Why does 'virtual' make a difference here ??

[John E]

I'm building a dll (let's call it DLL A) which uses a 3rd party dll (DLL B). But DLL A has never needed to link to the link library for DLL B. Recently, a new name() function got added to one of the classes in DLL B:-

class LIBCONTROLCP_API ControlProtocol : public PBD::Stateful, public PBD::ScopedConnectionList, public BasicUI
{
public:
	ControlProtocol (Session&, std::string name);
	virtual ~ControlProtocol();

	std::string name() const { return _name; } // <--- This function got added

	virtual int set_feedback (bool /*yn*/) { return 0; }
	virtual bool get_feedback () const { return false; }

protected:
	std::string _name;
};

(assume that LIBCONTROLCP_API is either __dllimport or __dllexport, as appropriate).

After the new function got added, I realised that I couldn't build DLL A any more without linking it to the link lib for DLL B. But the functions get_feedback() and set_feedback() have always worked without me needing the link lib. So I added virtual to name's declaration, like so:-

	virtual std::string name() const { return _name; } // I declared it as virtual

and the linker seems to accept this now without needing DLL B's link lib any more. I'm just curious to know why virtual would make a difference here

[John E]

I added virtual to name's declaration, like so:-

	virtual std::string name() const { return _name; } // I declared it as virtual

and the linker seems to accept this now without needing DLL B's link lib any more. I'm just curious to know why virtual would make a difference here

I'm always amazed how things seem clearer after a good night's sleep! Presumably, virtual enforces late binding. Does that seem like the explanation?