When are stateless class functors useful in place of a c style function?

Question

I\'ve found some good examples of functors on SO like this one, and all the convincing examples seem to use state in the class that defines operator().

Answer 1

The typical reason is that when you do this:

bool less_than(const Point&, const Point&);
// ...
std::sort(..., &less_than);

The template argument for the predicate is the following:

bool(const Point&,const Point&)

Since the sort function receives a function pointer, it is more difficult for the compiler to inline the predicate use inside std::sort(). This happens because you could have another function

bool greater_than(const Point&, const Point&);

which has the exact same type, meaning the std::sort() instatiation would be shared between the two predicates. (remember that I said that it makes inlining more difficult, not impossible).

In contrast, when you do this:

struct less_than {
    bool operator()(const Point&, const Point&) const;
};
// ...
std::sort(..., less_than());


struct greater_than {
    bool operator()(const Point&, const Point&) const;
};
// ...
std::sort(..., greater_than());

The compiler generates a unique template instantiation for std::sort() for each predicate, making it easier to inline the predicate's definition.