[Discuss-gnuradio] Puzzle about pure visual functions in UHD

[baobaonanpo D]

Dear madam/sir,
  It is my great honor to appreciate your works about GNURadio and USRP.
  
  When I try to use the UHD driver,when I read the source code I have puzzle
 about the use of the pure visual function for examples send(), 

    virtual size_t send(
        const send_buffs_type &buffs,
        size_t nsamps_per_buff,
        const tx_metadata_t &metadata,
        const io_type_t &io_type,
        send_mode_t send_mode,

        double timeout = 0.1
    ) = 0;

The URL: http://files.ettus.com/uhd_docs/doxygen/html/classuhd_1_1tx__streamer.html#aeb2e0f44810693d9da99ea1e04fad21f

  The function send is a pure virtual function and I can not find it is to be overridden anywhere else so I can not understand how it can to be 
called by other functions and send the message.
  
For example:

            //send a single packet
            size_t num_tx_samps = tx_stream->send(
                buffs, samps_to_send, md, timeout
            );
from the programe tx_burst.cpp

 
  I use the USRP2 and do not burn the SD card so can not use debug tools to see how it works.

  I am so appreciate if you can give me a hand!
  
Best Regards!

Sent by:Ding Yuzhen from Beijing,China