What are pointers to class members used for?

Question

I have read about pointers to class members, but I have never seen them being used in any practical applications. Can someone explain what are the use cases of such pointers? Is it really necessary to have such pointers?

Eg.

class abc
{
public:
    int a;
    abc(int val) { a = val; }
};

 int main()
{
   int abc::*data;
   abc obj(5);

   data = &abc::a;

   cout << "Value of a is " << obj.*data << endl;

   return 0;
}

In the above eg. why is the value of 'a' accessed in this manner? What is the advantage of using pointers to class members?

Answer 1

The biggest advantage of a pointer-to-member or pointer-to-member-function is that you

• don't have to bind to a specific instance right away
• don't need to place any restrictions on the member names, only the type has to match.

This can be used for e.g. call-backs or abstract algorithms:

std::map<int,int> m;
m.insert(std::make_pair(1,2));
m.insert(std::make_pair(3,4));
m.insert(std::make_pair(5,6));
std::ptrdiff_t s = 
    std::count_if(m.begin(), m.end(),
                  boost::bind(&std::map<int,int>::value_type::first, _1) > 2);
std::cout << s << std::endl; // 2

Note that Boost.Bind, Boost.Function and their TR1 equivalents already encapsulate that nicely for you. To a certain degree the current standard also includes tools like std::mem_fun in <functional>.

Answer 2

If you have used MFC, you will see pointers to member function concept is heavily used (internally)

DECLARE_MESSAGE_MAP, BEGIN_MESSAGE_MAP, END_MESSAGE_MAP See Message Maps