Flush all data from the buffer to the next layer. Returns the number of bytes written to the next layer on the last write operation. Throws an exception on failure.

std::size_t flush();
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Flush all data from the buffer to the next layer. Returns the number of bytes written to the next layer on the last write operation, or 0 if an error occurred.

std::size_t flush(
    asio::error_code & ec);
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