INFO: Casting a Pointer to Type Equivalent to Multidimensional

 Last reviewed: September 2, 1997
Article ID: Q43703

The information in this article applies to:
  • Microsoft C for MS-DOS, versions 5.1, 6.0, 6.0a, 6.0ax
  • Microsoft C/C++ for MS-DOS, version 7.0
  • Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51, 1.52
  • Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0, 5.0

SUMMARY

In some situations you may need to cast a pointer or an address to a type that is equivalent to a multidimensional array. The following example demonstrates such a situation: 

   typedef char Arr2Dim [][20] ;
   void myfunc (Arr2Dim) ;
   char * ptr ;
   ...
   void main (void)
   {
       ...
       myfunc ( (Arr2Dim) ptr ) ;  /* illegal */
       ...
   }
Casting the variable "ptr" to the array type "Arr2Dim" is not correct. The Microsoft C Compiler displays the following error message: 

   error C2067: cast to array type is illegal
The correct procedure is to cast the pointer "ptr" to a pointer type equivalent to the array type Arr2Dim. This pointer type can be defined as follows: 

   typedef char (*Ptr2Dim) [20] ;
Casting "ptr" to the type of "Ptr2Dim", as follows, is correct and produces no warning messages when compiled at warning level 3: 

   myfunc ( (Ptr2Dim) ptr ) ;
The address (or pointer) passed to the function is used correctly.

MORE INFORMATION

A similar solution may be applied to the problem of dynamically allocating a multidimensional array. For example, the following code fragment allocates a memory block, which can be used as a 10 x 20 x 30 three- dimensional array: 

   #include <malloc.h>

   typedef char (*Ptr3Dim) [20][30] ;
   Ptr3Dim ptr3arr ;
   ...
   void main (void)
   {
       ...
       ptr3arr = (Ptr3Dim) malloc (10 * sizeof(char) * 20 * 30) ;
       ...
   }
After the allocation, "ptr3arr" can be used as a three-dimensional array, as follows, provided i, j, and k are integers within the proper range: 

   ptr3arr [i][j][k] = 'a' ;

Keywords          : CLngIss kbfasttip
Version           : MS-DOS:6.0,6.00a,6.00ax,7.0; WINDOWS:1.0,1.5,1.51,1.52;  WINDOWS NT:1.0,2.0,2.1,4.0,5.0
Platform          : MS-DOS NT WINDOWS
Issue type        : kbinfo

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Last reviewed: September 2, 1997
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