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Thinking in C++
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Pointers to members

A pointer is a variable that holds the address of some location. You can change what a pointer selects at runtime, and the destination of the pointer can be either data or a function. The C++ pointer-to-member follows this same concept, except that what it selects is a location inside a class. The dilemma here is that a pointer needs an address, but there is no “address” inside a class; selecting a member of a class means offsetting into that class. You can’t produce an actual address until you combine that offset with the starting address of a particular object. The syntax of pointers to members requires that you select an object at the same time you’re dereferencing the pointer to member.

To understand this syntax, consider a simple structure, with a pointer sp and an object so for this structure. You can select members with the syntax shown:

//: C11:SimpleStructure.cpp
struct Simple { int a; };
int main() {
  Simple so, *sp = &so;
} ///:~

Now suppose you have an ordinary pointer to an integer, ip. To access what ip is pointing to, you dereference the pointer with a ‘*’:

*ip = 4;

Finally, consider what happens if you have a pointer that happens to point to something inside a class object, even if it does in fact represent an offset into the object. To access what it’s pointing at, you must dereference it with *. But it’s an offset into an object, so you must also refer to that particular object. Thus, the * is combined with the object dereference. So the new syntax becomes –>* for a pointer to an object, and .* for the object or a reference, like this:

objectPointer->*pointerToMember = 47;
object.*pointerToMember = 47;

Now, what is the syntax for defining pointerToMember? Like any pointer, you have to say what type it’s pointing at, and you use a * in the definition. The only difference is that you must say what class of objects this pointer-to-member is used with. Of course, this is accomplished with the name of the class and the scope resolution operator. Thus,

int ObjectClass::*pointerToMember;

defines a pointer-to-member variable called pointerToMember that points to any int inside ObjectClass. You can also initialize the pointer-to-member when you define it (or at any other time):

int ObjectClass::*pointerToMember = &ObjectClass::a;

There is actually no “address” of ObjectClass::a because you’re just referring to the class and not an object of that class. Thus, &ObjectClass::a can be used only as pointer-to-member syntax.

Here’s an example that shows how to create and use pointers to data members:

//: C11:PointerToMemberData.cpp
#include <iostream>
using namespace std;

class Data {
  int a, b, c; 
  void print() const {
    cout << "a = " << a << ", b = " << b
         << ", c = " << c << endl;

int main() {
  Data d, *dp = &d;
  int Data::*pmInt = &Data::a;
  dp->*pmInt = 47;
  pmInt = &Data::b;
  d.*pmInt = 48;
  pmInt = &Data::c;
  dp->*pmInt = 49;
} ///:~

Obviously, these are too awkward to use anywhere except for special cases (which is exactly what they were intended for).

Also, pointers to members are quite limited: they can be assigned only to a specific location inside a class. You could not, for example, increment or compare them as you can with ordinary pointers.

Thinking in C++
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire