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### BodyReader

A BodyReader provides an online algorithm to transfer a series of zero or more buffers containing parsed body octets into a message container. The `parser` creates an instance of this type when needed, and calls into it zero or more times to transfer buffers. The interface of BodyReader is intended to allow the conversion of buffers into these scenarios for representation:

- Storing a body in a dynamic buffer
- Storing a body in a user defined container with a custom allocator
- Transformation of incoming body data before storage, for example to compress it first.
- Saving body data to a file

##### Associated Types

- `is_body_reader`
- Body

##### Requirements

| Warning
These requirements may undergo non-backward compatible changes in subsequent versions.

In this table:

- `R` denotes a type meeting the requirements of BodyReader.
- `B` denotes a Body where `std::is_same<R, B::reader>::value == true`.
- `a` denotes a value of type `R`.
- `b` is an object whose type meets the requirements of ConstBufferSequence
- `h` denotes a value of type `header<isRequest, Fields>&`.
- `v` denotes a value of type `Body::value_type&`.
- `n` is a value of type `boost::optional<std::uint64_t>`.
- `ec` is a value of type `error_code&`.

Table 1.37. Valid expressions

Expression | Type | Semantics, Pre/Post-conditions
--- | --- | ---
`R{h,v};` |  | Constructible from `h` and `v`. The lifetime of `h` and `v` is guaranteed to end no earlier than after the `R` is destroyed. The reader shall not access the contents of `h` or `v` before the first call to `init`, permitting lazy construction of the message.
`a.init(n, ec)` |  | Called once to fully initialize the object before any calls to `put`. The message body is valid before entering this function, and remains valid until the reader is destroyed. The value of `n` will be set to the content length of the body if known, otherwise `n` will be equal to `boost::none`. Implementations of BodyReader may use this information to optimize allocation. 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.put(b,ec)` | `std::size_t` | This function is called to append some or all of the buffers specified by `b` into the body representation. The number of bytes inserted from `b` is returned. If the number of bytes inserted is less than the total input, the remainder of the input will be presented in the next call to `put`. 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.finish(ec)` |  | This function is called when no more body octets are remaining. The function will ensure that `!ec` is `true` if there was no error or set to the appropriate error code if there was one.
`is_body_reader<B>` | `std::true_type` | An alias for `std::true_type` for `B`, otherwise an alias for `std::false_type`.

##### Exemplar

    struct BodyReader
    {
        /** Construct the reader.

            @param h The header for the message being parsed

            @param body The body to store the parsed results into
        */
        template<bool isRequest, class Fields>
        BodyReader(header<isRequest, Fields>& h, value_type& body);

        /** Initialize the reader.

            This is called after construction and before the first
            call to `put`. The message is valid and complete upon
            entry.

            @param ec Set to the error, if any occurred.
        */
        void
        init(
            boost::optional<std::uint64_t> const& content_length,
            error_code& ec)
        {
            boost::ignore_unused(content_length);

            // The specification requires this to indicate "no error"
            ec = {};
        }

        /** Store buffers.

            This is called zero or more times with parsed body octets.

            @param buffers The constant buffer sequence to store.

            @param ec Set to the error, if any occurred.

            @return The number of bytes transferred from the input buffers.
        */
        template<class ConstBufferSequence>
        std::size_t
        put(ConstBufferSequence const& buffers, error_code& ec)
        {
            // The specification requires this to indicate "no error"
            ec = {};

            return buffer_bytes(buffers);
        }

        /** Called when the body is complete.

            @param ec Set to the error, if any occurred.
        */
        void
        finish(error_code& ec)
        {
            // The specification requires this to indicate "no error"
            ec = {};
        }
    };

##### Models

- `basic_dynamic_body::reader`
- `basic_file_body::reader`
- `basic_string_body::reader`
- `buffer_body::reader`
- `empty_body::reader`
- `span_body::reader`
- `vector_body::reader`