Pointers to Members

Declarations of pointers to members are special cases of pointer declarations.

Syntax

decl-specifiers class-name :: * cv-qualifier-list_opt dname ;

A pointer to a member of a class differs from a normal pointer because it has type information for the type of the member and for the class to which the member belongs. A normal pointer identifies (has the address of) only a single object in memory. A pointer to a member of a class identifies that member in any instance of the class. The following example declares a class, Window, and some pointers to member data.

class Window
{
public:
    Window();                               // Default constructor.
    Window( int x1, int y1,                 // Constructor specifying
        int x2, int y2 );                   //  window size.
    BOOL SetCaption( const char *szTitle ); // Set window caption.
    const char *GetCaption();               // Get window caption.
    char *szWinCaption;                     // Window caption.
};

// Declare a pointer to the data member szWinCaption.
char * Window::* pwCaption = &Window::szWinCaption;

In the preceding example, pwCaption is a pointer to any member of class Window that has type char*. The type of pwCaption is char * Window::*. The next code fragment declares pointers to the SetCaption and GetCaption member functions.

const char * (Window::*pfnwGC)() = &Window::GetCaption;
BOOL (Window::*pfnwSC)( const char * ) = &Window::SetCaption;

The pointers pfnwGC and pfnwSC point to GetCaption and SetCaption of the Window class, respectively. The code copies information to the window caption directly using the pointer to member pwCaption:

Window wMainWindow;
Window *pwChildWindow = new Window;
char   *szUntitled    = "Untitled -  ";
int    cUntitledLen   = strlen( szUntitled );

strcpy( wMainWindow.*pwCaption, szUntitled );
(wMainWindow.*pwCaption)[cUntitledLen - 1] = '1';     //same as
//wMainWindow.SzWinCaption [ ] = '1';
strcpy( pwChildWindow->*pwCaption, szUntitled );
(pwChildWindow->*pwCaption)[szUntitledLen - 1] = '2'; //same as
//pwChildWindow->szWinCaption[ ] = '2';

The difference between the .* and –>* operators (the pointer-to-member operators) is that the .* operator selects members given an object or object reference, while the –>* operator selects members through a pointer. (For more about these operators, see Expressions with Pointer-to-Member Operators in Chapter 4.)

The result of the pointer-to-member operators is the type of the member — in this case, char *.

The following code fragment invokes the member functions GetCaption and SetCaption using pointers to members:

// Allocate a buffer.
char szCaptionBase[100];

// Copy the main window caption into the buffer
//  and append " [View 1]".
strcpy( szCaptionBase, (wMainWindow.*pfnwGC)() );
strcat( szCaptionBase, " [View 1]" );

// Set the child window's caption.
(pwChildWindow->*pfnwSC)( szCaptionBase );