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#### http::async_read_some

Read part of a message asynchronously from a stream using a parser.

##### Synopsis

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

    template<
        class AsyncReadStream,
        class DynamicBuffer,
        bool isRequest,
        class ReadHandler = net::default_completion_token_t<            executor_type<AsyncReadStream>>>
    DEDUCED
    async_read_some(
        AsyncReadStream& stream,
        DynamicBuffer& buffer,
        basic_parser< isRequest >& parser,
        ReadHandler&& handler = net::default_completion_token_t< executor_type< AsyncReadStream > >{});

##### Description

This function is used to asynchronously read part of a message from a stream into an instance of `basic_parser`. The function call always returns immediately. The asynchronous operation will continue until one of the following conditions is true:

- A call to `basic_parser::put` with a non-empty buffer sequence is successful.
- An error occurs.

This operation is implemented in terms of zero or more calls to the next layer'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 bytes from the stream that lie past the end of the message being read. These additional bytes are stored in the dynamic buffer, which must be preserved for subsequent reads.

If the end of file error is received while reading from the stream, then the error returned from this function will be:

- `error::end_of_stream` if no bytes were parsed, or
- `error::partial_message` if any bytes were parsed but the message was incomplete, otherwise:
- A successful result. The next attempt to read will return `error::end_of_stream`

##### Parameters

Name | Description
--- | ---
`stream` | The stream from which the data is to be read. The type must meet the AsyncReadStream requirements.
`buffer` | Storage for additional bytes read by the implementation from the stream. This is both an input and an output parameter; on entry, the parser will be presented with any remaining data in the dynamic buffer's readable bytes sequence first. The type must meet the DynamicBuffer requirements. The object must remain valid at least until the handler is called; ownership is not transferred.
`parser` | The parser to use. The object must remain valid at least until the handler is called; ownership is not transferred.
`handler` | The completion handler to invoke when the operation completes. The implementation takes ownership of the handler by performing a decay-copy. 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 consumed by the parser
    );
If the handler has an associated immediate executor, an immediate completion will be dispatched to it. Otherwise, the handler will not be invoked from within this function. Invocation of the handler will be performed in a manner equivalent to using `net::post`.

##### Per-Operation Cancellation

This asynchronous operation supports cancellation for the following net::cancellation_type values:

- `net::cancellation_type::terminal`

if the `stream` also supports terminal cancellation, `terminal` cancellation leaves the stream in an undefined state, so that only closing it is guaranteed to succeed.