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

* `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>>`.

`W::const_buffers_type`

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.

* [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`]