C++ Cookbook

15.2. Using Pointers to Class Members

Problem

You need to refer to a data member or a member function with its address.

Solution

Use the class name and the scope operator (::) with an asterisk to correctly qualify the name. Example 15-2 shows how.

Example 15-2. Obtaining a pointer to a member

#include <iostream>
#include <string>

class MyClass {

public:
   MyClass() : ival_(0), sval_("foo") {}
  ~MyClass() {}

   void incr() {++ival_;}
   void decr() {ival_--;}

private:
   std::string sval_;
   int ival_;
};

int main() {

   MyClass obj;

   int         MyClass::* mpi = &MyClass::ival_;  // Data member
   std::string MyClass::* mps = &MyClass::sval_;  // pointers

   void (MyClass::*mpf)(); // A pointer to a member function that
                           // takes no params and returns void
   void (*pf)();           // A normal function pointer

   int* pi = &obj.ival_;   // int pointer referring to int member--no
                           // problem.

   mpf = &MyClass::incr;   // A pointer to a member function. You can't
                           // write this value to a stream. Look at it
                           // in your debugger to see what its
                           // representation looks like.

   pf = &MyClass::incr;    // Error: &MyClass::incr is not an instance
                           // of a function

   std::cout << "mpi = " << mpi << '\n';
   std::cout << "mps = " << mps << '\n';
   std::cout << "pi =  " << pi << '\n';
   std::cout << "*pi = " << *pi << '\n';

   obj.*mpi = 5;
   obj.*mps = "bar";

   (obj.*mpf)(); // now obj.ival_ is 6

   std::cout << "obj.ival_ = " << obj.ival_ << '\n';
   std::cout << "obj.sval_ = " << obj.sval_ << '\n';
}

Discussion

Pointers to members look and act differently than ordinary pointers. ...