If a class type is defined to represent an operation, the function call operator can be overloaded for this class type to invoke this operation. For example:

#include <iostream>
#include <vector>
#include <algorithm>
   
struct absInt {
   int operator()( int iv ){
      return (iv > 0) ? iv : -iv;
   }
};
   
int main(){
   std::vector<int> ia = {-1, 3, -31, 6};
   std::transform( ia.begin(), ia.end(), ia.begin(), absInt() );
   for ( const int& e : ia ){
      std::cout << e << "\n";
   }
}

An overloaded operator() must be declared as a member function.
The fourth argument of transform() is a temporary object of class absInt created by invoking the default constructor of absInt.