Virtual protected function error

I have Packet class contains virtual method and I have LogInRequest class which extends the packet class

Packet.h file

class Packet {

public:
    Packet();
    virtual ~Packet();

protected:
    virtual char* toByte() = 0;
    virtual void fromByte(char *d) =开发者_如何学Go 0;
    virtual Packet* handle() = 0;

    short m_bodySize;
    int64_t m_deviceId;
};


/*
 * LogInRequestPacket
 */
class LogInRequestPacket: public Packet{

public:

    LogInRequestPacket();
    virtual ~LogInRequestPacket();

    virtual char* toByte();
    virtual void fromByte(char *d);
    virtual Packet* handle();
};

Packet.cpp file

#include "Packet.h"

Packet::Packet() {
    // TODO Auto-generated constructor stub

}

Packet::~Packet() {
    // TODO Auto-generated destructor stub
}

/*
 * LogInRequestPacket
 */
LogInRequestPacket::LogInRequestPacket(){
    printf("LogInRequestPacket is being created... \n");
}

LogInRequestPacket::~LogInRequestPacket(){

}

char* LogInRequestPacket::toByte(){

}

void LogInRequestPacket::fromByte(char *d){

}

Packet* LogInRequestPacket::handle(){

}

I am trying to create this LogInRequestPacket from factory like this

Packet *packet = m_packetFactory->createInstance(static_cast<PACKET_TYPES>(type));
packet->fromByte(pdata);

And it seems like trying to call the protected virtual fromByte function instead of the child class's fromByte function.

I am getting a red line at the line declaring "virtual void fromByte(char *d) = 0" in Packet class scope in Packet.h file and it complains "... is protected"

Also, it complains too with x within context

packet->fromByte(pdata);

How do I fix this problem. Thanks in advance...

When you try to call a method through a pointer or reference, the compiler will verify the access specifier of the function in the static type of the object, even if it will dispatch the call to the dynamic type.

In your Packet class you have stated that you don't want user code (other than Packet, friends and derived classes) to be able to call the methods, and the compiler is just telling you that. If those functions should be accessible from other code, make them public:

class Packet {
public:
    Packet();
    virtual ~Packet();

    virtual char* toByte() = 0;
    virtual void fromByte(char *d) = 0;
    virtual Packet* handle() = 0;
protected:
    short m_bodySize;
    int64_t m_deviceId;
};

Called function access specifier is always deduced from the calling object/pointer/reference at compile time.

If base class (in your case) is calling the function, then compiler will consider its access specifier, which is protected and thus error is coming.

Suppose, you make that method access specifier public in base class and make protected in derived class and run the same code. It will work fine (this is something different than Java access speicifier in C++).

packet is declared as Packet*. Since in Packet it's not public - you can't do that.

Generally, I think changing access permissions of interface functions when inheriting is not such a good idea. If a function is hidden - there probably was a reason for that. If there's none - change at the parent object.