Predicate for STL algorithms: template function vs functor

[Robertlzw]

I found for such predicate, both of the following works:

// template function
template<typename T>
bool Pred(const T& x, const T& y) {
	return x == 3*y;
}

// functor
struct Pred : public std::binary_function<int, int, bool> {
	bool operator()(int x, int y) {
		return x == 3 * y;
	}
};

Which one is better in term of performance. Why should we prefer one over the other? Thanks.

[webJose]

The functor will always be slower because an object must be constructed, I would say. Furthermore, STL algorithms take the #"%&%$!! functor by value, meaning you must make sure you have a proper copy constructor.

Now, how much more slower functors are? Probably not noticeable by many standards. I bet only the most demanding of scenarios would care about performance difference.

[Robertlzw]

It seems that STL algorithms DO NOT take the functor by value. I tested the following code which works:

struct Pred : public std::binary_function<int, int, bool> {
	bool operator()(int x, int y) {
		return x == 3 * y;
	}
private:
	Pred (const ThreeTimes&);
};

[webJose]

I have used functors twice and the algorithm I used took them by value both times.