160 lines
4.5 KiB
C++
160 lines
4.5 KiB
C++
//
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// use_future.hpp
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// ~~~~~~~~~~~~~~
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//
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// Copyright (c) 2003-2018 Christopher M. Kohlhoff (chris at kohlhoff dot com)
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//
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// Distributed under the Boost Software License, Version 1.0. (See accompanying
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// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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//
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#ifndef ASIO_USE_FUTURE_HPP
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#define ASIO_USE_FUTURE_HPP
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#if defined(_MSC_VER) && (_MSC_VER >= 1200)
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# pragma once
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#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
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#include "asio/detail/config.hpp"
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#if defined(ASIO_HAS_STD_FUTURE) \
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|| defined(GENERATING_DOCUMENTATION)
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#include <memory>
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#include "asio/detail/type_traits.hpp"
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#include "asio/detail/push_options.hpp"
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namespace asio {
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namespace detail {
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template <typename Function, typename Allocator>
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class packaged_token;
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template <typename Function, typename Allocator, typename Result>
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class packaged_handler;
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} // namespace detail
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/// Class used to specify that an asynchronous operation should return a future.
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/**
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* The use_future_t class is used to indicate that an asynchronous operation
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* should return a std::future object. A use_future_t object may be passed as a
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* handler to an asynchronous operation, typically using the special value @c
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* asio::use_future. For example:
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*
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* @code std::future<std::size_t> my_future
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* = my_socket.async_read_some(my_buffer, asio::use_future); @endcode
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*
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* The initiating function (async_read_some in the above example) returns a
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* future that will receive the result of the operation. If the operation
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* completes with an error_code indicating failure, it is converted into a
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* system_error and passed back to the caller via the future.
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*/
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template <typename Allocator = std::allocator<void> >
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class use_future_t
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{
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public:
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/// The allocator type. The allocator is used when constructing the
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/// @c std::promise object for a given asynchronous operation.
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typedef Allocator allocator_type;
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/// Construct using default-constructed allocator.
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ASIO_CONSTEXPR use_future_t()
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{
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}
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/// Construct using specified allocator.
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explicit use_future_t(const Allocator& allocator)
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: allocator_(allocator)
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{
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}
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#if !defined(ASIO_NO_DEPRECATED)
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/// (Deprecated: Use rebind().) Specify an alternate allocator.
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template <typename OtherAllocator>
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use_future_t<OtherAllocator> operator[](const OtherAllocator& allocator) const
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{
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return use_future_t<OtherAllocator>(allocator);
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}
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#endif // !defined(ASIO_NO_DEPRECATED)
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/// Specify an alternate allocator.
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template <typename OtherAllocator>
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use_future_t<OtherAllocator> rebind(const OtherAllocator& allocator) const
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{
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return use_future_t<OtherAllocator>(allocator);
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}
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/// Obtain allocator.
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allocator_type get_allocator() const
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{
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return allocator_;
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}
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/// Wrap a function object in a packaged task.
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/**
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* The @c package function is used to adapt a function object as a packaged
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* task. When this adapter is passed as a completion token to an asynchronous
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* operation, the result of the function object is retuned via a std::future.
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*
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* @par Example
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*
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* @code std::future<std::size_t> fut =
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* my_socket.async_read_some(buffer,
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* use_future([](asio::error_code ec, std::size_t n)
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* {
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* return ec ? 0 : n;
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* }));
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* ...
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* std::size_t n = fut.get(); @endcode
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*/
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template <typename Function>
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#if defined(GENERATING_DOCUMENTATION)
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unspecified
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#else // defined(GENERATING_DOCUMENTATION)
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detail::packaged_token<typename decay<Function>::type, Allocator>
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#endif // defined(GENERATING_DOCUMENTATION)
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operator()(ASIO_MOVE_ARG(Function) f) const;
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private:
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// Helper type to ensure that use_future can be constexpr default-constructed
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// even when std::allocator<void> can't be.
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struct std_allocator_void
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{
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ASIO_CONSTEXPR std_allocator_void()
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{
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}
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operator std::allocator<void>() const
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{
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return std::allocator<void>();
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}
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};
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typename conditional<
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is_same<std::allocator<void>, Allocator>::value,
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std_allocator_void, Allocator>::type allocator_;
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};
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/// A special value, similar to std::nothrow.
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/**
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* See the documentation for asio::use_future_t for a usage example.
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*/
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#if defined(ASIO_HAS_CONSTEXPR) || defined(GENERATING_DOCUMENTATION)
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constexpr use_future_t<> use_future;
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#elif defined(ASIO_MSVC)
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__declspec(selectany) use_future_t<> use_future;
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#endif
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} // namespace asio
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#include "asio/detail/pop_options.hpp"
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#include "asio/impl/use_future.hpp"
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#endif // defined(ASIO_HAS_STD_FUTURE)
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// || defined(GENERATING_DOCUMENTATION)
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#endif // ASIO_USE_FUTURE_HPP
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