[Ns-developers] C++ virtual method overloading and inheritance = disaster?

[Gustavo Carneiro]

-----------
struct Socket
{
    virtual int Bind () { return -1; }
    virtual int Bind (int address) { return address; }
};

struct UdpSocket : public Socket
{
    virtual int Bind () { return 0; }
};


int main(void)
{
    UdpSocket sock;
    sock.Bind (123);
}

---------------

Compiling the above program I get:

gjc at dark-tower:tmp$ g++ test.cc
test.cc: In function 'int main()':
test.cc:18: error: no matching function for call to 'UdpSocket::Bind(int)'
test.cc:11: note: candidates are: virtual int UdpSocket::Bind()

WTF?  How come a Bind() method in UdpSocket hides the other Bind() method
defined in the base Socket class?

Commenting out the Bind() method in UdpSocket makes the program compile.

I bring this example this because this problem appears in NS-3, and Python
bindings suffer.

Let me explain (easier than creating a bug report).  In
ns-3-dev/src/node/socket.h,

class Socket {
[...]
  virtual int Bind (const Address &address) = 0;
  virtual int Bind () = 0;
[...]
};

In ns-3-dev/src/node/udp-socket.h:

class UdpSocket : Socket {
[...]
  // missing: virtual int Bind (const Address &address) = 0;
  virtual int Bind () = 0;
[...]
};

Now, what happens next.  When I call socketFactory.CreateSocket () from
Python, the Python bindings know that the method is supposed to return
Socket*.  However, they also know that there exists a known public class
UdpSocket, and use C++ RTTI (typeid(*pointer)) to discover this is actually
a UdpSocket object, not a Socket.  Finally, when I call socket.Bind
(address) from Python, the call fails because apparently only one Bind()
method exists, and not the other.

A long story just to explain why we need to fix UdpSocket, adding the
missing Bind method.  And to vent some of my frustration at this (having
spent some hours trying to support this use case in PyBindGen only to find
out that C++ itself does not support it :P).

-- 
Gustavo J. A. M. Carneiro
INESC Porto, Telecommunications and Multimedia Unit
"The universe is always one step beyond logic." -- Frank Herbert