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Why would you create a class with one single member that is an operator ()?

I tried to figure out -out there- a good technical reason for defining a class that contains only one member, and that member is operator().

I stumbled on who -for regardless of reason- created a namespace containing a couple of classes, but each class contains only one operator()as a member.

It is clear to me that these classes can then be used as if they were methods (very likely), but why is this a good technical approach (I believe it is good), and not just defining a set of different methods inside the singleton class, which in this the specific case will belong to the namespace that I mentioned above.

The namespace looks something like this:

namespace myNamespace {
   class ClassA {
   public:
      void operator()();
   };

   class ClassB {
   public:
      int operator()(arg1, arg2);
   };

   ...
}

Is it just a matter of personal taste / style or is it an expression of advanced / sophisticated design? I assume that this design includes some wise knowledge that I have not yet collected, but on the other hand, I have not found a single article or question discussing this - which makes me doubt such "wise knowledge" in design.

What do experts say here?

Thank you in advance!

EDIT

Hello again! I accept this question as a REPEAT question, the answer to which can be found here .

The term "Functor" was unknown to me, and, trying to find an answer to my doubts (before submitting this question), I did not see any references to this term.

SergeyA, , - . -from view- , , .

!

+6
1

. , operator(), , functor, , . :

template<class F>
void call_f(F f) {
    f();
}

, . STL, find_if, copy_if, remove_if .. , , ( ) a .

, . - , . (, - ) , . , , std::less std::map. , .

+5

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