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#### basic_stream

A stream socket wrapper with timeouts, an executor, and a rate limit policy.

##### Synopsis

Defined in header `<boost/beast/core/basic_stream.hpp>`

    template<
        class Protocol,
        class Executor = net::any_io_executor,
        class RatePolicy = unlimited_rate_policy>
    class basic_stream

##### Types

Name | Description
--- | ---
endpoint_type | The endpoint type.
executor_type | The type of the executor associated with the stream.
protocol_type | The protocol type.
rebind_executor | Rebinds the stream type to another executor.
socket_type | The type of the underlying socket.

##### Member Functions

Name | Description
--- | ---
async_connect | Connect the stream to the specified endpoint asynchronously. — Establishes a connection by trying each endpoint in a sequence asynchronously.
async_read_some | Read some data asynchronously.
async_write_some | Write some data asynchronously.
basic_stream [constructor] | Constructor. — Move constructor.
cancel | Cancel all asynchronous operations associated with the socket.
close | Close the timed stream.
connect | Connect the stream to the specified endpoint. — Establishes a connection by trying each endpoint in a sequence.
expires_after | Set the timeout for subsequent logical operations.
expires_at | Set the timeout for subsequent logical operations.
expires_never | Disable the timeout for subsequent logical operations.
get_executor | Get the executor associated with the object.
operator= | Move assignment (deleted).
rate_policy | Returns the rate policy associated with the object.
read_some | Read some data.
release_socket | Release ownership of the underlying socket.
socket | Return a reference to the underlying socket.
write_some | Write some data.
~basic_stream [destructor] | Destructor.

##### Description

This stream wraps a `net::basic_stream_socket` to provide the following features:

- An Executor may be associated with the stream, which will be used to invoke any completion handlers which do not already have an associated executor. This achieves support for [P1322R0] Networking TS enhancement to enable custom I/O executors.
- Timeouts may be specified for each logical asynchronous operation performing any reading, writing, or connecting.
- A RatePolicy may be associated with the stream, to implement rate limiting through the policy's interface.

Although the stream supports multiple concurrent outstanding asynchronous operations, the stream object is not thread-safe. The caller is responsible for ensuring that the stream is accessed from only one thread at a time. This includes the times when the stream, and its underlying socket, are accessed by the networking implementation. To meet this thread safety requirement, all asynchronous operations must be performed by the stream within the same implicit strand (only one thread `net::io_context::run`) or within the same explicit strand, such as an instance of `net::strand`.

Completion handlers with explicit associated executors (such as those arising from use of `net::bind_executor`) will be invoked by the stream using the associated executor. Otherwise, the completion handler will be invoked by the executor associated with the stream upon construction. The type of executor used with this stream must meet the following requirements:

- Function objects submitted to the executor shall never run concurrently with each other.

The executor type `net::strand` meets these requirements. Use of a strand as the executor in the stream class template offers an additional notational convenience: the strand does not need to be specified in each individual initiating function call.

Unlike other stream wrappers, the underlying socket is accessed through the `socket` member function instead of `next_layer`. This causes the `basic_stream` to be returned in calls to `get_lowest_layer`.

##### Usage

To use this stream declare an instance of the class. Then, before each logical operation for which a timeout is desired, call `expires_after` with a duration, or call `expires_at` with a time point. Alternatively, call `expires_never` to disable the timeout for subsequent logical operations. A logical operation is any series of one or more direct or indirect calls to the timeout stream's asynchronous read, asynchronous write, or asynchronous connect functions.

When a timeout is set and a mixed operation is performed (one that includes both reads and writes, for example) the timeout applies to all of the intermediate asynchronous operations used in the enclosing operation. This allows timeouts to be applied to stream algorithms which were not written specifically to allow for timeouts, when those algorithms are passed a timeout stream with a timeout set.

When a timeout occurs the socket will be closed, canceling any pending I/O operations. The completion handlers for these canceled operations will be invoked with the error `beast::error::timeout`.

##### Examples

This function reads an HTTP request with a timeout, then sends the HTTP response with a different timeout.

    void process_http_1 (tcp_stream& stream, net::yield_context yield)
    {
        flat_buffer buffer;
        http::request<http::empty_body> req;

        // Read the request, with a 15 second timeout
        stream.expires_after(std::chrono::seconds(15));
        http::async_read(stream, buffer, req, yield);

        // Calculate the response
        http::response<http::string_body> res = make_response(req);

        // Send the response, with a 30 second timeout.
        stream.expires_after (std::chrono::seconds(30));
        http::async_write (stream, res, yield);
    }

The example above could be expressed using a single timeout with a simple modification. The function that follows first reads an HTTP request then sends the HTTP response, with a single timeout that applies to the entire combined operation of reading and writing:

    void process_http_2 (tcp_stream& stream, net::yield_context yield)
    {
        flat_buffer buffer;
        http::request<http::empty_body> req;

        // Require that the read and write combined take no longer than 30 seconds
        stream.expires_after(std::chrono::seconds(30));

        http::async_read(stream, buffer, req, yield);

        http::response<http::string_body> res = make_response(req);
        http::async_write (stream, res, yield);
    }

Some stream algorithms, such as `ssl::stream::async_handshake` perform both reads and writes. A timeout set before calling the initiating function of such composite stream algorithms will apply to the entire composite operation. For example, a timeout may be set on performing the SSL handshake thusly:

    void do_ssl_handshake (net::ssl::stream<tcp_stream>& stream, net::yield_context yield)
    {
        // Require that the SSL handshake take no longer than 10 seconds
        stream.expires_after(std::chrono::seconds(10));

        stream.async_handshake(net::ssl::stream_base::client, yield);
    }

##### Blocking I/O

Synchronous functions behave identically as that of the wrapped `net::basic_stream_socket`. Timeouts are not available when performing blocking calls.

##### Template Parameters

Type | Description
--- | ---
`Protocol` | A type meeting the requirements of Protocol representing the protocol the protocol to use for the basic stream socket. A common choice is `net::ip::tcp`.
`Executor` | A type meeting the requirements of Executor to be used for submitting all completion handlers which do not already have an associated executor. If this type is omitted, the default of `net::any_io_executor` will be used.

##### Thread Safety

Distinct objects: Safe.

Shared objects: Unsafe. The application must also ensure that all asynchronous operations are performed within the same implicit or explicit strand.

##### See Also

- [P1322R0] Networking TS enhancement to enable custom I/O executors.