jrtplib example3程序问题

看了example1和example2程序都是可以理解的,现在再看example3,以前对C++用的较少感觉有问题不好理解

继承类MyRTPSession中的三个函数明明在main函数中没有调用,但是为什么在后台运行了呢,为什么可以自动添加AddDestination,希望看到的博友给解释一下,谢谢!

/*

   This IPv4 example listens for incoming packets and automatically adds destinations
   for new sources.
*/


#include "rtpsession.h"
#include "rtppacket.h"
#include "rtpudpv4transmitter.h"
#include "rtpipv4address.h"
#include "rtpsessionparams.h"
#include "rtperrors.h"
#ifndef WIN32
#include <netinet/in.h>
#include <arpa/inet.h>
#else
#include <winsock2.h>
#endif // WIN32
#include "rtpsourcedata.h"
#include <stdlib.h>
#include <stdio.h>
#include <iostream>
#include <string>


using namespace jrtplib;


//
// This function checks if there was a RTP error. If so, it displays an error
// message and exists.
//


void checkerror(int rtperr)
{
if (rtperr < 0)
{
std::cout << "ERROR: " << RTPGetErrorString(rtperr) << std::endl;
exit(-1);
}
}


//
// The new class routine
//


class MyRTPSession : public RTPSession
{
protected:
void OnNewSource(RTPSourceData *dat)
{
if (dat->IsOwnSSRC())
return;


uint32_t ip;
uint16_t port;

if (dat->GetRTPDataAddress() != 0)
{
const RTPIPv4Address *addr = (const RTPIPv4Address *)(dat->GetRTPDataAddress());
ip = addr->GetIP();
port = addr->GetPort();
}
else if (dat->GetRTCPDataAddress() != 0)
{
const RTPIPv4Address *addr = (const RTPIPv4Address *)(dat->GetRTCPDataAddress());
ip = addr->GetIP();
port = addr->GetPort()-1;
}
else
return;

RTPIPv4Address dest(ip,port);
AddDestination(dest);


struct in_addr inaddr;
inaddr.s_addr = htonl(ip);
std::cout << "Adding destination " << std::string(inet_ntoa(inaddr)) << ":" << port << std::endl;
}


void OnBYEPacket(RTPSourceData *dat)
{
if (dat->IsOwnSSRC())
return;

uint32_t ip;
uint16_t port;

if (dat->GetRTPDataAddress() != 0)
{
const RTPIPv4Address *addr = (const RTPIPv4Address *)(dat->GetRTPDataAddress());
ip = addr->GetIP();
port = addr->GetPort();
}
else if (dat->GetRTCPDataAddress() != 0)
{
const RTPIPv4Address *addr = (const RTPIPv4Address *)(dat->GetRTCPDataAddress());
ip = addr->GetIP();
port = addr->GetPort()-1;
}
else
return;

RTPIPv4Address dest(ip,port);
DeleteDestination(dest);

struct in_addr inaddr;
inaddr.s_addr = htonl(ip);
std::cout << "Deleting destination " << std::string(inet_ntoa(inaddr)) << ":" << port << std::endl;
}


void OnRemoveSource(RTPSourceData *dat)
{
if (dat->IsOwnSSRC())
return;
if (dat->ReceivedBYE())
return;

uint32_t ip;
uint16_t port;

if (dat->GetRTPDataAddress() != 0)
{
const RTPIPv4Address *addr = (const RTPIPv4Address *)(dat->GetRTPDataAddress());
ip = addr->GetIP();
port = addr->GetPort();
}
else if (dat->GetRTCPDataAddress() != 0)
{
const RTPIPv4Address *addr = (const RTPIPv4Address *)(dat->GetRTCPDataAddress());
ip = addr->GetIP();
port = addr->GetPort()-1;
}
else
return;

RTPIPv4Address dest(ip,port);
DeleteDestination(dest);

struct in_addr inaddr;
inaddr.s_addr = htonl(ip);
std::cout << "Deleting destination " << std::string(inet_ntoa(inaddr)) << ":" << port << std::endl;
}
};


//
// The main routine
// 


int main(void)
{
#ifdef WIN32
WSADATA dat;
WSAStartup(MAKEWORD(2,2),&dat);
#endif // WIN32

MyRTPSession sess;
uint16_t portbase;
std::string ipstr;
int status,i,num;


        // First, we'll ask for the necessary information

std::cout << "Enter local portbase:" << std::endl;
std::cin >> portbase;
std::cout << std::endl;

std::cout << std::endl;
std::cout << "Number of seconds you wish to wait:" << std::endl;
std::cin >> num;

// Now, we'll create a RTP session, set the destination
// and poll for incoming data.

RTPUDPv4TransmissionParams transparams;
RTPSessionParams sessparams;

// IMPORTANT: The local timestamp unit MUST be set, otherwise
//            RTCP Sender Report info will be calculated wrong
// In this case, we'll be just use 8000 samples per second.
sessparams.SetOwnTimestampUnit(1.0/8000.0);

sessparams.SetAcceptOwnPackets(true);
transparams.SetPortbase(portbase);
status = sess.Create(sessparams,&transparams);
checkerror(status);

for (i = 1 ; i <= num ; i++)
{
sess.BeginDataAccess();

// check incoming packets
if (sess.GotoFirstSourceWithData())
{
do
{
RTPPacket *pack;

while ((pack = sess.GetNextPacket()) != NULL)
{
// You can examine the data here
printf("Got packet !\n");

// we don't longer need the packet, so
// we'll delete it
sess.DeletePacket(pack);
}
} while (sess.GotoNextSourceWithData());
}

sess.EndDataAccess();


#ifndef RTP_SUPPORT_THREAD
status = sess.Poll();
checkerror(status);
#endif // RTP_SUPPORT_THREAD

RTPTime::Wait(RTPTime(1,0));
}

sess.BYEDestroy(RTPTime(10,0),0,0);


#ifdef WIN32
WSACleanup();
#endif // WIN32
return 0;
}

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