C ++中的operator（）和operator <有什么区别？

Question

Whenever I have C++ entities that I want to compare I just implement operator< . However, reading other people's code I saw that the same can be achieved by implementing operator() .

What is the difference between the two? When should each of these operators be used?

Answer 1

operator< is the canonical way to define a comparison operator:

struct A { /* some members */ };
struct B { /* some members */ };

// comparison operator
bool operator<(const A&, const B&);

This gives you the conventional usage:

int main()
{
   A a;
   B b;

   const bool result = (a < b);
}

---

What you've seen is people creating functors ; that is, entire classes whose sole purpose is to wrap a comparison. To make these look a bit like functions to calling code, they use operator() :

struct A { /* some members */ };
struct B { /* some members */ };

// comparison functor
struct comparator
{
   bool operator()(const A&, const B&);
};

This gives you a less conventional usage in code equivalent to my previous example:

int main()
{
   A a;
   B b;

   comparator c;
   const bool result = c(a,b);
}

However, you wouldn't use it for this. Functors are for passing to algorithms (particularly in generic code). They also have the added benefit of being able to hold state , since you have constructor arguments to play with. This makes them more flexible than a simple function pointer.

int main()
{
   std::vector<A> a(5);
   B b;

   comparator c;
   std::sort(a.begin(), a.end(), c);

   // or, simply:
   std::sort(a.begin(), a.end(), comparator());

   // all more easily extensible than:
   std::sort(a.begin(), a.end(), &someComparisonFunction);
}