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.\n* A body produced incrementally from coroutine output.\n* A body represented by zero or more buffers already in memory.\n* A body whose size is not known ahead of time.\n* Body data generated dynamically from other threads.\n* Body data computed algorithmically. Associated Types * [link beast.ref.boost__beast__http__is_body_writer `is_body_writer`]\n* __Body__ Requirements These requirements may undergo non-backward compatible In this table: * `W` denotes a type meeting the requirements of [*BodyWriter].\n* `B` denotes a __Body__ where * `a` denotes a value of type `W`.\n* `h` denotes a const value of type `header<isRequest, Fields> const&`.\n* `v` denotes a possibly const value of type `Body::value_type&`.\n* `ec` is a value of type [link beast.ref.boost__beast__error_code `error_code&`].\n* `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.\n\nThe constructor may optionally require that `h` and `v` are `const` references, with these consequences:\n\n* 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:\n\n* 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 * [link beast.ref.boost__beast__http__basic_dynamic_body.writer `basic_dynamic_body::writer`]\n* [link beast.ref.boost__beast__http__basic_file_body__writer `basic_file_body::writer`]\n* [link beast.ref.boost__beast__http__basic_string_body.writer `basic_string_body::writer`]\n* [link beast.ref.boost__beast__http__buffer_body.writer `buffer_body::writer`]\n* [link beast.ref.boost__beast__http__empty_body.writer `empty_body::writer`]\n* [link beast.ref.boost__beast__http__span_body.writer `span_body::writer`]\n* [link beast.ref.boost__beast__http__vector_body.writer `vector_body::writer`]