Correct Way to Define a Predicate Function in C++

Question

I\'m trying to write predicate function for use with STL algorithms. I see that they are two ways to define a predicate:

(1) Use a simple function as below:

Answer 1

checker::isEven is not a function; it is a member function. And you cannot call a non-static member function without a reference to a checker object. So you can't just use a member function in any old place that you could pass a function pointer. Member pointers have special syntax that requires more than just () to call.

That's why functors use operator(); this makes the object callable without having to use a member function pointer.