C++ writing a modify_if function

Question

Hi, I have a hw question which confuses me on what i have to do. The question is below:

1. The idea is to design a generic function called Modify_If that will take an input x (passed by reference), and two functors f1 and f2. The function Modify_If will use functor f1 to determine whether x obeys a certain condition. If it does, Modify_if will change the value of x, by applying functor f2 to it.

The prototype for Modify_If is as follows:

template <class C, class Tester, class Transform>
void Modify_If(C & a, Tester f, Transform g)

I have to write the Modify_If function but I have no idea where to start so if anyone can help me out I would appreciate it.

Answer 1

From the description it sounds like all you're supposed to do is to execute g(a) if f(a) returns true. That would look like this:

if(f(a)) {
  g(a);
}

Answer 2

template <class C, class Tester, class Transform>
void Modify_If(C& a, Tester f1, Transform f2) {
    if (f1(a)) // Apply f1 to a - Check whether result is true
        a = f2(a); // Transform with f2; save
}

Answer 3

In both functors Tester and Transform, all what you have to do is to create a class for each, and overload the () operator. Then write the body of the functor as if it were a function. This is a simple functor skeleton:

class Functor
{
public:
   template <class Type>
   void operator()(Type& value) // it doesn't have to be void.
   {
     // functor body, as if it were a function.
   }
};

Then, you can pass the functor as a parameter, by creating an object of it, or easily by creating it on the fly:

template <class Func>
void fun(Func f); // a function hat accepts a functor.
Functor f;
fun(f);
fun(Functor()); // creating the functor on the fly.

Answer 4

Function prototype could look as follows:

template<class ForwardIterator, class Predicate, class Type>
void transform_if(
      ForwardIterator First, 
      ForwardIterator Last,
      Predicate Pred, 
      UnaryFunction Func
   );

In its implementation you should iterate through all set of elements and apply Func to all elements that will return true for Pred(*element_iterator).