Can you cache a virtual function lookup in C++?

Question

Say I have a virtual function call foo() on an abstract base class pointer, mypointer->foo(). When my app starts up, based on the contents of a file, it chooses to instantiate a

Answer 1

I have seen situations where avoiding a virtual function call is beneficial. This does not look to me to be one of those cases because you really are using the function polymorphically. You are just chasing one extra address indirection, not a huge hit, and one that might be partially optimized away in some situations. If it really does matter, you may want to restructure your code so that type-dependent choices such as virtual function calls are made fewer times, pulled outside of loops.

If you really think it's worth giving it a shot, you can set a separate function pointer to a non-virtual function specific to the class. I might (but probably wouldn't) consider doing it this way.

class MyConcrete : public MyBase
{
public:
  static void foo_nonvirtual(MyBase* obj);
  virtual void foo()
  { foo_nonvirtual(this); }
};

void (*f_ptr)(MyBase* obj) = &MyConcrete::foo_nonvirtual;
// Call f_ptr instead of obj->foo() in your code.
// Still not as good a solution as restructuring the algorithm.

Other than making the algorithm itself a bit wiser, I suspect any attempt to manually optimize the virtual function call will cause more problems than it solves.

Answer 2

All answers are dealing with the most simple scenario, where calling a virtual method only requires getting the address of the actual method to call. In the general case, when multiple and virtual inheritance come into play, calling a virtual method requires shifting the this pointer.

The method dispatch mechanism can be implemented in more than one way, but it is common to find that the entry in the virtual table is not the actual method to call, but rather some intermediate 'trampoline' code inserted by the compiler that relocates the this pointer prior to calling the actual method.

When the dispatch is the simplest, just an extra pointer redirection, then trying to optimize it does not make sense. When the problem is more complex, then any solution will be compiler dependent and hackerish. Moreover, you do not even know in what scenario you are: if the objects are loaded from dlls then you don't really know whether the actual instance returned belongs to a simple linear inheritance hierarchy or a more complex scenario.

Answer 3

You can't use a method pointer because pointers to member functions aren't considered covariant return types. See the example below:

#include <iostream>

struct base;
struct der;

typedef void(base::*pt2base)();
typedef void(der::*pt2der)();

struct base {
    virtual pt2base method() = 0;
    virtual void testmethod() = 0;
    virtual ~base() {}
};

struct der : base {
    void testmethod() {
        std::cout << "Hello from der" << std::endl;
    }
    pt2der method() { **// this is invalid because pt2der isn't a covariant of pt2base**
        return &der::testmethod;
    }
};

The other option would be to have the method declared pt2base method() but then the return would be invalid because der::testmethod is not of type pt2base.

Also even if you had a method that received a ptr or reference to the base type you would have to dynamically cast it to the derived type in that method to do anything particularly polymorphic which adds back in the cost we're trying to save.