Wednesday, February 18, 2009

C++: Pointer to members

Pointer to member operators are not an everyday operators that we use but those can be useful if used in the correct way. Pointer to member operator ::* is used to declare a pointer that points to a member variable, pointer or function of a class. The operators .* and ->* are used to access it.

Following is an example of how we can use Pointer to member function (also called member function pointer).

  1 #include <iostream>
2 using namespace std;
3
4 class A{
5 public:
6 A(){
7 }
8 int Func(int a){
9 return a * 2;
10 }
11
12 };
13
14 int main(){
15 int (A::*fp)(int) = &A::Func;
16
17 A a;
18 cout << (a.*fp)(10) << endl;
19
20 return 0;
21 }
22

Following is another (almost random) example of how we can write generic classes to wrap pointer to member function to use with multiple classes.

  1 #include <iostream>
2 using namespace std;
3
4 class A{
5 public:
6 int Func1(int a){
7 return a * 2;
8 }
9
10 };
11
12 class B{
13 public:
14 int Func2(int a){
15 return a / 2;
16 }
17
18 };
19
20 template <class T>
21 class MemFnPtr
22 {
23 public:
24 MemFnPtr(int (T::*fp)(int), T &t) : m_fp(fp), m_t(t){
25 }
26 int Fn(int n)
27 {
28 return ((m_t).*(m_fp))(n);
29 }
30
31 //member vars
32 int (T::*m_fp)(int);
33 T &m_t;
34 };
35
36 int main(){
37 A a_obj;
38
39 MemFnPtr<A> fp_a(&A::Func1, a_obj);
40 cout << fp_a.Fn(10) << endl;
41
42 B b_obj;
43
44 MemFnPtr<B> fp_b(&B::Func2, b_obj);
45 cout << fp_b.Fn(10) << endl;
46
47 return 0;
48 }
49

3 comments:

Giancarlo Niccolai said...

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luc said...

Time to time, I still look into your old C/C++ blog posts, to refresh my memory of some intricate stuffs. Thanx for the posts, it's really is of great help to me.

Mohammad Kaisar Ul Haque said...

@luc: hey, thanks. That inspires me to write more :).