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BodyWriter
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:
* 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.
Associated Types
* [link beast.ref.boost__beast__http__is_body_writer `is_body_writer`]
* __Body__
Requirements
These requirements may undergo non-backward compatible
In this table:
* `W` denotes a type meeting the requirements of [*BodyWriter].
* `B` denotes a __Body__ where
* `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>>`.
Valid expressions
Expression
Type
Semantics, Pre/Post-conditions
`W::const_buffers_type`
A type which meets the requirements of __ConstBufferSequence__. This is the type of buffer returned by `W::get`.
`W{h,v};`
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.
`a.init(ec)`
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.
`a.get(ec)`
`W<W::const_buffers_type>`
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.
Exemplar
Models
English Actions
BodyWriter
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:
* 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.
Associated Types
* [link beast.ref.boost__beast__http__is_body_writer `is_body_writer`]
* __Body__
Requirements
These requirements may undergo non-backward compatible
In this table:
* `W` denotes a type meeting the requirements of [*BodyWriter].
* `B` denotes a __Body__ where
* `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>>`.
Valid expressions
Expression
Type
Semantics, Pre/Post-conditions
`W::const_buffers_type`
A type which meets the requirements of __ConstBufferSequence__. This is the type of buffer returned by `W::get`.
`W{h,v};`
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.
`a.init(ec)`
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.
`a.get(ec)`
`W<W::const_buffers_type>`
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.
Exemplar

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String added in the repository
* `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>>`.
* `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>>`.
06/05/2026
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String information

Component
Boost Beast Translation (zh_Hans)/Doc / Qbk / 07 Concepts / Bodywriter (qbk)
Source string description
type: list
Flags
read-only
Unit identifier
56501
Source string location
string ID 44
String age
06/05/2026
Last updated
06/05/2026
Source string age
06/05/2026
Translation file QuickBook file
doc/qbk/07_concepts/BodyWriter.qbk, string 10