INFO: Creating a Function Pointer to a C++ Member Function

• Microsoft C/C++ for MS-DOS, versions 7.0, 7.0a
• Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51
• Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0, 5.0

SUMMARY

The text below describes generating a function pointer to a class member function in code compiled with Microsoft C/C++ version 7.0 or later. The declaration of a pointer to a class member function must include the class name. However, the class name is omitted from the declaration of a pointer to a static member function.

MORE INFORMATION

In the C and C++ languages, an application can define a variable that contains the address of a function. You can call the function using this variable instead of through the function name. In C++, a pointer to a nonstatic member function contains the address of the function in the class, not in the object. To call the function, use the appropriate member selection operator (. or ->), the indirection operator (*), and the name of an object of the class.

In most cases in the C language, function pointer declarations take the following form:

   int (*ptr)();

To create a pointer to a C++ class member function, specify the class name in the function pointer declaration, as follows:

   int (Sample::*ptr)();

The different ways to interpret the empty parentheses in function declarations is a major difference between the C and C++ languages. In C, a function pointer declaration that has no arguments is syntactically identical to the following:

   int (*ptr)(...);

   int (Sample::*ptr)(void);

   int (Sample::*ptr2)(int, int);

To determine the procedure address to assign to a pointer variable, use the class name and the scope resolution operator (::). This syntax provides flexibility because a declared variable can contain the address of any object of the class. The object name in the function call determines the copy of the function used.

The sample code below demonstrates declaring and using pointers to a class member function and to a static member function. Note that when the arguments in the function pointer declaration do not match the arguments of the function assigned to the pointer, the compiler generates errors C2440 and C2564. For example, if the "int" declaration is omitted from the argument list declaration for the function pointer, the compiler generates the following error messages:

16-bit

   error C2440: 'initializing' : cannot convert from 'void (__far __pascal
     Data::*)(int )__near ' to 'void (__far __pascal Data::*)(void )__near'

   error C2564: formal/actual parameters mismatch in call through pointer
     to function

32-bit

   error C2440: 'initializing' : cannot convert from 'void (Data::*)(int)
     to 'void (Data::*)(void)"

   error C2197: 'void (Data::*)(void)' : too many actual parameters

Sample Code

   /*
    * Compile options needed: None
    */

   #include <iostream.h>

   class Data
   {
   private:
      int y;
      static int x;

   public:
      void SetData(int value) {y = value; return;};
      int GetData() {return y;};
      static void SSetData(int value) {x = value; return;};
      static int SGetData() {return x;};
   };

   int Data::x = 0;

   void main(void)
   {
      Data mydata, mydata2;

      // Initialize pointer.
      void (Data::*pmfnP)(int) = &Data::SetData; // mydata.SetData;

      // Initialize static pointer.
      void (*psfnP)(int) = &Data::SSetData;

      mydata.SetData(5); // Set initial value for private data.
      cout << "mydata.data = " << mydata.GetData() << endl;

      (mydata.*pmfnP)(20); // Call member function through pointer.
      cout << "mydata.data = " << mydata.GetData() << endl;

      (mydata2.*pmfnP)(10) ; // Call member function through pointer.
      cout << "mydata2.data = " << mydata2.GetData() << endl;

      (*psfnP)(30) ; // Call static member function through pointer.
      cout << "static data = " << Data::SGetData() << endl ;
   }

   ARTICLE-ID: Q79845
   TITLE     : INFO: Old Style (K&R) Declarations Are Not Supported in C++