c function pointer example

 

  
    

   
     /*
  
    
  
    

   
     

  
    
  
    

   
     
   
     In the following example we regard the task to perform one of the four
  
    
  
    

   
     basic arithmetic operations. The task 
   
     is
   
      first solved 
   
     using
   
      a 
   
     switch
   
     -
   
     statement.
  
    
  
    

   
     Then it 
   
     is
   
      shown, how the same can be done 
   
     using
   
      a function pointer. 
  
    
  
    

   
     It
   
     '
   
     s only an example and the task is so easy that I suppose nobody will 
  
    
  
    

   
     ever use a function pointer for it ;-)
   
     

   
      
  
    
  
    

   
     */


   
     //
   
     ------------------------------------------------------------------------------------

   
     //
   
      1.2 Introductory Example or How to Replace a Switch-Statement

   
     //
   
      Task: Perform one of the four basic arithmetic operations specified by the

   
     //
   
      characters '+', '-', '*' or '/'.



   
     //
   
      The four arithmetic operations ... one of these functions is selected

   
     //
   
      at runtime with a swicth or a function pointer
   
     

   
     float
   
      Plus (
   
     float
   
      a, 
   
     float
   
      b) { 
   
     return
   
      a
   
     +
   
     b; }

   
     float
   
      Minus (
   
     float
   
      a, 
   
     float
   
      b) { 
   
     return
   
      a
   
     -
   
     b; }

   
     float
   
      Multiply(
   
     float
   
      a, 
   
     float
   
      b) { 
   
     return
   
      a
   
     *
   
     b; }

   
     float
   
      Divide (
   
     float
   
      a, 
   
     float
   
      b) { 
   
     return
   
      a
   
     /
   
     b; }



   
     //
   
      Solution with a switch-statement - <opCode> specifies which operation to execute
   
     

   
     void
   
      Switch(
   
     float
   
      a, 
   
     float
   
      b, 
   
     char
   
      opCode)
{
 
   
     float
   
      result;

 
   
     //
   
      execute operation
   
     

   
      
   
     switch
   
     (opCode)
 {
 
   
     case
   
      
   
     '
   
     +
   
     '
   
      : result 
   
     =
   
      Plus (a, b); 
   
     break
   
     ;
 
   
     case
   
      
   
     '
   
     -
   
     '
   
      : result 
   
     =
   
      Minus (a, b); 
   
     break
   
     ;
 
   
     case
   
      
   
     '
   
     *
   
     '
   
      : result 
   
     =
   
      Multiply (a, b); 
   
     break
   
     ;
 
   
     case
   
      
   
     '
   
     /
   
     '
   
      : result 
   
     =
   
      Divide (a, b); 
   
     break
   
     ;
 }

 cout 
   
     <<
   
      
   
     "
   
     Switch: 2+5=
   
     "
   
      
   
     <<
   
      result 
   
     <<
   
      endl; 
   
     //
   
      display result
   
     

   
     }



   
     //
   
      Solution with a function pointer - <pt2Func> is a function pointer and points to

   
     //
   
      a function which takes two floats and returns a float. The function pointer

   
     //
   
      "specifies" which operation shall be executed.
   
     

   
     void
   
      Switch_With_Function_Pointer(
   
     float
   
      a, 
   
     float
   
      b, 
   
     float
   
      (
   
     *
   
     pt2Func)(
   
     float
   
     , 
   
     float
   
     ))
{
 
   
     float
   
      result 
   
     =
   
      pt2Func(a, b); 
   
     //
   
      call using function pointer
   
     

   
     
 cout 
   
     <<
   
      
   
     "
   
     Switch replaced by function pointer: 2-5=
   
     "
   
     ; 
   
     //
   
      display result
   
     

   
      cout 
   
     <<
   
      result 
   
     <<
   
      endl;
}



   
     //
   
      Execute example code
   
     

   
     void
   
      Replace_A_Switch()
{
 cout 
   
     <<
   
      endl 
   
     <<
   
      
   
     "
   
     Executing function 'Replace_A_Switch'
   
     "
   
      
   
     <<
   
      endl;

 Switch(
   
     2
   
     , 
   
     5
   
     , 
   
     /*
   
      '+' specifies function 'Plus' to be executed 
   
     */
   
      
   
     '
   
     +
   
     '
   
     );
 Switch_With_Function_Pointer(
   
     2
   
     , 
   
     5
   
     , 
   
     /*
   
      pointer to function 'Minus' 
   
     */
   
      
   
     &
   
     Minus);
}