Multiple definition of virtual function while implementing Factory Pattern

Question

I've a set of possible connections in my SW so I decided to use the Factory Pattern, so I created the base class (both .h and .cpp)

The following the content of Connection.h header file

namespace Connection
{
    class Connection
    {
        public: 
             Connection();
             ~Connection();

             virtual void sendPacket(Packet* p) = 0;
             virtual void receivePacket() = 0;
             virtual int connect() = 0;
             virtual void disconnect() = 0; 
     }
}

Even a Connection.cpp file exists but it has only an empty constructor and deconstructor.

Next, I created the derivated class (both .h and .cpp files)

namespace Connection
{
    class SocketConnection : public Connection
    {
        public:
            SocketConnection();
            ~SocketConnection();
    }
}

and its relative SocketConnection.cpp file where I'm trying to define the pure virtual methods defined in Connection.h

namespace Connection
{
    SocketConnection::SocketConnection() { }
    SocketConnection::~SocketConnection() { }

    int connect()
    {
        //Design of socket connection
    }
}

Next, I created a new Connection, DatabaseConnection defined pratically in the same way of the SocketConnection().

The error I get is

multiple definition of Connection::connect();

while invoking the Cross G++ Linker but I can't find out the reason. Can anyone tell me where I'm wrong? Thank you!

You need to implement all of the pure virtual functions in the derived classes of the interface. Read [this](https://stackoverflow.com/questions/1306778/c-virtual-pure-virtual-explained). — Incomputable, Oct 27 '17 at 13:45
First of all you have to declare virtual function you override in derived class, second that function has to belong to that class, you define function in namespace, not method. — Slava, Oct 27 '17 at 13:47
Please don't have a namespace that contains a class with the same name. — Caleth, Oct 27 '17 at 13:56

score 4 · Answer 1 · answered Oct 27 '17 at 13:48

4

If you want to override connect in SocketConnection then you have to declare it in the class definition:

namespace Connection
{
    class SocketConnection : public Connection
    {
        public:
            SocketConnection();
            ~SocketConnection();
             virtual int connect(); // virtual is not required here but it doesn't hurt
    }
}

Then the definition has to be properly scoped:

namespace Connection
{
    SocketConnection::SocketConnection() { }
    SocketConnection::~SocketConnection() { }

    int SocketConnection::connect()
    {
        //Design of socket connection
    }
}

answered Oct 27 '17 at 13:48

Tomek

4,554
1
19
19

I would add `override` to it as well – Slava Oct 27 '17 at 13:49
1

Yeah, was about to say the same thing. `override` would probably have helped give a better error message too (maybe). – Nir Friedman Oct 27 '17 at 13:50
I still work on MSVC 1.52 from 1993, so I don't have access to such advanced features :(. – Tomek Oct 27 '17 at 13:51

score 0 · Answer 2 · answered Oct 27 '17 at 13:47

Inside your SocketConnection.h you should add:

virtual int connect();

And inside your SocketConnection.cpp you should define a scoped SocketConnection::connect() instead of connect(), like this:

int SocketConnection::connect()
{
    //Design of socket connection
}

Same for all other connection types.

Multiple definition of virtual function while implementing Factory Pattern

2 Answers2