Question

I have a function that needs to call a virtual method many times in a loop and was hoping there would be a way to avoid the overhead of a vtable lookup every time. I thought maybe obtaining a pointer to the method would be a good way round this. The code below shows what I’m trying to do, the problem being that the address of a method in Derived cannot be assigned to a Base member function pointer.

class Base
{
    public:

    typedef float ( Base::*MFP )( float const & x ) const;

    virtual MFP getMFP( void ) const = 0;

    virtual float function( float const & x ) const = 0;
};

class Derived : public Base
{
    public:

    virtual MFP getMFP( void ) const
    {
        return &Derived::function;
    }

    virtual float function( float const & x ) const
    {
        return x * x;
    }
};

class Other
{
    public:

    float calculate( float const & x, Base * pBase ) const
    {
        Base::MFP function = pBase->getMFP();
        return ( ( *pBase ).*( function ) )( x );
    }
};

Is there a way to do what I’m trying to do here?

---

EDIT:

For anyone who’s still interested I ran a timed test of my code. Turns out dynamic dispatch only slowed down my calculation method by 0.004%, so pretty much nothing. Compiled using MSVC 2010 with full optimisation.

Answer 1

Don’t do it. The processor has deep pipelines, and the compiler is almost certainly already caching the function pointer. You can do this with some effort. But I guarantee that on any compiler less than 10 years old this will make no difference.