Function Object in C++

Function object is very userful to use member function or non-member function as callback mechanism,
Same as event or delegate in C#.


For pointer to object

template <class Return, class Type, class Argument>
class MemFunctionObject : public binary_function <Type*,Argument,Return>
{
   Return (Type::*pMemFunction)(Argument);
public:
  explicit MemFunctionObject( Return (Type::*pMF)(Argument) ) : pMemFunction (pMF) 
  { 
  }
  Return operator() (Type* pObject, A x) const
  { 
     return (pObject->*pMemFunction)(x); 
  }
};

For reference to object

template <class Result, class Type, class Argument>
class MemFunctionObjectRef: public binary_function <Type,Argument,S>
{
private:
  Result (Type::*pMemMethod)(Argument);
public:
  explicit MemFunctionObjectRef ( Result (Type::*p)(Argument) ) : pMemMethod (p) {}
  Result operator() (Type& refObject, Argument x) const
    { 
         return (refObject.*pMemMethod)(x); 
    }
};

 binary_function  define type's alias

template <class Arg1, class Arg2, class Result>
struct binary_function {
    typedef Arg1   first_argument_type;
    typedef Arg2   second_argument_type;
    typedef Result result_type;
};

 Now, wo can define a function to generate a function object

template <class Result, class Type, class Argument> 
MemFunctionObject<Result,Type,Argument> MemFunction(Result (Type::*f)(Argument))
{ 
   return MemFunctionObject<Result,Type,Argument>(f); 
}

How to use:

// mem_fun example
#include <iostream>
#include <functional>
#include <vector>
#include <algorithm>
#include <string>
using namespace std;

int main () {
  vector <string*> numbers;

  // populate vector of pointers:
  numbers.push_back ( new string ("one") );
  numbers.push_back ( new string ("two") );
  numbers.push_back ( new string ("three") );
  numbers.push_back ( new string ("four") );
  numbers.push_back ( new string ("five") );

  vector <int> lengths ( numbers.size() );

  transform (numbers.begin(), numbers.end(), lengths.begin(),MemFunction(&string::append,_1,"_test!").length());
    
  for (int i=0; i<5; i++) {
      cout << *numbers[i] << " has " << lengths[i] << " letters.
";
  }
  return 0;
}