Lesson 14. Pattern 6. Changing an array's type

24.01.2012

Sometimes it is necessary (or simply convenient) to present array items in the form of items of another type. The following code example shows dangerous and safe type conversions:

int array[4] = { 1, 2, 3, 4 };
enum ENumbers { ZERO, ONE, TWO, THREE, FOUR };
//safe cast (for MSVC)
ENumbers *enumPtr = (ENumbers *)(array);
cout << enumPtr[1] << " ";
//unsafe cast
size_t *sizetPtr = (size_t *)(array);
cout << sizetPtr[1] << endl;
//Output on 32-bit system: 2 2
//Output on 64-bit system: 2 17179869187

As you may see, the output result of the program differs in the 32-bit and 64-bit versions. On the 32-bit system, the access to the array items is correct because the sizes of the types size_t and "int" coincide, so we see the result "2 2".

On the 64-bit system, the output result is "2 17179869187" because it is the value 17179869187 which is located in the 1-st item of the array sizetPtr (see Figure 1). Sometimes it is this behavior you need but usually it is considered an error.

Note. The type enum in Visual C++ compiler by default coincides with the type int, i.e. it is a 32-bit type. You may use enum of another size only with the help of an extension considered non-standard in Visual C++. So the example above is correct from the viewpoint of Visual C++ compiler but from the viewpoint of other compilers conversion of a pointer to int items to a pointer to enum items may be also incorrect.

Picture 1073343

Figure 1 - Arrangement of array items in memory

To get rid of this incorrectness you should refuse to use unsafe type conversions and modify the program. Another way is to create a new array and copy the values from the original array into it.

You may encounter the described error pattern most often in the code fragments where programmers try to use pointer values as unique 32-bit identifiers.

Diagnosis

Unsafe changes of an array's type are diagnosed by the tool PVS-Studio. The analyzer warns you about potentially dangerous type conversions with the diagnostic warning V114. Accordingly, the analyzer responds only to those constructs that may cause an error on a 64-bit system. For example, the following code sample is correct and the analyzer will not call the programmer's attention to it:

void **pointersArray;
ptrdiff_t correctID = ((ptrdiff_t *)pointersArray)[index];

The course authors: Andrey Karpov (karpov@viva64.com), Evgeniy Ryzhkov (evg@viva64.com).

The rightholder of the course "Lessons on development of 64-bit C/C++ applications" is OOO "Program Verification Systems". The company develops software in the sphere of source program code analysis. The company's site: http://www.viva64.com.

Contacts: e-mail: support@viva64.com, Tula, 300027, PO box 1800.