Pointers to Members

 

The latest version of this topic can be found at 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];  

1. 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.

2. 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 );  

Restrictions on Pointers to Members

The address of a static member is not a pointer to a member. It is a regular pointer to the one instance of the static member. Because only one instance of a static member exists for all objects of a given class, the ordinary address-of (&) and dereference (*) operators can be used.

Pointers to Members and Virtual Functions

Invoking a virtual function through a pointer-to-member function works as if the function had been called directly; the correct function is looked up in the v-table and invoked.

The key to virtual functions working, as always, is invoking them through a pointer to a base class. (For more information about virtual functions, see Virtual Functions.)

The following code shows how to invoke a virtual function through a pointer-to-member function:

// virtual_functions.cpp  
// compile with: /EHsc  
#include <iostream>  
using namespace std;  
  
class Base  
{  
public:  
virtual void Print();  
};  
void (Base ::* bfnPrint)() = &Base :: Print;  
void Base :: Print()  
{  
cout << "Print function for class Base\n";  
}  
  
class Derived : public Base  
{  
public:  
void Print();  // Print is still a virtual function.  
};  
  
void Derived :: Print()  
{  
cout << "Print function for class Derived\n";  
}  
  
int main()  
{  
    Base   *bPtr;  
    Base    bObject;  
Derived dObject;  
bPtr = &bObject;    // Set pointer to address of bObject.  
(bPtr->*bfnPrint)();  
bPtr = &dObject;    // Set pointer to address of dObject.  
(bPtr->*bfnPrint)();  
}  
  
//Output: Print function for class Base  
Print function for class Derived  

See Also

C++ Abstract Declarators