Question

There is a simple POD data type like

struct Item {
   int value1;
   double value2;
   bool value3;
}

Now I would like to write different count functions like it could be done with the following code (or some std method):

typedef bool Selector(const Item& i);
int count(const vector<Item>& items, Selector f) {
  int sum = 0;
  BOOST_FOREACH(const Item& i, items) {
    if(f(i)) {
      sum++;
    }
  }
  return sum;
}

with f e.g.

bool someSimpleSelector(const Item& i) {
  return i.value1 > 0; // quite simple criterion
}

However in this approach the compiler can not inline the function call and thus will not inline my (trivial) selection code.

My question would be: is there a possibility to implement the above code in a way, where the compiler can inline my selection code, but without implementing the whole count function again and again explicitely (for example by using templates)?

Answer 1

Replace the typedef with a template parameter, to allow generic functors:

template <typename Selector>
int count(const vector<Item>& items, const Selector &f)

Then replace your functions with function objects:

struct someSimpleSelector
{
    bool operator()(const Item& i) const { return i.value1 > 0; }
};

You should find that this gets inlined, with suitable compiler optimisation settings.

By the way, this particular problem can be solved using std::count_if().