Q43703: INFO: Casting a Pointer to Type Equivalent to Multidimensional

Article: Q43703
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,1.51,1.52,2.0,2.1,4.0,5.0,6.0
Operating System(s): 
Keyword(s): kbLangC kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600
Last Modified: 29-NOV-2001

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The information in this article applies to:

- Microsoft C for MS-DOS 
- Microsoft C/C++ for MS-DOS 
- Microsoft Visual C++, versions 1.0, 1.5, 1.51, 1.52, 2.0, 2.1, 4.0 
- Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
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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' ;

Additional query words:

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Keywords          : kbLangC kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600 
Technology        : kbVCsearch kbVC400 kbAudDeveloper kbZNotKeyword8 kbvc150 kbvc100 kbCCompSearch kbZNotKeyword3 kbVC500 kbVC600 kbVC151 kbVC200 kbVC210 kbVC32bitSearch kbVC152 kbVC500Search
Version           : :1.0,1.5,1.51,1.52,2.0,2.1,4.0,5.0,6.0
Issue type        : kbinfo

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