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

 1 template <class Return, class Type, class Argument>
 2 class MemFunctionObject : public binary_function <Type*,Argument,Return>
 3 {
 4    Return (Type::*pMemFunction)(Argument);
 5 public:
 6   explicit MemFunctionObject( Return (Type::*pMF)(Argument) ) : pMemFunction (pMF) 
 7   { 
 8   }
 9   Return operator() (Type* pObject, A x) const
10   { 
11      return (pObject->*pMemFunction)(x); 
12   }
13 };

For reference to object

 1 template <class Result, class Type, class Argument>
 2 class MemFunctionObjectRef: public binary_function <Type,Argument,S>
 3 {
 4 private:
 5   Result (Type::*pMemMethod)(Argument);
 6 public:
 7   explicit MemFunctionObjectRef ( Result (Type::*p)(Argument) ) : pMemMethod (p) {}
 8   Result operator() (Type& refObject, Argument x) const
 9     { 
10          return (refObject.*pMemMethod)(x); 
11     }
12 };

binary_function define type's alias

1 template <class Arg1, class Arg2, class Result>
2 struct binary_function {
3     typedef Arg1   first_argument_type;
4     typedef Arg2   second_argument_type;
5     typedef Result result_type;
6 };

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

1 template <class Result, class Type, class Argument> 
2 MemFunctionObject<Result,Type,Argument> MemFunction(Result (Type::*f)(Argument))
3 { 
4    return MemFunctionObject<Result,Type,Argument>(f); 
5 }

How to use:

 1 // mem_fun example
 2 #include <iostream>
 3 #include <functional>
 4 #include <vector>
 5 #include <algorithm>
 6 #include <string>
 7 using namespace std;
 8 
 9 int main () {
10   vector <string*> numbers;
11 
12   // populate vector of pointers:
13   numbers.push_back ( new string ("one") );
14   numbers.push_back ( new string ("two") );
15   numbers.push_back ( new string ("three") );
16   numbers.push_back ( new string ("four") );
17   numbers.push_back ( new string ("five") );
18 
19   vector <int> lengths ( numbers.size() );
20 
21   transform (numbers.begin(), numbers.end(), lengths.begin(),MemFunction(&string::append,_1,"_test!").length());
22     
23   for (int i=0; i<5; i++) {
24       cout << *numbers[i] << " has " << lengths[i] << " letters.
";
25   }
26   return 0;
27 }