Boost ASIO SSL get the number of available bytes for read

From this example, the size of the buffer available for reading is 1024 bytes. However, in the boost::asio::buffer() of handle_write(), I can't put that number. The program would give an error:

Write failed: uninitialized

The program would only work if I put exactly the number of bytes sent from the server.

So I tried to use the function available() which I found defined in the include file basic_socket.hpp to tell the number of available bytes to read, but I'm not sure how to call this function. Please assist!

void handle_write(const boost::system::error_code& error, size_t bytes_transferred)
{
    if (!error)
    {
        size_t len = socket_.available();
        boost::asio::async_read(socket_,
                                boost::asio::buffer(reply_, len),
                                boost::bind(&SSLClient::handle_read, this,
                                            boost::asio::placeholders::error,
                                            boost::asio::placeholders::bytes_transferred));
    }
    else
    {
        std::cout << "Write failed: " << error.message() << "\n";
    }
}

The question is: In this example, how can I read all the bytes available?