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A.4 Pointer conversions

Visual Studio .NET 2003

In an unsafe context, the set of available implicit conversions (Section 6.1) is extended to include the following implicit pointer conversions:

  • From any pointer-type to the type void*.
  • From the null type to any pointer-type.

Additionally, in an unsafe context, the set of available explicit conversions (Section 6.2) is extended to include the following explicit pointer conversions:

  • From any pointer-type to any other pointer-type.
  • From sbyte, byte, short, ushort, int, uint, long, or ulong to any pointer-type.
  • From any pointer-type to sbyte, byte, short, ushort, int, uint, long, or ulong.

Finally, in an unsafe context, the set of standard implicit conversions (Section 6.3.1) includes the following pointer conversion:

  • From any pointer-type to the type void*.

Conversions between two pointer types never change the actual pointer value. In other words, a conversion from one pointer type to another has no effect on the underlying address given by the pointer.

When one pointer type is converted to another, if the resulting pointer is not correctly aligned for the pointed-to type, the behavior is undefined if the result is dereferenced. In general, the concept "correctly aligned" is transitive: if a pointer to type A is correctly aligned for a pointer to type B, which, in turn, is correctly aligned for a pointer to type C, then a pointer to type A is correctly aligned for a pointer to type C.

Consider the following case in which a variable having one type is accessed via a pointer to a different type:

char c = 'A';
char* pc = &c;
void* pv = pc;
int* pi = (int*)pv;
int i = *pi;      // undefined
*pi = 123456;      // undefined

When a pointer type is converted to a pointer to byte, the result points to the lowest addressed byte of the variable. Successive increments of the result, up to the size of the variable, yield pointers to the remaining bytes of that variable. For example, the following method displays each of the eight bytes in a double as a hexadecimal value:

using System;
class Test
{
   unsafe static void Main() {
      double d = 123.456e23;
      unsafe {
         byte* pb = (byte*)&d;
         for (int i = 0; i < sizeof(double); ++i)
            Console.Write("{0:X2} ", *pb++);
         Console.WriteLine();
      }
   }
}

Of course, the output produced depends on endianness.

Mappings between pointers and integers are implementation-defined. However, on 32- and 64-bit CPU architectures with a linear address space, conversions of pointers to or from integral types typically behave exactly like conversions of uint or ulong values, respectively, to or from those integral types.

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