Skip Unused Virtual Function

I have main executable and two functions that are dereferenced to DLL.

class CPluginInterface
{
public:
    virtual void A(void) = 0;
    virtual void B(void) = 0;
};

I created DLL like this

//Main.h
#include "Function.h"

class CForward : public CPluginInterface
{
public:
    void A(void);
    void B(void);
};

//Main.cpp
#include "Main.h"

void CForward::B(void)
{
    //Do something here
}

extern "C"
{
    // Plugin factory function
    //void __declspec(dllexport) __cdecl A(void) { }
    void __declspec(dllexport) __cdecl B(void) { }
}

However the program is crashed because A(void) doesn't exist when the main executable dereferencing it. How to skip A(void)?

NB: I create Plugin Interface.