Puzzle about pure visual functions in UHD

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
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:

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

To understand what is happening, you will need to read up a bit on C++
abstract base classes.