Pointers to Members

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 );

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.

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

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