en	BodyWriter

en	A [*BodyWriter] provides an [@https://en.wikipedia.org/wiki/Online_algorithm online algorithm] to obtain a sequence of zero or more buffers from a body during serialization. The implementation creates an instance of this type when needed, and calls into it one or more times to retrieve buffers holding body octets. The interface of [*BodyWriter] is intended to obtain buffers for these scenarios:

en	* A body that does not entirely fit in memory. * A body produced incrementally from coroutine output. * A body represented by zero or more buffers already in memory. * A body whose size is not known ahead of time. * Body data generated dynamically from other threads. * Body data computed algorithmically.

en	Associated Types

en	* [link beast.ref.boost__beast__http__is_body_writer `is_body_writer`] * __Body__

en	Requirements

en	These requirements may undergo non-backward compatible

en	In this table:

en	* `W` denotes a type meeting the requirements of [*BodyWriter]. * `B` denotes a __Body__ where

en	* `a` denotes a value of type `W`. * `h` denotes a const value of type `header<isRequest, Fields> const&`. * `v` denotes a possibly const value of type `Body::value_type&`. * `ec` is a value of type [link beast.ref.boost__beast__error_code `error_code&`]. * `W<T>` is the type `boost::optional<std::pair<T, bool>>`.

en	Valid expressions

en	Expression

en	Type

en	Semantics, Pre/Post-conditions

en	`W::const_buffers_type`

en	A type which meets the requirements of __ConstBufferSequence__. This is the type of buffer returned by `W::get`.

en	`W{h,v};`

en	Constructible from `h` and `v`. The lifetime of `h` and `v` are guaranteed to end no earlier than after the `W` is destroyed. The writer shall not access the contents of `h` or `v` before the first call to `init`, permitting lazy construction of the message. The constructor may optionally require that `h` and `v` are `const` references, with these consequences: * If `W` requires that `h` and `v` are const references, then the corresponding serializer constructors for messages with this body type will will accept a const reference to a message, otherwise: * If `W` requires that `h` and `v` are non-const references, then the corresponding serializer constructors for messages with this body type will require a non-const reference to a message.

en	`a.init(ec)`

en	Called once to fully initialize the object before any calls to `get`. The message body becomes valid before entering this function, and remains valid until the writer is destroyed. The function will ensure that `!ec` is `true` if there was no error or set to the appropriate error code if there was one.

en	`a.get(ec)`

en	`W<W::const_buffers_type>`

en	Called one or more times after `init` succeeds. This function returns `boost::none` if all buffers representing the body have been returned in previous calls or if it sets `ec` to indicate an error. Otherwise, if there are buffers remaining the function should return a pair with the first element containing a non-zero length buffer sequence representing the next set of octets in the body, while the second element is a `bool` meaning `true` if there may be additional buffers returned on a subsequent call, or `false` if the buffer returned on this call is the last buffer representing the body. The function will ensure that `!ec` is `true` if there was no error or set to the appropriate error code if there was one.

en	Exemplar

en	Models

en	* [link beast.ref.boost__beast__http__basic_dynamic_body.writer `basic_dynamic_body::writer`] * [link beast.ref.boost__beast__http__basic_file_body__writer `basic_file_body::writer`] * [link beast.ref.boost__beast__http__basic_string_body.writer `basic_string_body::writer`] * [link beast.ref.boost__beast__http__buffer_body.writer `buffer_body::writer`] * [link beast.ref.boost__beast__http__empty_body.writer `empty_body::writer`] * [link beast.ref.boost__beast__http__span_body.writer `span_body::writer`] * [link beast.ref.boost__beast__http__vector_body.writer `vector_body::writer`]