Microsoft KB Archive/43703

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

Article ID: 43703

Article Last Modified on 7/5/2005

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APPLIES TO


 * Microsoft C Professional Development System 6.0a
 * Microsoft Visual C++ 1.0 Professional Edition
 * Microsoft Visual C++ 1.5 Professional Edition
 * Microsoft Visual C++ 1.51
 * Microsoft Visual C++ 1.52 Professional Edition
 * Microsoft Visual C++ 2.0 Professional Edition
 * Microsoft Visual C++ 2.1
 * Microsoft Visual C++ 4.0 Standard Edition
 * Microsoft Visual C++ 5.0 Enterprise Edition
 * Microsoft Visual C++ 6.0 Enterprise Edition
 * Microsoft Visual C++ 5.0 Professional Edition
 * Microsoft Visual C++ 6.0 Professional Edition
 * Microsoft Visual C++ 6.0 Standard Edition

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This article was previously published under Q43703





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 &quot;ptr&quot; to the array type &quot;Arr2Dim&quot; 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 &quot;ptr&quot; to a pointer type equivalent to the array type Arr2Dim. This pointer type can be defined as follows: typedef char (*Ptr2Dim) [20] ; Casting &quot;ptr&quot; to the type of &quot;Ptr2Dim&quot;, 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:
 * 1) include 

typedef char (*Ptr3Dim) [20][30] ; Ptr3Dim ptr3arr ; ... void main (void) {   ...    ptr3arr = (Ptr3Dim) malloc (10 * sizeof(char) * 20 * 30) ; ... } After the allocation, &quot;ptr3arr&quot; 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: kbinfo kblangc KB43703

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