(C++) Functor

Any class that overload the function call operator (i.e. operator()) is a functor class[1]

Also commonly named Function object[1]).

A simple std::cout functor:

template <typename T> 
struct Couter 
{ 
  void operator()(const T& t) 
  { 
    std::cout << t << std::endl; 
  } 
};

There are some STL functors:

The example below shows three more complex functors in the (common) combination with std::for_each.

include <iostream>

include <vector>

include <algorith>

pragma hdrstop


class MyInitializer
{
  public:
  MyInitializer()
  : index(0) {}
  template<class T>
  void operator () (T & a)
  {
    a = index;
    ++index;
  }
  private:
  int index;
};
class MyIndexCout
{
  public:
  MyIndexCout() : index(0) {}
  template<class T>
  void operator () (const T & a)
  {
    std::cout << index << " : " << a << std::endl;
    ++index;
  }
  private:
  int index;
};
class MySquarer
{
  public:
  MySquarer() {}
  template<class T> void operator () (T & a)
  {
    a*=a;
  }
};
int main()
{
  const int size = 10;
  std::vector<int> myVector(size);
  std::for_each(myVector.begin(), myVector.end(), MyInitializer());
  std::for_each(myVector.begin(), myVector.end(), MyIndexCout());
  std::cout << "-----------" << std::endl;
  std::for_each(myVector.begin(), myVector.end(), MySquarer());
  std::for_each(myVector.begin(), myVector.end(), MyIndexCout());
}

Reference

1) Scott Meyers. Effective STL. ISBN: 0-201-74962-9. Chapter 'Introduction', paragraph 'Terms, terms, terms'.

last edited (December 16, 2006) by bilderbikkel, Number of views: 3043, Current Rev: 8 (Diff)

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