The :: Scope Resolution Operator

The .* and –>* Pointer-to-Member Operators 

C++ allows you to generate a special type of pointer that “points” generically to a member of a class, not to a specific instance of that member in an object. This sort of pointer is called a pointer to a member, or pointer-to- member, for short. A pointer to a member is not the same as a normal C++ pointer. Instead, a pointer to a member provides only an offset into an object of the member’s class at which that member can be found. Since member pointers are not true pointers, the . and > operators cannot be applied to them. To access a member of a class given a pointer to it, you must use the special pointer-to-member operators .* and >*. They allow you to access a member of a class given a pointer to that member.

When you are accessing a member of an object given an object or a reference to an object, use the .* operator. When accessing a member given a pointer to an object, use the >* operator.

A pointer to a member is declared by using the general form shown here:

type class-name::*ptr;

Here, type is the base type of the member, class-name is the name of the class, and ptr is the name of the pointer-to-member variable being created. Once created, ptr can point to any member of its class that is of type type.

Here is a short example that demonstrates the .* operator. Pay special attention to the way the member pointers are declared.

#include <iostream> using namespace std; class cl { public:   cl(int i) { val=i; }   int val;   int double_val() { return val+val; } }; int main() {   int cl::*data; // int data member pointer   int (cl::*func)(); // func member pointer   cl ob1(1), ob2(2); // create objects      data = &cl::val; // get offset of val   func = &cl::double_val;  // get offset   cout << "Here are values: ";   cout << ob1.*data << " " << ob2.*data << "\n";   cout << "Here they are doubled: ";   cout << (ob1.*func)() << " ";   cout << (ob2.*func)() << "\n";   return 0; }

The pointer-to-member operators are not supported by C.

C(s)C++ Programmer's Reference
C Programming on the IBM PC (C Programmers Reference Guide Series)
ISBN: 0673462897
EAN: 2147483647
Year: 2002
Pages: 539 © 2008-2017.
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