::broken_promise));
// This function is only called when the last asynchronous result
// provider is abandoning this shared state, so noone can be
// trying to make the shared state ready at the same time, and
// we can access _M_result directly instead of through call_once.
_M_result.swap(__res);
// Use release MO to synchronize with observers of the ready state.
_M_status._M_store_notify_all(_Status::__ready,
memory_order_release);
}
}
// Called when this object is first passed to a future.
void
_M_set_retrieved_flag()
{
if (_M_retrieved.test_and_set())
__throw_future_error(int(future_errc::future_already_retrieved));
}
template<typename _Res, typename _Arg>
struct _Setter;
// set lvalues
template<typename _Res, typename _Arg>
struct _Setter<_Res, _Arg&>
{
// check this is only used by promise<R>::set_value(const R&)
// or promise<R&>::set_value(R&)
static_assert(is_same<_Res, _Arg&>::value // promise<R&>
|| is_same<const _Res, _Arg>::value, // promise<R>
"Invalid specialisation");
// Used by std::promise to copy construct the result.
typename promise<_Res>::_Ptr_type operator()() const
{
_M_promise->_M_storage->_M_set(*_M_arg);
return std::move(_M_promise->_M_storage);
}
promise<_Res>* _M_promise;
_Arg* _M_arg;
};
// set rvalues
template<typename _Res>
struct _Setter<_Res, _Res&&>
{
// Used by std::promise to move construct the result.
typename promise<_Res>::_Ptr_type operator()() const
{
_M_promise->_M_storage->_M_set(std::move(*_M_arg));
return std::move(_M_promise->_M_storage);
}
promise<_Res>* _M_promise;
_Res* _M_arg;
};
// set void
template<typename _Res>
struct _Setter<_Res, void>
{
static_assert(is_void<_Res>::value, "Only used for promise<void>");
typename promise<_Res>::_Ptr_type operator()() const
{ return std::move(_M_promise->_M_storage); }
promise<_Res>* _M_promise;
};
struct __exception_ptr_tag { };
// set exceptions
template<typename _Res>
struct _Setter<_Res, __exception_ptr_tag>
{
// Used by std::promise to store an exception as the result.
typename promise<_Res>::_Ptr_type operator()() const
{
_M_promise->_M_storage->_M_error = *_M_ex;
return std::move(_M_promise->_M_storage);
}
promise<_Res>* _M_promise;
exception_ptr* _M_ex;
};
template<typename _Res, typename _Arg>
__attribute__((__always_inline__))
static _Setter<_Res, _Arg&&>
__setter(promise<_Res>* __prom, _Arg&& __arg) noexcept
{
return _Setter<_Res, _Arg&&>{ __prom, std::__addressof(__arg) };
}
template<typename _Res>
__attribute__((__always_inline__))
static _Setter<_Res, __exception_ptr_tag>
__setter(exception_ptr& __ex, promise<_Res>* __prom) noexcept
{
__glibcxx_assert(__ex != nullptr); // LWG 2276
return _Setter<_Res, __exception_ptr_tag>{ __prom, &__ex };
}
template<typename _Res>
__attribute__((__always_inline__))
static _Setter<_Res, void>
__setter(promise<_Res>* __prom) noexcept
{
return _Setter<_Res, void>{ __prom };
}
template<typename _Tp>
static void
_S_check(const shared_ptr<_Tp>& __p)
{
if (!static_cast<bool>(__p))
__throw_future_error((int)future_errc::no_state);
}
private:
// The function invoked with std::call_once(_M_once, ...).
void
_M_do_set(function<_Ptr_type()>* __f, bool* __did_set)
{
_Ptr_type __res = (*__f)();
// Notify the caller that we did try to set; if we do not throw an
// exception, the caller will be aware that it did set (e.g., see
// _M_set_result).
*__did_set = true;
_M_result.swap(__res); // nothrow
}
// Wait for completion of async function.
virtual void _M_complete_async() { }
// Return true if state corresponds to a deferred function.
virtual bool _M_is_deferred_future() const { return false; }
struct _Make_ready final : __at_thread_exit_elt
{
weak_ptr<_State_baseV2> _M_shared_state;
static void _S_run(void*);
void _M_set();
};
};
#ifdef _GLIBCXX_ASYNC_ABI_COMPAT
class _State_base;
class _Async_state_common;
#else
using _State_base = _State_baseV2;
class _Async_state_commonV2;
#endif
template<typename _BoundFn,
typename _Res = decltype(std::declval<_BoundFn&>()())>
class _Deferred_state;
template<typename _BoundFn,
typename _Res = decltype(std::declval<_BoundFn&>()())>
class _Async_state_impl;
template<typename _Signature>
class _Task_state_base;
template<typename _Fn, typename _Alloc, typename _Signature>
class _Task_state;
template<typename _Res_ptr, typename _Fn,
typename _Res = typename _Res_ptr::element_type::result_type>
struct _Task_setter;
template<typename _Res_ptr, typename _BoundFn>
static _Task_setter<_Res_ptr, _BoundFn>
_S_task_setter(_Res_ptr& __ptr, _BoundFn& __call)
{
return { std::__addressof(__ptr), std::__addressof(__call) };
}
};
/// Partial specialization for reference types.
template<typename _Res>
struct __future_base::_Result<_Res&> : __future_base::_Result_base
{
typedef _Res& result_type;
_Result() noexcept : _M_value_ptr() { }
void
_M_set(_Res& __res) noexcept
{ _M_value_ptr = std::addressof(__res); }
_Res& _M_get() noexcept { return *_M_value_ptr; }
private:
_Res* _M_value_ptr;
void _M_destroy() { delete this; }
};
/// Explicit specialization for void.
template<>
struct __future_base::_Result<void> : __future_base::_Result_base
{
typedef void result_type;
private:
void _M_destroy() { delete this; }
};
/// @endcond
#ifndef _GLIBCXX_ASYNC_ABI_COMPAT
/// @cond undocumented
// Allow _Setter objects to be stored locally in std::function
template<typename _Res, typename _Arg>
struct __is_location_invariant
<__future_base::_State_base::_Setter<_Res, _Arg>>
: true_type { };
// Allow _Task_setter objects to be stored locally in std::function
template<typename _Res_ptr, typename _Fn, typename _Res>
struct __is_location_invariant
<__future_base::_Task_setter<_Res_ptr, _Fn, _Res>>
: true_type { };
/// @endcond
/// Common implementation for future and shared_future.
template<typename _Res>
class __basic_future : public __future_base
{
protected:
typedef shared_ptr<_State_base> __state_type;
typedef __future_base::_Result<_Res>& __result_type;
private:
__state_type _M_state;
public:
// Disable copying.
__basic_future(const __basic_future&) = delete;
__basic_future& operator=(const __basic_future&) = delete;
bool
valid() const noexcept { return static_cast<bool>(_M_state); }
void
wait() const
{
_State_base::_S_check(_M_state);
_M_state->wait();
}
template<typename _Rep, typename _Period>
future_status
wait_for(const chrono::duration<_Rep, _Period>& __rel) const
{
_State_base::_S_check(_M_state);
return _M_state->wait_for(__rel);
}
template<typename _Clock, typename _Duration>
future_status
wait_until(const chrono::time_point<_Clock, _Duration>& __abs) const
{
_State_base::_S_check(_M_state);
return _M_state->wait_until(__abs);
}
protected:
/// Wait for the state to be ready and rethrow any stored exception
__result_type
_M_get_result() const
{
_State_base::_S_check(_M_state);
_Result_base& __res = _M_state->wait();
if (!(__res._M_error == nullptr))
rethrow_exception(__res._M_error);
return static_cast<__result_type>(__res);
}
void _M_swap(__basic_future& __that) noexcept
{
_M_state.swap(__that._M_state);
}
// Construction of a future by promise::get_future()
explicit
__basic_future(const __state_type& __state) : _M_state(__state)
{
_State_base::_S_check(_M_state);
_M_state->_M_set_retrieved_flag();
}
// Copy construction from a shared_future
explicit
__basic_future(const shared_future<_Res>&) noexcept;
// Move construction from a shared_future
explicit
__basic_future(shared_future<_Res>&&) noexcept;
// Move construction from a future
explicit
__basic_future(future<_Res>&&) noexcept;
constexpr __basic_future() noexcept : _M_state() { }
struct _Reset
{
explicit _Reset(__basic_future& __fut) noexcept : _M_fut(__fut) { }
~_Reset() { _M_fut._M_state.reset(); }
__basic_future& _M_fut;
};
};
/// Primary template for future.
template<typename _Res>
class future : public __basic_future<_Res>
{
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// 3458. Is shared_future intended to work with arrays or function types?
static_assert(!is_array<_Res>{}, "result type must not be an array");
static_assert(!is_function<_Res>{}, "result type must not be a function");
static_assert(is_destructible<_Res>{},
"result type must be destructible");
friend class promise<_Res>;
template<typename> friend class packaged_task;
template<typename _Fn, typename... _Args>
friend future<__async_result_of<_Fn, _Args...>>
async(launch, _Fn&&, _Args&&...);
typedef __basic_future<_Res> _Base_type;
typedef typename _Base_type::__state_type __state_type;
explicit
future(const __state_type& __state) : _Base_type(__state) { }
public:
constexpr future() noexcept : _Base_type() { }
/// Move constructor
future(future&& __uf) noexcept : _Base_type(std::move(__uf)) { }
// Disable copying
future(const future&) = delete;
future& operator=(const future&) = delete;
future& operator=(future&& __fut) noexcept
{
future(std::move(__fut))._M_swap(*this);
return *this;
}
/// Retrieving the value
_Res
get()
{
typename _Base_type::_Reset __reset(*this);
return std::move(this->_M_get_result()._M_value());
}
shared_future<_Res> share() noexcept;
};
/// Partial specialization for future<R&>
template<typename _Res>
class future<_Res&> : public __basic_future<_Res&>
{
friend class promise<_Res&>;
template<typename> friend class packaged_task;
template<typename _Fn, typename... _Args>
friend future<__async_result_of<_Fn, _Args...>>
async(launch, _Fn&&, _Args&&...);
typedef __basic_future<_Res&> _Base_type;
typedef typename _Base_type::__state_type __state_type;
explicit
future(const __state_type& __state) : _Base_type(__state) { }
public:
constexpr future() noexcept : _Base_type() { }
/// Move constructor
future(future&& __uf) noexcept : _Base_type(std::move(__uf)) { }
// Disable copying
future(const future&) = delete;
future& operator=(const future&) = delete;
future& operator=(future&& __fut) noexcept
{
future(std::move(__fut))._M_swap(*this);
return *this;
}
/// Retrieving the value
_Res&
get()
{
typename _Base_type::_Reset __reset(*this);
return this->_M_get_result()._M_get();
}
shared_future<_Res&> share() noexcept;
};
/// Explicit specialization for future<void>
template<>
class future<void> : public __basic_future<void>
{
friend class promise<void>;
template<typename> friend class packaged_task;
template<typename _Fn, typename... _Args>
friend future<__async_result_of<_Fn, _Args...>>
async(launch, _Fn&&, _Args&&...);
typedef __basic_future<void> _Base_type;
typedef typename _Base_type::__state_type __state_type;
explicit
future(const __state_type& __state) : _Base_type(__state) { }
public:
constexpr future() noexcept : _Base_type() { }
/// Move constructor
future(future&& __uf) noexcept : _Base_type(std::move(__uf)) { }
// Disable copying
future(const future&) = delete;
future& operator=(const future&) = delete;
future& operator=(future&& __fut) noexcept
{
future(std::move(__fut))._M_swap(*this);
return *this;
}
/// Retrieving the value
void
get()
{
typename _Base_type::_Reset __reset(*this);
this->_M_get_result();
}
shared_future<void> share() noexcept;
};
/// Primary template for shared_future.
template<typename _Res>
class shared_future : public __basic_future<_Res>
{
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// 3458. Is shared_future intended to work with arrays or function types?
static_assert(!is_array<_Res>{}, "result type must not be an array");
static_assert(!is_function<_Res>{}, "result type must not be a function");
static_assert(is_destructible<_Res>{},
"result type must be destructible");
typedef __basic_future<_Res> _Base_type;
public:
constexpr shared_future() noexcept : _Base_type() { }
/// Copy constructor
shared_future(const shared_future& __sf) noexcept : _Base_type(__sf) { }
/// Construct from a future rvalue
shared_future(future<_Res>&& __uf) noexcept
: _Base_type(std::move(__uf))
{ }
/// Construct from a shared_future rvalue
shared_future(shared_future&& __sf) noexcept
: _Base_type(std::move(__sf))
{ }
shared_future& operator=(const shared_future& __sf) noexcept
{
shared_future(__sf)._M_swap(*this);
return *this;
}
shared_future& operator=(shared_future&& __sf) noexcept
{
shared_future(std::move(__sf))._M_swap(*this);
return *this;
}
/// Retrieving the value
const _Res&
get() const { return this->_M_get_result()._M_value(); }
};
/// Partial specialization for shared_future<R&>
template<typename _Res>
class shared_future<_Res&> : public __basic_future<_Res&>
{
typedef __basic_future<_Res&> _Base_type;
public:
constexpr shared_future() noexcept : _Base_type() { }
/// Copy constructor
shared_future(const shared_future& __sf) : _Base_type(__sf) { }
/// Construct from a future rvalue
shared_future(future<_Res&>&& __uf) noexcept
: _Base_type(std::move(__uf))
{ }
/// Construct from a shared_future rvalue
shared_future(shared_future&& __sf) noexcept
: _Base_type(std::move(__sf))
{ }
shared_future& operator=(const shared_future& __sf)
{
shared_future(__sf)._M_swap(*this);
return *this;
}
shared_future& operator=(shared_future&& __sf) noexcept
{
shared_future(std::move(__sf))._M_swap(*this);
return *this;
}
/// Retrieving the value