What's the difference between struct mycompare and static bool mycompare?

Question

struct mycompare1 {
    bool operator() (const Interval a, const Interval b) {
        return a.start < b.start;
    }
} mycompare1_instance;

static bool mycompare2(const Interval a, const Interval b) {
    return a.start < b.start;
}

Q1: What is the difference between these two. It seems that mycompare1_instance somehow equals to mycompare2 like this. The following two lines seem to do the same thing.

sort(intervals.begin(), intervals.end(), mycompare1);
sort(intervals.begin(), intervals.end(), mycompare2);

Could anyone explain it?

Q2: How is "static" useful here?

Thx ahead!

You should try learning C++ from the beginnings. Your question is nonsensical. — gnasher729, Nov 25 '14 at 22:16
Especially the `static` part. It's completely unrelated to the other part of the question. — Columbo, Nov 25 '14 at 22:17
**Books:** http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list — Galik, Nov 25 '14 at 22:21
user3692521 - how do you know that `mycompare1_instance` is equal to `mycompare2`? Do you have some sort of code that shows it? Or do you mean one can replace the other in some situations? — anatolyg, Nov 25 '14 at 22:33

score 1 · Answer 1 · answered Nov 25 '14 at 22:20

The first case defines operator() so you could could use it as a function - in short they are functors. The advantage is that it can have a state and depending how you define the operator() you could use the instance of that class as a function. Thse second case is simply a static functionn and does not hold the state in the sense of a class/struct.

score 0 · Answer 2 · answered Nov 25 '14 at 22:29

I suspect the question is related to functors

ie you have seen

mycompare_instance1(x,y)

vs

mycompare2(x,y)

The first one is a functor. It used where we need to make things that are both objects with state and can be invoked like functions. The second one is a simple function

What's the difference between struct mycompare and static bool mycompare?

2 Answers2