Pointers to Members
Declarations of pointers to members are special cases of pointer declarations. They are declared using the following sequence:
[storage-class-specifiers] [cv-qualifiers] type-specifiers [ms-modifier]qualified-name ::* [cv-qualifiers] identifier
[= & qualified-name :: member-name];
The declaration specifier:
An optional storage class specifier.
Optional const and/or volatile specifiers.
The type specifier: the name of a type. This is the type of the member to be pointed to, not the class.
The declarator:
An optional Microsoft specific modifier. For more information, see Microsoft-Specific Modifiers.
The qualified name of the class containing the members to be pointed to. See Names and Qualified Names.
The :: operator.
The * operator.
Optional const and/or volatile specifiers.
The identifier naming the pointer to member.
An optional initializer:
The = operator.
The & operator.
The qualified name of the class.
The :: operator.
The name of a nonstatic member of the class of the appropriate type.
As always, multiple declarators (and any associated initializers) are allowed in a single declaration.
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.
// pointers_to_members1.cpp
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;
int main()
{
}
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_s( wMainWindow.*pwCaption, cUntitledLen, szUntitled );
(wMainWindow.*pwCaption)[cUntitledLen - 1] = '1'; //same as
//wMainWindow.SzWinCaption [cUntitledLen - 1] = '1';
strcpy_s( pwChildWindow->*pwCaption, cUntitledLen, szUntitled );
(pwChildWindow->*pwCaption)[cUntitledLen - 1] = '2'; //same as //pwChildWindow->szWinCaption[cUntitledLen - 1] = '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.)
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.
enum {
sizeOfBuffer = 100
};
char szCaptionBase[sizeOfBuffer];
// Copy the main window caption into the buffer
// and append " [View 1]".
strcpy_s( szCaptionBase, sizeOfBuffer, (wMainWindow.*pfnwGC)() );
strcat_s( szCaptionBase, sizeOfBuffer, " [View 1]" );
// Set the child window's caption.
(pwChildWindow->*pfnwSC)( szCaptionBase );