Using STL algorithms, is it better to pass a function pointer or a functor?

Question

Which of these 2 methods is better and why?

Method 1:

void fun(int i) {
  //do stuff
}

...
for_each(a.begin(), a.end(), fun);

Method 2:

class functor {
public:
  void operator()(int i);
};

...
for_each(a.begin(), a.end(), functor());

Edit: Should have formulated it this way, in what situation is one of the above method preferable to the other?

Thanks a lot!

Answer 1

Functors may (and will) be trivially inlined – this isn't done for regular function pointers.

Thus, functors have a real performance benefit which may be huge in tight loops. Furthermore, functors are generally more easily composable and in particuler play nicer with the STL: std::bindx doesn't work on function pointers, for instance.

I hate how they clutter the code but given all the advantages, I'd prefer them over function pointers any time.

Answer 2

To clear up a misconception of what a compiler can inline, a good enough compiler can inline function pointers. It can just inline function objects more easily since there is more static information available. E.g., a pointer to a function that takes no parameters and returns a bool is of type bool (*)(), while a functor has an explicit type, namely, the functor, and the template instantiation can statically call the functor operator, rather than having to call through a function pointer.

In practice, though, it's mainly a matter of giving the compiler enough information to optimize effectively.

For example, Visual C++ 2008, given the following code with full optimizations:

#include "stdafx.h"
#include <algorithm>

const char print_me[]= "hello!";

class print_functor
{
public:
    void operator()(char c)
    {
        printf("%c", c);
    }
};

void print_function(char c)
{
    printf("%c", c);
}

int _tmain(int argc, _TCHAR* argv[])
{
    std::for_each(print_me, print_me + sizeof(print_me)/sizeof(print_me[0]), print_functor());
    printf("\n");

    std::for_each(print_me, print_me + sizeof(print_me)/sizeof(print_me[0]), print_function);

    return 0;
}

inlines both std::for_each calls completely. Incidentally, on the PC, the first for_each has an unnecessary lea ecx, [ecx].