Virtual Functions and Performance C++

Question

Before you cringe at the duplicate title, the other question wasn't suited to what I ask here (IMO). So.

I am really wanting to use virtual functions in my application to make things a hundred times easier (isn't that what OOP is all about ;)). But I read somewhere they came at a performance cost, seeing nothing but the same old contrived hype of premature optimization, I decided to give it a quick whirl in a small benchmark test using:

CProfiler.cpp

#include "CProfiler.h"

CProfiler::CProfiler(void (*func)(void), unsigned int iterations) {
    gettimeofday(&a, 0);
    for (;iterations > 0; iterations --) {
        func();
    }
    gettimeofday(&b, 0);
    result = (b.tv_sec * (unsigned int)1e6 + b.tv_usec) - (a.tv_sec * (unsigned int)1e6 + a.tv_usec);
};

main.cpp

#include "CProfiler.h"

#include <iostream>

class CC {
  protected:
    int width, height, area;
  };

class VCC {
  protected:
    int width, height, area;
  public:
    virtual void set_area () {}
  };

class CS: public CC {
  public:
    void set_area () { area = width * height; }
  };

class VCS: public VCC {
  public:
    void set_area () {  area = width * height; }
  };

void profileNonVirtual() {
    CS *abc = new CS;
    abc->set_area();
    delete abc;
}

void profileVirtual() {
    VCS *abc = new VCS;
    abc->set_area();
    delete abc;
}

int main() {
    int iterations = 5000;
    CProfiler prf2(&profileNonVirtual, iterations);
    CProfiler prf(&profileVirtual, iterations);

    std::cout << prf.result;
    std::cout << "\n";
    std::cout << prf2.result;

    return 0;
}

At first I only did 100 and 10000 iterations, and the results were worrying: 4ms for non virtualised, and 250ms for the virtualised! I almost went "nooooooo" inside, but then I upped the iterations to around 500,000; to see the results become almost completely identical (maybe 5% slower without optimization flags enabled).

My question is, why was there such a significant change with a low amount of iterations compared to high amount? Was it purely because the virtual functions are hot in cache at that many iterations?

Disclaimer
I understand that my 'profiling' code is not perfect, but it, as it has, gives an estimate of things, which is all that matters at this level. Also I am asking these questions to learn, not to solely optimize my application.

Answer 1

I believe that your test case is too artificial to be of any great value.

First, inside your profiled function you dynamically allocate and deallocate an object as well as call a function, if you want to profile just the function call then you should do just that.

Second, you are not profiling a case where a virtual function call represents a viable alternative to a given problem. A virtual function call provides dynamic dispatch. You should try profiling a case such as where a virtual function call is used as an alternative to something using a switch-on-type anti-pattern.