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Start an asynchronous operation to read a certain amount of data from a stream.
template<
typename AsyncReadStream,
typename Allocator,
typename ReadHandler>
DEDUCED async_read(
AsyncReadStream & s,
basic_streambuf< Allocator > & b,
ReadHandler && handler);
This function is used to asynchronously read a certain number of bytes of data from a stream. 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. The program must ensure that the stream performs no other read operations (such as async_read, 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.
A basic_streambuf
object into which the data will be read. Ownership of the streambuf
is retained by the caller, which must guarantee that it remains valid
until the handler is called.
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(
const asio::error_code& error, // Result of operation.
std::size_t bytes_transferred // Number of bytes copied into the
// buffers. If an error occurred,
// this will be the number of
// bytes successfully transferred
// prior to the error.
);
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 asio::io_context::post().
This overload is equivalent to calling:
asio::async_read(
s, b,
asio::transfer_all(),
handler);