问题
I have been trying to create a TCP Server model based on inheritance, with varying success. These servers are managed by a singleton whose task it is to shut these servers down and other simple maintenance functions:
class TCPServer {
public:
TCPServer();
~TCPServer();
void Bind(TCPDaemon *daemon) {
if(!daemon->IsRunning()) {
throw TCPBindException("Daemon is inactive");
}
// if the port is not taken, bind this daemon to it
if(this->servers.count(daemon->port())==0) {
this->servers[daemon->port()]=daemon;
...
} else {
throw TCPBindException("Port is taken");
}
}
void Shutdown() {
MASON::UINT16 i;
for(i=0;i<this->servers.size();i++) {
this->Shutdown((*this->servers.begin()).first);
}
}
void Shutdown(unsigned short port) {
if(this->servers.count(port)) {
if(this->servers[port]->IsRunning()) {
this->servers[port]->Stop();
}
delete this->servers[port];
this->servers.erase(port);
}
}
private:
std::map<unsigned short, TCPDaemon*> servers;
};
The TCPDaemon class's Stop() function is a pure virtual. My problem is that when the Shutdown() function is called, it is attempting to call this pure virtual instead of the derived class' version. How can I force it to do the right thing?
Thanks in advance
[edit] sorry I did not include the TCPDaemon code before, it derives from a TCPSocket class (which I have checked to be 100% working and is fairly self-explanatory). Here it is:
class TCPDaemon: public TCPSocket {
public:
TCPDaemon(unsigned short port) {
this->_enabled=false;
this->_host.ipaddr(INADDR_ANY);
this->_host.port(port);
this->paused=false;
struct sockaddr_in opts=this->_host.Compile();
#ifdef PLATFORM_WINDOWS
WSADATA wsaData;
if(WSAStartup(0x0202, &wsaData)) {
throw TCPDaemonException("Failed to start WSA");
}
#endif
this->raw_socket=socket(AF_INET, SOCK_STREAM, 0);
if(this->raw_socket<=0) {
throw TCPDaemonException("Failed to create socket");
}
if(int status=bind(this->raw_socket, (sockaddr*)&opts, sizeof(sockaddr))) {
printf("error [%i]\r\n", status);
throw TCPDaemonException("Failed to bind to port");
}
if(listen(this->raw_socket, 5)) {
throw TCPDaemonException("Failed to listen on port");
}
this->_enabled=true;
}
virtual ~TCPDaemon() {
this->Shutdown();
}
virtual void Start()=0;
virtual void Run(TCPSocket*)=0;
virtual void Stop()=0;
unsigned short port() {
return this->host().port();
}
bool IsRunning() {
return this->_enabled;
}
TCPSocket *Accept() {
SOCKET client;
struct sockaddr client_addr;
int len=sizeof(client_addr);
client=accept(this->raw_socket, &client_addr, &len);
return new TCPSocket(client, &client_addr);
}
void Shutdown() {
}
private:
bool _enabled;
bool paused;
};
and here is a sample derived server and its method of creation:
class EchoServer: public TCPDaemon {
public:
EchoServer(MASON::UINT16 port): TCPDaemon(port) {
}
~EchoServer() {}
virtual void Start() {
}
virtual void Run(TCPSocket *client) {
printf("RUN\r\n");
Accessor<TCPSocket> acc_client=client;
acc_client->Write(Accessor<Blob> (new Blob(std::string("hello!"))));
acc_client->Disconnect();
}
virtual void Stop() {
}
};
myTCPServer->Bind(new EchoServer(8008));
[edit+1] I think what the problem boils down to is this (i could easily be wrong): I have a std::map of the base class, TCPDaemon, which has a pure virtual/abstract function Stop(). It appears that when I call Stop() through one of the entries in the map, it is attempting to call TCPDaemon::Stop(), as opposed to the overriding function EchoServer::Stop(). Could this be the issue? If so, how do I resolve it?
回答1:
I'm not sure if this is the problem you're seeing, but there's certainly a problem in the following function.
void Shutdown() {
MASON::UINT16 i;
for(i=0;i<this->servers.size();i++) {
this->Shutdown((*this->servers.begin()).first);
}
}
After each loop iterator, i will increase by one AND size will decrease, so you'll only ShutDown half your servers. Maybe keeping some open is the cause of further errors.
You could do either:
void Shutdown() {
MASON::UINT16 i;
std::size_t nrServers = this->servers.size();
for(i=0;i<nrServers;i++) {
this->Shutdown((*this->servers.begin()).first);
}
}
or, which I would prefer as it better shows the intention of the code:
void Shutdown() {
while (!this->servers.empty()) {
this->Shutdown((*this->servers.begin()).first);
}
}
回答2:
Check the syntax of what you are declaring:
class TCPDaemon
{
virtual void stop() = 0;
};
class MyDaemon : public TCPDaemon
{
virtual void stop()
{
//Do stuff here.
}
};
That's the best I can do without more code.
EDIT:
Ok, so it seems like you are using abstract functions. Next question is this: what is the error you are getting? I can say for certain that it is not attempting to call Stop() from TCPDeamon. That would be impossible, as it is not even implemented.
回答3:
I finally worked it out in the end, thanks to the input I have received. The problem was with the delete call in TCPServer::Shutdown(unsigned short), which was causing a memory access violation in a completely different part of the code... A rather noobie mistake, I shall be wrapping smart pointers around everything ASAP.
Thanks for all your feedback!
回答4:
If you say that it calls pure virtual method instead of derived class' version, that means that your derived class calls it. E.g.
class t1
{
public:
virtual ~t1(){};
virtual void foo()=0
{
std::cout << "pure virtual";
};
};
class t2:public t1
{
public :
virtual void foo()
{
t1::foo();
std::cout << "derived class ";
};
};
As far as I remember only derived class' object can call base class' pure virtual function (surely, if it's implemented in base class)
来源：https://stackoverflow.com/questions/3526275/c-calling-derived-function-from-abstract-base-pointer