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 //
 // write_at.hpp
 // ~~~~~~~~~~~~
 //
 // Copyright (c) 2003-2024 Christopher M. Kohlhoff (chris at kohlhoff dot com)
 //
 // Distributed under the Boost Software License, Version 1.0. (See accompanying
 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 //
 
 #ifndef BOOST_ASIO_WRITE_AT_HPP
 #define BOOST_ASIO_WRITE_AT_HPP
 
 #if defined(_MSC_VER) && (_MSC_VER >= 1200)
 # pragma once
 #endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
 
 #include <boost/asio/detail/config.hpp>
 #include <cstddef>
 #include <boost/asio/async_result.hpp>
 #include <boost/asio/completion_condition.hpp>
 #include <boost/asio/detail/cstdint.hpp>
 #include <boost/asio/error.hpp>
 
 #if !defined(BOOST_ASIO_NO_EXTENSIONS)
 # include <boost/asio/basic_streambuf_fwd.hpp>
 #endif // !defined(BOOST_ASIO_NO_EXTENSIONS)
 
 #include <boost/asio/detail/push_options.hpp>
 
 namespace boost {
 namespace asio {
 namespace detail {
 
 template <typename> class initiate_async_write_at;
 #if !defined(BOOST_ASIO_NO_IOSTREAM)
 template <typename> class initiate_async_write_at_streambuf;
 #endif // !defined(BOOST_ASIO_NO_IOSTREAM)
 
 } // namespace detail
 
 /**
  * @defgroup write_at boost::asio::write_at
  *
  * @brief The @c write_at function is a composed operation that writes a
  * certain amount of data at a specified offset before returning.
  */
 /*@{*/
 
 /// Write all of the supplied data at the specified offset before returning.
 /**
  * This function is used to write a certain number of bytes of data to a random
  * access device at a specified offset. The call will block until one of the
  * following conditions is true:
  *
  * @li All of the data in the supplied buffers has been written. That is, the
  * bytes transferred is equal to the sum of the buffer sizes.
  *
  * @li An error occurred.
  *
  * This operation is implemented in terms of zero or more calls to the device's
  * write_some_at function.
  *
  * @param d The device to which the data is to be written. The type must support
  * the SyncRandomAccessWriteDevice concept.
  *
  * @param offset The offset at which the data will be written.
  *
  * @param buffers One or more buffers containing the data to be written. The sum
  * of the buffer sizes indicates the maximum number of bytes to write to the
  * device.
  *
  * @returns The number of bytes transferred.
  *
  * @throws boost::system::system_error Thrown on failure.
  *
  * @par Example
  * To write a single data buffer use the @ref buffer function as follows:
  * @code boost::asio::write_at(d, 42, boost::asio::buffer(data, size)); @endcode
  * See the @ref buffer documentation for information on writing multiple
  * buffers in one go, and how to use it with arrays, boost::array or
  * std::vector.
  *
  * @note This overload is equivalent to calling:
  * @code boost::asio::write_at(
  *     d, offset, buffers,
  *     boost::asio::transfer_all()); @endcode
  */
 template <typename SyncRandomAccessWriteDevice, typename ConstBufferSequence>
 std::size_t write_at(SyncRandomAccessWriteDevice& d,
     uint64_t offset, const ConstBufferSequence& buffers);
 
 /// Write all of the supplied data at the specified offset before returning.
 /**
  * This function is used to write a certain number of bytes of data to a random
  * access device at a specified offset. The call will block until one of the
  * following conditions is true:
  *
  * @li All of the data in the supplied buffers has been written. That is, the
  * bytes transferred is equal to the sum of the buffer sizes.
  *
  * @li An error occurred.
  *
  * This operation is implemented in terms of zero or more calls to the device's
  * write_some_at function.
  *
  * @param d The device to which the data is to be written. The type must support
  * the SyncRandomAccessWriteDevice concept.
  *
  * @param offset The offset at which the data will be written.
  *
  * @param buffers One or more buffers containing the data to be written. The sum
  * of the buffer sizes indicates the maximum number of bytes to write to the
  * device.
  *
  * @param ec Set to indicate what error occurred, if any.
  *
  * @returns The number of bytes transferred.
  *
  * @par Example
  * To write a single data buffer use the @ref buffer function as follows:
  * @code boost::asio::write_at(d, 42,
  *     boost::asio::buffer(data, size), ec); @endcode
  * See the @ref buffer documentation for information on writing multiple
  * buffers in one go, and how to use it with arrays, boost::array or
  * std::vector.
  *
  * @note This overload is equivalent to calling:
  * @code boost::asio::write_at(
  *     d, offset, buffers,
  *     boost::asio::transfer_all(), ec); @endcode
  */
 template <typename SyncRandomAccessWriteDevice, typename ConstBufferSequence>
 std::size_t write_at(SyncRandomAccessWriteDevice& d,
     uint64_t offset, const ConstBufferSequence& buffers,
     boost::system::error_code& ec);
 
 /// Write a certain amount of data at a specified offset before returning.
 /**
  * This function is used to write a certain number of bytes of data to a random
  * access device at a specified offset. The call will block until one of the
  * following conditions is true:
  *
  * @li All of the data in the supplied buffers has been written. That is, the
  * bytes transferred is equal to the sum of the buffer sizes.
  *
  * @li The completion_condition function object returns 0.
  *
  * This operation is implemented in terms of zero or more calls to the device's
  * write_some_at function.
  *
  * @param d The device to which the data is to be written. The type must support
  * the SyncRandomAccessWriteDevice concept.
  *
  * @param offset The offset at which the data will be written.
  *
  * @param buffers One or more buffers containing the data to be written. The sum
  * of the buffer sizes indicates the maximum number of bytes to write to the
  * device.
  *
  * @param completion_condition The function object to be called to determine
  * whether the write operation is complete. The signature of the function object
  * must be:
  * @code std::size_t completion_condition(
  *   // Result of latest write_some_at operation.
  *   const boost::system::error_code& error,
  *
  *   // Number of bytes transferred so far.
  *   std::size_t bytes_transferred
  * ); @endcode
  * A return value of 0 indicates that the write operation is complete. A
  * non-zero return value indicates the maximum number of bytes to be written on
  * the next call to the device's write_some_at function.
  *
  * @returns The number of bytes transferred.
  *
  * @throws boost::system::system_error Thrown on failure.
  *
  * @par Example
  * To write a single data buffer use the @ref buffer function as follows:
  * @code boost::asio::write_at(d, 42, boost::asio::buffer(data, size),
  *     boost::asio::transfer_at_least(32)); @endcode
  * See the @ref buffer documentation for information on writing multiple
  * buffers in one go, and how to use it with arrays, boost::array or
  * std::vector.
  */
 template <typename SyncRandomAccessWriteDevice, typename ConstBufferSequence,
     typename CompletionCondition>
 std::size_t write_at(SyncRandomAccessWriteDevice& d,
     uint64_t offset, const ConstBufferSequence& buffers,
     CompletionCondition completion_condition,
     constraint_t<
       is_completion_condition<CompletionCondition>::value
     > = 0);
 
 /// Write a certain amount of data at a specified offset before returning.
 /**
  * This function is used to write a certain number of bytes of data to a random
  * access device at a specified offset. The call will block until one of the
  * following conditions is true:
  *
  * @li All of the data in the supplied buffers has been written. That is, the
  * bytes transferred is equal to the sum of the buffer sizes.
  *
  * @li The completion_condition function object returns 0.
  *
  * This operation is implemented in terms of zero or more calls to the device's
  * write_some_at function.
  *
  * @param d The device to which the data is to be written. The type must support
  * the SyncRandomAccessWriteDevice concept.
  *
  * @param offset The offset at which the data will be written.
  *
  * @param buffers One or more buffers containing the data to be written. The sum
  * of the buffer sizes indicates the maximum number of bytes to write to the
  * device.
  *
  * @param completion_condition The function object to be called to determine
  * whether the write operation is complete. The signature of the function object
  * must be:
  * @code std::size_t completion_condition(
  *   // Result of latest write_some_at operation.
  *   const boost::system::error_code& error,
  *
  *   // Number of bytes transferred so far.
  *   std::size_t bytes_transferred
  * ); @endcode
  * A return value of 0 indicates that the write operation is complete. A
  * non-zero return value indicates the maximum number of bytes to be written on
  * the next call to the device's write_some_at function.
  *
  * @param ec Set to indicate what error occurred, if any.
  *
  * @returns The number of bytes written. If an error occurs, returns the total
  * number of bytes successfully transferred prior to the error.
  */
 template <typename SyncRandomAccessWriteDevice, typename ConstBufferSequence,
     typename CompletionCondition>
 std::size_t write_at(SyncRandomAccessWriteDevice& d,
     uint64_t offset, const ConstBufferSequence& buffers,
     CompletionCondition completion_condition, boost::system::error_code& ec,
     constraint_t<
       is_completion_condition<CompletionCondition>::value
     > = 0);
 
 #if !defined(BOOST_ASIO_NO_EXTENSIONS)
 #if !defined(BOOST_ASIO_NO_IOSTREAM)
 
 /// Write all of the supplied data at the specified offset before returning.
 /**
  * This function is used to write a certain number of bytes of data to a random
  * access device at a specified offset. The call will block until one of the
  * following conditions is true:
  *
  * @li All of the data in the supplied basic_streambuf has been written.
  *
  * @li An error occurred.
  *
  * This operation is implemented in terms of zero or more calls to the device's
  * write_some_at function.
  *
  * @param d The device to which the data is to be written. The type must support
  * the SyncRandomAccessWriteDevice concept.
  *
  * @param offset The offset at which the data will be written.
  *
  * @param b The basic_streambuf object from which data will be written.
  *
  * @returns The number of bytes transferred.
  *
  * @throws boost::system::system_error Thrown on failure.
  *
  * @note This overload is equivalent to calling:
  * @code boost::asio::write_at(
  *     d, 42, b,
  *     boost::asio::transfer_all()); @endcode
  */
 template <typename SyncRandomAccessWriteDevice, typename Allocator>
 std::size_t write_at(SyncRandomAccessWriteDevice& d,
     uint64_t offset, basic_streambuf<Allocator>& b);
 
 /// Write all of the supplied data at the specified offset before returning.
 /**
  * This function is used to write a certain number of bytes of data to a random
  * access device at a specified offset. The call will block until one of the
  * following conditions is true:
  *
  * @li All of the data in the supplied basic_streambuf has been written.
  *
  * @li An error occurred.
  *
  * This operation is implemented in terms of zero or more calls to the device's
  * write_some_at function.
  *
  * @param d The device to which the data is to be written. The type must support
  * the SyncRandomAccessWriteDevice concept.
  *
  * @param offset The offset at which the data will be written.
  *
  * @param b The basic_streambuf object from which data will be written.
  *
  * @param ec Set to indicate what error occurred, if any.
  *
  * @returns The number of bytes transferred.
  *
  * @note This overload is equivalent to calling:
  * @code boost::asio::write_at(
  *     d, 42, b,
  *     boost::asio::transfer_all(), ec); @endcode
  */
 template <typename SyncRandomAccessWriteDevice, typename Allocator>
 std::size_t write_at(SyncRandomAccessWriteDevice& d,
     uint64_t offset, basic_streambuf<Allocator>& b,
     boost::system::error_code& ec);
 
 /// Write a certain amount of data at a specified offset before returning.
 /**
  * This function is used to write a certain number of bytes of data to a random
  * access device at a specified offset. The call will block until one of the
  * following conditions is true:
  *
  * @li All of the data in the supplied basic_streambuf has been written.
  *
  * @li The completion_condition function object returns 0.
  *
  * This operation is implemented in terms of zero or more calls to the device's
  * write_some_at function.
  *
  * @param d The device to which the data is to be written. The type must support
  * the SyncRandomAccessWriteDevice concept.
  *
  * @param offset The offset at which the data will be written.
  *
  * @param b The basic_streambuf object from which data will be written.
  *
  * @param completion_condition The function object to be called to determine
  * whether the write operation is complete. The signature of the function object
  * must be:
  * @code std::size_t completion_condition(
  *   // Result of latest write_some_at operation.
  *   const boost::system::error_code& error,
  *
  *   // Number of bytes transferred so far.
  *   std::size_t bytes_transferred
  * ); @endcode
  * A return value of 0 indicates that the write operation is complete. A
  * non-zero return value indicates the maximum number of bytes to be written on
  * the next call to the device's write_some_at function.
  *
  * @returns The number of bytes transferred.
  *
  * @throws boost::system::system_error Thrown on failure.
  */
 template <typename SyncRandomAccessWriteDevice, typename Allocator,
     typename CompletionCondition>
 std::size_t write_at(SyncRandomAccessWriteDevice& d, uint64_t offset,
     basic_streambuf<Allocator>& b, CompletionCondition completion_condition,
     constraint_t<
       is_completion_condition<CompletionCondition>::value
     > = 0);
 
 /// Write a certain amount of data at a specified offset before returning.
 /**
  * This function is used to write a certain number of bytes of data to a random
  * access device at a specified offset. The call will block until one of the
  * following conditions is true:
  *
  * @li All of the data in the supplied basic_streambuf has been written.
  *
  * @li The completion_condition function object returns 0.
  *
  * This operation is implemented in terms of zero or more calls to the device's
  * write_some_at function.
  *
  * @param d The device to which the data is to be written. The type must support
  * the SyncRandomAccessWriteDevice concept.
  *
  * @param offset The offset at which the data will be written.
  *
  * @param b The basic_streambuf object from which data will be written.
  *
  * @param completion_condition The function object to be called to determine
  * whether the write operation is complete. The signature of the function object
  * must be:
  * @code std::size_t completion_condition(
  *   // Result of latest write_some_at operation.
  *   const boost::system::error_code& error,
  *
  *   // Number of bytes transferred so far.
  *   std::size_t bytes_transferred
  * ); @endcode
  * A return value of 0 indicates that the write operation is complete. A
  * non-zero return value indicates the maximum number of bytes to be written on
  * the next call to the device's write_some_at function.
  *
  * @param ec Set to indicate what error occurred, if any.
  *
  * @returns The number of bytes written. If an error occurs, returns the total
  * number of bytes successfully transferred prior to the error.
  */
 template <typename SyncRandomAccessWriteDevice, typename Allocator,
     typename CompletionCondition>
 std::size_t write_at(SyncRandomAccessWriteDevice& d,
     uint64_t offset, basic_streambuf<Allocator>& b,
     CompletionCondition completion_condition, boost::system::error_code& ec,
     constraint_t<
       is_completion_condition<CompletionCondition>::value
     > = 0);
 
 #endif // !defined(BOOST_ASIO_NO_IOSTREAM)
 #endif // !defined(BOOST_ASIO_NO_EXTENSIONS)
 
 /*@}*/
 /**
  * @defgroup async_write_at boost::asio::async_write_at
  *
  * @brief The @c async_write_at function is a composed asynchronous operation
  * that writes a certain amount of data at the specified offset before
  * completion.
  */
 /*@{*/
 
 /// Start an asynchronous operation to write all of the supplied data at the
 /// specified offset.
 /**
  * This function is used to asynchronously write a certain number of bytes of
  * data to a random access device at a specified offset. It is an initiating
  * function for an @ref asynchronous_operation, and always returns immediately.
  * The asynchronous operation will continue until one of the following
  * conditions is true:
  *
  * @li All of the data in the supplied buffers has been written. That is, the
  * bytes transferred is equal to the sum of the buffer sizes.
  *
  * @li An error occurred.
  *
  * This operation is implemented in terms of zero or more calls to the device's
  * async_write_some_at function, and is known as a <em>composed operation</em>.
  * The program must ensure that the device performs no <em>overlapping</em>
  * write operations (such as async_write_at, the device's async_write_some_at
  * function, or any other composed operations that perform writes) until this
  * operation completes. Operations are overlapping if the regions defined by
  * their offsets, and the numbers of bytes to write, intersect.
  *
  * @param d The device to which the data is to be written. The type must support
  * the AsyncRandomAccessWriteDevice concept.
  *
  * @param offset The offset at which the data will be written.
  *
  * @param buffers One or more buffers containing the data to be written.
  * Although the buffers object may be copied as necessary, ownership of the
  * underlying memory blocks is retained by the caller, which must guarantee
  * that they remain valid until the completion handler is called.
  *
  * @param token The @ref completion_token that will be used to produce a
  * completion handler, which will be called when the write completes.
  * Potential completion tokens include @ref use_future, @ref use_awaitable,
  * @ref yield_context, or a function object with the correct completion
  * signature. The function signature of the completion handler must be:
  * @code void handler(
  *   // Result of operation.
  *   const boost::system::error_code& error,
  *
  *   // Number of bytes written from the buffers. If an error
  *   // occurred, this will be less than the sum of the buffer sizes.
  *   std::size_t bytes_transferred
  * ); @endcode
  * Regardless of whether the asynchronous operation completes immediately or
  * not, the completion handler will not be invoked from within this function.
  * On immediate completion, invocation of the handler will be performed in a
  * manner equivalent to using boost::asio::async_immediate().
  *
  * @par Completion Signature
  * @code void(boost::system::error_code, std::size_t) @endcode
  *
  * @par Example
  * To write a single data buffer use the @ref buffer function as follows:
  * @code
  * boost::asio::async_write_at(d, 42, boost::asio::buffer(data, size), handler);
  * @endcode
  * See the @ref buffer documentation for information on writing multiple
  * buffers in one go, and how to use it with arrays, boost::array or
  * std::vector.
  *
  * @par Per-Operation Cancellation
  * This asynchronous operation supports cancellation for the following
  * boost::asio::cancellation_type values:
  *
  * @li @c cancellation_type::terminal
  *
  * @li @c cancellation_type::partial
  *
  * if they are also supported by the @c AsyncRandomAccessWriteDevice type's
  * async_write_some_at operation.
  */
 template <typename AsyncRandomAccessWriteDevice, typename ConstBufferSequence,
     BOOST_ASIO_COMPLETION_TOKEN_FOR(void (boost::system::error_code,
       std::size_t)) WriteToken = default_completion_token_t<
         typename AsyncRandomAccessWriteDevice::executor_type>>
 inline auto async_write_at(AsyncRandomAccessWriteDevice& d,
     uint64_t offset, const ConstBufferSequence& buffers,
     WriteToken&& token = default_completion_token_t<
       typename AsyncRandomAccessWriteDevice::executor_type>(),
     constraint_t<
       !is_completion_condition<WriteToken>::value
     > = 0)
   -> decltype(
     async_initiate<WriteToken,
       void (boost::system::error_code, std::size_t)>(
         declval<detail::initiate_async_write_at<
           AsyncRandomAccessWriteDevice>>(),
         token, offset, buffers, transfer_all()))
 {
   return async_initiate<WriteToken,
     void (boost::system::error_code, std::size_t)>(
       detail::initiate_async_write_at<AsyncRandomAccessWriteDevice>(d),
       token, offset, buffers, transfer_all());
 }
 
 /// Start an asynchronous operation to write a certain amount of data at the
 /// specified offset.
 /**
  * This function is used to asynchronously write a certain number of bytes of
  * data to a random access device at a specified offset. It is an initiating
  * function for an @ref asynchronous_operation, and always returns immediately.
  * The asynchronous operation will continue until one of the following
  * conditions is true:
  *
  * @li All of the data in the supplied buffers has been written. That is, the
  * bytes transferred is equal to the sum of the buffer sizes.
  *
  * @li The completion_condition function object returns 0.
  *
  * This operation is implemented in terms of zero or more calls to the device's
  * async_write_some_at function, and is known as a <em>composed operation</em>.
  * The program must ensure that the device performs no <em>overlapping</em>
  * write operations (such as async_write_at, the device's async_write_some_at
  * function, or any other composed operations that perform writes) until this
  * operation completes. Operations are overlapping if the regions defined by
  * their offsets, and the numbers of bytes to write, intersect.
  *
  * @param d The device to which the data is to be written. The type must support
  * the AsyncRandomAccessWriteDevice concept.
  *
  * @param offset The offset at which the data will be written.
  *
  * @param buffers One or more buffers containing the data to be written.
  * Although the buffers object may be copied as necessary, ownership of the
  * underlying memory blocks is retained by the caller, which must guarantee
  * that they remain valid until the completion handler is called.
  *
  * @param completion_condition The function object to be called to determine
  * whether the write operation is complete. The signature of the function object
  * must be:
  * @code std::size_t completion_condition(
  *   // Result of latest async_write_some_at operation.
  *   const boost::system::error_code& error,
  *
  *   // Number of bytes transferred so far.
  *   std::size_t bytes_transferred
  * ); @endcode
  * A return value of 0 indicates that the write operation is complete. A
  * non-zero return value indicates the maximum number of bytes to be written on
  * the next call to the device's async_write_some_at function.
  *
  * @param token The @ref completion_token that will be used to produce a
  * completion handler, which will be called when the write completes.
  * Potential completion tokens include @ref use_future, @ref use_awaitable,
  * @ref yield_context, or a function object with the correct completion
  * signature. The function signature of the completion handler must be:
  * @code void handler(
  *   // Result of operation.
  *   const boost::system::error_code& error,
  *
  *   // Number of bytes written from the buffers. If an error
  *   // occurred, this will be less than the sum of the buffer sizes.
  *   std::size_t bytes_transferred
  * ); @endcode
  * Regardless of whether the asynchronous operation completes immediately or
  * not, the completion handler will not be invoked from within this function.
  * On immediate completion, invocation of the handler will be performed in a
  * manner equivalent to using boost::asio::async_immediate().
  *
  * @par Completion Signature
  * @code void(boost::system::error_code, std::size_t) @endcode
  *
  * @par Example
  * To write a single data buffer use the @ref buffer function as follows:
  * @code boost::asio::async_write_at(d, 42,
  *     boost::asio::buffer(data, size),
  *     boost::asio::transfer_at_least(32),
  *     handler); @endcode
  * See the @ref buffer documentation for information on writing multiple
  * buffers in one go, and how to use it with arrays, boost::array or
  * std::vector.
  *
  * @par Per-Operation Cancellation
  * This asynchronous operation supports cancellation for the following
  * boost::asio::cancellation_type values:
  *
  * @li @c cancellation_type::terminal
  *
  * @li @c cancellation_type::partial
  *
  * if they are also supported by the @c AsyncRandomAccessWriteDevice type's
  * async_write_some_at operation.
  */
 template <typename AsyncRandomAccessWriteDevice,
     typename ConstBufferSequence, typename CompletionCondition,
     BOOST_ASIO_COMPLETION_TOKEN_FOR(void (boost::system::error_code,
       std::size_t)) WriteToken = default_completion_token_t<
         typename AsyncRandomAccessWriteDevice::executor_type>>
 inline auto async_write_at(AsyncRandomAccessWriteDevice& d,
     uint64_t offset, const ConstBufferSequence& buffers,
     CompletionCondition completion_condition,
     WriteToken&& token = default_completion_token_t<
       typename AsyncRandomAccessWriteDevice::executor_type>(),
     constraint_t<
       is_completion_condition<CompletionCondition>::value
     > = 0)
   -> decltype(
     async_initiate<WriteToken,
       void (boost::system::error_code, std::size_t)>(
         declval<detail::initiate_async_write_at<
           AsyncRandomAccessWriteDevice>>(),
         token, offset, buffers,
         static_cast<CompletionCondition&&>(completion_condition)))
 {
   return async_initiate<WriteToken,
     void (boost::system::error_code, std::size_t)>(
       detail::initiate_async_write_at<AsyncRandomAccessWriteDevice>(d),
       token, offset, buffers,
       static_cast<CompletionCondition&&>(completion_condition));
 }
 
 #if !defined(BOOST_ASIO_NO_EXTENSIONS)
 #if !defined(BOOST_ASIO_NO_IOSTREAM)
 
 /// Start an asynchronous operation to write all of the supplied data at the
 /// specified offset.
 /**
  * This function is used to asynchronously write a certain number of bytes of
  * data to a random access device at a specified offset. It is an initiating
  * function for an @ref asynchronous_operation, and always returns immediately.
  * The asynchronous operation will continue until one of the following
  * conditions is true:
  *
  * @li All of the data in the supplied basic_streambuf has been written.
  *
  * @li An error occurred.
  *
  * This operation is implemented in terms of zero or more calls to the device's
  * async_write_some_at function, and is known as a <em>composed operation</em>.
  * The program must ensure that the device performs no <em>overlapping</em>
  * write operations (such as async_write_at, the device's async_write_some_at
  * function, or any other composed operations that perform writes) until this
  * operation completes. Operations are overlapping if the regions defined by
  * their offsets, and the numbers of bytes to write, intersect.
  *
  * @param d The device to which the data is to be written. The type must support
  * the AsyncRandomAccessWriteDevice concept.
  *
  * @param offset The offset at which the data will be written.
  *
  * @param b A basic_streambuf object from which data will be written. Ownership
  * of the streambuf is retained by the caller, which must guarantee that it
  * remains valid until the completion handler is called.
  *
  * @param token The @ref completion_token that will be used to produce a
  * completion handler, which will be called when the write completes.
  * Potential completion tokens include @ref use_future, @ref use_awaitable,
  * @ref yield_context, or a function object with the correct completion
  * signature. The function signature of the completion handler must be:
  * @code void handler(
  *   // Result of operation.
  *   const boost::system::error_code& error,
  *
  *   // Number of bytes written from the buffers. If an error
  *   // occurred, this will be less than the sum of the buffer sizes.
  *   std::size_t bytes_transferred
  * ); @endcode
  * Regardless of whether the asynchronous operation completes immediately or
  * not, the completion handler will not be invoked from within this function.
  * On immediate completion, invocation of the handler will be performed in a
  * manner equivalent to using boost::asio::async_immediate().
  *
  * @par Completion Signature
  * @code void(boost::system::error_code, std::size_t) @endcode
  *
  * @par Per-Operation Cancellation
  * This asynchronous operation supports cancellation for the following
  * boost::asio::cancellation_type values:
  *
  * @li @c cancellation_type::terminal
  *
  * @li @c cancellation_type::partial
  *
  * if they are also supported by the @c AsyncRandomAccessWriteDevice type's
  * async_write_some_at operation.
  */
 template <typename AsyncRandomAccessWriteDevice, typename Allocator,
     BOOST_ASIO_COMPLETION_TOKEN_FOR(void (boost::system::error_code,
       std::size_t)) WriteToken = default_completion_token_t<
         typename AsyncRandomAccessWriteDevice::executor_type>>
 inline auto async_write_at(AsyncRandomAccessWriteDevice& d,
     uint64_t offset, basic_streambuf<Allocator>& b,
     WriteToken&& token = default_completion_token_t<
       typename AsyncRandomAccessWriteDevice::executor_type>(),
     constraint_t<
       !is_completion_condition<WriteToken>::value
     > = 0)
   -> decltype(
     async_initiate<WriteToken,
       void (boost::system::error_code, std::size_t)>(
         declval<detail::initiate_async_write_at_streambuf<
           AsyncRandomAccessWriteDevice>>(),
         token, offset, &b, transfer_all()))
 {
   return async_initiate<WriteToken,
     void (boost::system::error_code, std::size_t)>(
       detail::initiate_async_write_at_streambuf<
         AsyncRandomAccessWriteDevice>(d),
       token, offset, &b, transfer_all());
 }
 
 /// Start an asynchronous operation to write a certain amount of data at the
 /// specified offset.
 /**
  * This function is used to asynchronously write a certain number of bytes of
  * data to a random access device at a specified offset. It is an initiating
  * function for an @ref asynchronous_operation, and always returns immediately.
  * The asynchronous operation will continue until one of the following
  * conditions is true:
  *
  * @li All of the data in the supplied basic_streambuf has been written.
  *
  * @li The completion_condition function object returns 0.
  *
  * This operation is implemented in terms of zero or more calls to the device's
  * async_write_some_at function, and is known as a <em>composed operation</em>.
  * The program must ensure that the device performs no <em>overlapping</em>
  * write operations (such as async_write_at, the device's async_write_some_at
  * function, or any other composed operations that perform writes) until this
  * operation completes. Operations are overlapping if the regions defined by
  * their offsets, and the numbers of bytes to write, intersect.
  *
  * @param d The device to which the data is to be written. The type must support
  * the AsyncRandomAccessWriteDevice concept.
  *
  * @param offset The offset at which the data will be written.
  *
  * @param b A basic_streambuf object from which data will be written. Ownership
  * of the streambuf is retained by the caller, which must guarantee that it
  * remains valid until the completion handler is called.
  *
  * @param completion_condition The function object to be called to determine
  * whether the write operation is complete. The signature of the function object
  * must be:
  * @code std::size_t completion_condition(
  *   // Result of latest async_write_some_at operation.
  *   const boost::system::error_code& error,
  *
  *   // Number of bytes transferred so far.
  *   std::size_t bytes_transferred
  * ); @endcode
  * A return value of 0 indicates that the write operation is complete. A
  * non-zero return value indicates the maximum number of bytes to be written on
  * the next call to the device's async_write_some_at function.
  *
  * @param token The @ref completion_token that will be used to produce a
  * completion handler, which will be called when the write completes.
  * Potential completion tokens include @ref use_future, @ref use_awaitable,
  * @ref yield_context, or a function object with the correct completion
  * signature. The function signature of the completion handler must be:
  * @code void handler(
  *   // Result of operation.
  *   const boost::system::error_code& error,
  *
  *   // Number of bytes written from the buffers. If an error
  *   // occurred, this will be less than the sum of the buffer sizes.
  *   std::size_t bytes_transferred
  * ); @endcode
  * Regardless of whether the asynchronous operation completes immediately or
  * not, the completion handler will not be invoked from within this function.
  * On immediate completion, invocation of the handler will be performed in a
  * manner equivalent to using boost::asio::async_immediate().
  *
  * @par Completion Signature
  * @code void(boost::system::error_code, std::size_t) @endcode
  *
  * @par Per-Operation Cancellation
  * This asynchronous operation supports cancellation for the following
  * boost::asio::cancellation_type values:
  *
  * @li @c cancellation_type::terminal
  *
  * @li @c cancellation_type::partial
  *
  * if they are also supported by the @c AsyncRandomAccessWriteDevice type's
  * async_write_some_at operation.
  */
 template <typename AsyncRandomAccessWriteDevice,
     typename Allocator, typename CompletionCondition,
     BOOST_ASIO_COMPLETION_TOKEN_FOR(void (boost::system::error_code,
       std::size_t)) WriteToken = default_completion_token_t<
         typename AsyncRandomAccessWriteDevice::executor_type>>
 inline auto async_write_at(AsyncRandomAccessWriteDevice& d, uint64_t offset,
     basic_streambuf<Allocator>& b, CompletionCondition completion_condition,
     WriteToken&& token = default_completion_token_t<
       typename AsyncRandomAccessWriteDevice::executor_type>(),
     constraint_t<
       is_completion_condition<CompletionCondition>::value
     > = 0)
   -> decltype(
     async_initiate<WriteToken,
       void (boost::system::error_code, std::size_t)>(
         declval<detail::initiate_async_write_at_streambuf<
           AsyncRandomAccessWriteDevice>>(),
         token, offset, &b,
         static_cast<CompletionCondition&&>(completion_condition)))
 {
   return async_initiate<WriteToken,
     void (boost::system::error_code, std::size_t)>(
       detail::initiate_async_write_at_streambuf<
         AsyncRandomAccessWriteDevice>(d),
       token, offset, &b,
       static_cast<CompletionCondition&&>(completion_condition));
 }
 
 #endif // !defined(BOOST_ASIO_NO_IOSTREAM)
 #endif // !defined(BOOST_ASIO_NO_EXTENSIONS)
 
 /*@}*/
 
 } // namespace asio
 } // namespace boost
 
 #include <boost/asio/detail/pop_options.hpp>
 
 #include <boost/asio/impl/write_at.hpp>
 
 #endif // BOOST_ASIO_WRITE_AT_HPP

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