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##### http::async_read (2 of 2 overloads)

Read a complete message from a stream asynchronously.

###### Synopsis

Defined in header `<boost/beast/http/read.hpp>`

    template<
        class AsyncReadStream,
        class DynamicBuffer,
        bool isRequest,
        class Body,
        class Allocator,
        class ReadHandler>
    DEDUCED
    async_read(
        AsyncReadStream& stream,
        DynamicBuffer& buffer,
        message< isRequest, Body, basic_fields< Allocator >>& msg,
        ReadHandler&& handler);

###### Description

This function is used to asynchronously read a complete message from a stream using HTTP/1. The function call always returns immediately. The asynchronous operation will continue until one of the following conditions is true:

- The entire message is read.
- An error occurs.

This operation is implemented in terms of one or more calls to the stream's `async_read_some` function, and is known as a composed operation. The program must ensure that the stream performs no other reads until this operation completes. The implementation may read additional octets that lie past the end of the message being read. This additional data is stored in the dynamic buffer, which must be retained for subsequent reads.

If the stream returns the error `boost::asio::error::eof` indicating the end of file during a read, the error returned from this function will be:

- `http::end_of_stream` if no octets were parsed, or
- `http::partial_message` if any octets were parsed but the message was incomplete, otherwise:
- A successful result. A subsequent attempt to read will return `http::end_of_stream`

###### Parameters

Name | Description
--- | ---
`stream` | The stream from which the data is to be read. The type must support the AsyncReadStream concept.
`buffer` | A DynamicBuffer holding additional bytes read by the implementation from the stream. This is both an input and an output parameter; on entry, any data in the dynamic buffer's input sequence will be given to the parser first.
`msg` | An object in which to store the message contents. This object should not have previous contents, otherwise the behavior is undefined. The type must be MoveAssignable and MoveConstructible.

The object must remain valid at least until the handler is called; ownership is not transferred.

###### Parameters

Name | Description
--- | ---
`handler` | Invoked when the operation completes. The handler may be moved or copied as needed. The equivalent function signature of the handler must be:
     void handler(
        error_code const& error,        // result of operation,
        std::size_t bytes_transferred   // the number of bytes transferred to the parser
    );
Regardless of whether the asynchronous operation completes immediately or not, the handler will not be invoked from within this function. Invocation of the handler will be performed in a manner equivalent to using `boost::asio::io_context::post`.

Convenience header `<boost/beast/http.hpp>`