Thread: Why am I getting this error?

I keep getting this error (I'm using Visual C++ 2005 Express Edition by the way):

error C2440: '=' : cannot convert from 'void *' to 'ListNodePtr'

Code:

/* Operating and maintaining a list */
#include<stdio.h>
#include<stdlib.h>

/* self-referential structure */
struct listNode {            
   char data; /* each listNode contains a character */
   struct listNode *nextPtr; /* pointer to next node*/ 
}; /* end structure listNode */

typedef struct listNode ListNode; /* synonym for struct listNode */
typedef struct listNode *ListNodePtr; /* synonym for ListNode* */

/* prototypes */
void insert( ListNodePtr *sPtr, char value );
char del( ListNodePtr *sPtr, char value );
int isEmpty( ListNodePtr sPtr );
void printList( ListNodePtr currentPtr );
void instructions( void );

int main( void )
{ 
   ListNodePtr startPtr = NULL; /* initially there are no nodes */
   int choice; /* user's choice */
   char item;  /* char entered by user */

   instructions(); /* display the menu */
   printf("? ");
   scanf_s("%d", &choice);

   /* loop while user does not choose 3 */
   while ( choice != 3 ) { 

      switch ( choice ) { 

         case 1:
            printf( "Enter a character: " );
            scanf_s( "\n%c", &item );
            insert( &startPtr, item ); /* insert item in list */
            printList( startPtr );
            break;

         case 2:

            /* if list is not empty */
            if ( !isEmpty( startPtr ) ) { 
               printf( "Enter character to be deleted: " );
               scanf_s( "\n%c", &item );

               /* if character is found, remove it */
               if ( del( &startPtr, item ) ) { /* remove item */
                  printf( "%c deleted.\n", item );
                  printList( startPtr );
               } /* end if */
               else {
                  printf( "%c not found.\n\n", item );
               } /* end else */

            } /* end if */
            else {
               printf( "List is empty.\n\n" );
            } /* end else */

            break;

         default:
            printf( "Invalid choice.\n\n" );
            instructions();
            break;
      
      } /* end switch */

      printf( "? " );
      scanf_s( "%d", &choice );
   } /* end while */

   printf( "End of run.\n" );
   
   return 0; /* indicates successful termination */

} /* end main */

/* display program instructions to user */
void instructions( void )
{ 
   printf( "Enter your choice:\n"
      "   1 to insert an element into the list.\n"
      "   2 to delete an element from the list.\n"
      "   3 to end.\n" );
} /* end function instructions */

/* Insert a new value into the list in sorted order */
void insert( ListNodePtr *sPtr, char value )
{ 
   ListNodePtr newPtr;      /* pointer to new node */
   ListNodePtr previousPtr; /* pointer to previous node in list */
   ListNodePtr currentPtr;  /* pointer to current node in list */

   newPtr = malloc( sizeof(ListNode) ); /* create node */  <- this is where I start getting the error

   if ( newPtr != NULL ) { /* is space available */
      newPtr->data = value; /* place value in node */
      newPtr->nextPtr = NULL; /* node does not link to another node */

      previousPtr = NULL;
      currentPtr = *sPtr;

      /* loop to find the correct location in the list */
      while ( currentPtr != NULL && value > currentPtr->data ) { 
         previousPtr = currentPtr;          /* walk to ...   */
         currentPtr = currentPtr->nextPtr;  /* ... next node */
      } /* end while */

      /* insert new node at beginning of list */
      if ( previousPtr == NULL ) { 
         newPtr->nextPtr = *sPtr;
         *sPtr = newPtr;
      } /* end if */
      else { /* insert new node between previousPtr and currentPtr */
         previousPtr->nextPtr = newPtr;
         newPtr->nextPtr = currentPtr;
      } /* end else */
   
   } /* end if */
   else {
      printf( "%c not inserted. No memory available.\n", value );
   } /* end else */

} /* end function insert */

/* Delete a list element */
char del( ListNodePtr *sPtr, char value )
{ 
   ListNodePtr previousPtr; /* pointer to previous node in list */
   ListNodePtr currentPtr;  /* pointer to current node in list */
   ListNodePtr tempPtr;     /* temporary node pointer */

   /* delete first node */
   if ( value == ( *sPtr )->data ) { 
      tempPtr = *sPtr; /* hold onto node being removed */
      *sPtr = ( *sPtr )->nextPtr; /* de-thread the node */
      free( tempPtr ); /* free the de-threaded node */
      return value;
   } /* end if */
   else { 
      previousPtr = *sPtr;
      currentPtr = ( *sPtr )->nextPtr;

      /* loop to find the correct location in the list */
      while ( currentPtr != NULL && currentPtr->data != value ) { 
         previousPtr = currentPtr;         /* walk to ...   */
         currentPtr = currentPtr->nextPtr; /* ... next node */  
      } /* end while */

      /* delete node at currentPtr */
      if ( currentPtr != NULL ) { 
         tempPtr = currentPtr;
         previousPtr->nextPtr = currentPtr->nextPtr;
         free( tempPtr );
         return value;
      } /* end if */
     
   } /* end else */

   return '\0';

} /* end function delete */

/* Return 1 if the list is empty, 0 otherwise */
int isEmpty( ListNodePtr sPtr )
{ 
   return sPtr == NULL;

} /* end function isEmpty */

/* Print the list */
void printList( ListNodePtr currentPtr )
{ 

   /* if list is empty */
   if ( currentPtr == NULL ) {
      printf( "List is empty.\n\n" );
   } /* end if */
   else { 
      printf( "The list is:\n" );

      /* while not the end of the list */
      while ( currentPtr != NULL ) { 
         printf( "%c --> ", currentPtr->data );
         currentPtr = currentPtr->nextPtr;   
      } /* end while */

      printf( "NULL\n\n" );
   } /* end else */

} /* end function printList */

It looks like you are compiling as C++. Change the source file extension to .c and configure the project settings to compile as C.

Why? When I ran my other C programs, I didn't have to specify their extension as ".c". They simply ran. =/
Also, there is only one error in the program I posted. =/

Alright. You're right. I finally got it! Yay! Thanks much. ^^,

Originally Posted by We'reGeeks

Also, there is only one error in the program I posted. =/

It is precisely the nature of that error that allowed me to deduce that you are compiling the code as C++

In C, an implicit conversion from void* (i.e., the return type of malloc) to ListNodePtr is allowed. In C++, it requires an explicit cast.