...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
Start an asynchronous operation to read data into a dynamic buffer sequence until it contains a specified delimiter.
template< typename AsyncReadStream, typename DynamicBuffer_v1, typename ReadHandler = DEFAULT> DEDUCED async_read_until( AsyncReadStream & s, DynamicBuffer_v1 && buffers, string_view delim, ReadHandler && handler = DEFAULT, typename constraint< is_dynamic_buffer_v1< typename decay< DynamicBuffer_v1 >::type >::value >::type = 0, typename constraint< !is_dynamic_buffer_v2< typename decay< DynamicBuffer_v1 >::type >::value >::type = 0);
This function is used to asynchronously read data into the specified dynamic buffer sequence until the dynamic buffer sequence's get area contains the specified delimiter. The function call always returns immediately. The asynchronous operation will continue until one of the following conditions is true:
This operation is implemented in terms of zero or more calls to the stream's async_read_some function, and is known as a composed operation. If the dynamic buffer sequence's get area already contains the delimiter, this asynchronous operation completes immediately. The program must ensure that the stream performs no other read operations (such as async_read, async_read_until, the stream's async_read_some function, or any other composed operations that perform reads) until this operation completes.
The stream from which the data is to be read. The type must support the AsyncReadStream concept.
The dynamic buffer sequence into which the data will be read. Although the buffers object may be copied as necessary, ownership of the underlying memory blocks is retained by the caller, which must guarantee that they remain valid until the handler is called.
The delimiter string.
The handler to be called when the read operation completes. Copies will be made of the handler as required. The function signature of the handler must be:
void handler( // Result of operation. const boost::system::error_code& error, // The number of bytes in the dynamic buffer sequence's // get area up to and including the delimiter. // 0 if an error occurred. std::size_t bytes_transferred );
Regardless of whether the asynchronous operation completes immediately
or not, the handler will not be invoked from within this function.
On immediate completion, invocation of the handler will be performed
in a manner equivalent to using post
.
After a successful async_read_until operation, the dynamic buffer sequence may contain additional data beyond the delimiter. An application will typically leave that data in the dynamic buffer sequence for a subsequent async_read_until operation to examine.
To asynchronously read data into a std::string
until a CR-LF
sequence is encountered:
std::string data; ... void handler(const boost::system::error_code& e, std::size_t size) { if (!e) { std::string line = data.substr(0, n); data.erase(0, n); ... } } ... boost::asio::async_read_until(s, data, "\r\n", handler);
After the async_read_until
operation completes successfully,
the string data
contains the delimiter:
{ 'a', 'b', ..., 'c', '\r', '\n', 'd', 'e', ... }
The call to substr
then extracts the data up to and including
the delimiter, so that the string line
contains:
{ 'a', 'b', ..., 'c', '\r', '\n' }
After the call to erase
, the remaining data is left in the
string data
as follows:
{ 'd', 'e', ... }
This data may be the start of a new line, to be extracted by a subsequent
async_read_until
operation.
This asynchronous operation supports cancellation for the following cancellation_type
values:
cancellation_type::terminal
cancellation_type::partial
if they are also supported by the AsyncReadStream
type's
async_read_some
operation.