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=; i<; i++) {
       cout << *numbers[i] << " has " << lengths[i] << " letters.\n";
   }
   return ;
 }