C++ virtual function call versus boost::function call speedwise

Question

Hello,

I wanted to know how fast is a single-inheritance virtual function call when compared to one same boost::function call. Are they almost the same in performance or is boost::function slower?

I'm aware that performance may vary from case to case, but, as a general rule, which is faster, and to a how large degree is that so?

Thanks, Guilherme

-- edit

KennyTM's test was sufficiently convincing for me. boost::function doesn't seem to be that much slower than a vcall for my own purposes. Thanks.

Answer 1

As a very special case, consider calling an empty function 10⁹ times.

---

Code A:

struct X {
        virtual void do_x() {};
};
struct Y : public X {}; // for the paranoid.

int main () {
        Y* x = new Y;
        for (int i = 100000000; i >= 0; -- i)
                x->do_x();
        delete x;
        return 0;
}

Code B: (with boost 1.41):

#include <boost/function.hpp>

struct X {
    void do_x() {};
};

int main () {
    X* x = new X;
    boost::function<void (X*)> f;
    f = &X::do_x;
    for (int i = 100000000; i >= 0; -- i)
        f(x);
    delete x;
    return 0;
}

Compile with g++ -O3, then time with time,

• Code A takes 0.30 seconds.
• Code B takes 0.54 seconds.

---

Inspecting the assembly code, it seems that the slowness may be due to exceptions and handling the possibility and that f can be NULL. But given the price of one boost::function call is only 2.4 nanoseconds (on my 2 GHz machine), the actual code in your do_x() could shadow this pretty much. I would say, it's not a reason to avoid boost::function.