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13.7.2 The Package System.Address_To_Access_Conversions

Static Semantics

1
The following language-defined generic library package exists: 
2
generic
    type Object(<>) is limited private;
package System.Address_To_Access_Conversions is
   pragma Preelaborate(Address_To_Access_Conversions);
3/3
{AI05-0229-1}    type Object_Pointer is access all Object;
   function To_Pointer(Value : Address) return Object_Pointer
      with Convention => Intrinsic;
   function To_Address(Value : Object_Pointer) return Address
      with Convention => Intrinsic;
4/3
{AI05-0229-1} end System.Address_To_Access_Conversions;
5/2
{AI95-00230-01} The To_Pointer and To_Address subprograms convert back and forth between values of types Object_Pointer and Address. To_Pointer(X'Address) is equal to X'Unchecked_Access for any X that allows Unchecked_Access. To_Pointer(Null_Address) returns null. For other addresses, the behavior is unspecified. To_Address(null) returns Null_Address. To_Address(Y), where Y /= null, returns Y.all'Address. 
5.a/3
Discussion: {AI95-00114-01} {AI05-0005-1} The programmer should ensure that the address passed to To_Pointer is either Null_Address, or the address of an object of type Object. (If Object is not a by-reference type, the object ought to be aliased; recall that the Address attribute is not required to provide a useful result for other objects.) Otherwise, the behavior of the program is unspecified; it might raise an exception or crash, for example. 
5.b
Reason: Unspecified is almost the same thing as erroneous; they both allow arbitrarily bad behavior. We don't say erroneous here, because the implementation might allow the address passed to To_Pointer to point at some memory that just happens to “look like” an object of type Object. That's not necessarily an error; it's just not portable. However, if the actual type passed to Object is (for example) an array type, the programmer would need to be aware of any dope that the implementation expects to exist, when passing an address that did not come from the Address attribute of an object of type Object.
5.c
One might wonder why To_Pointer and To_Address are any better than unchecked conversions. The answer is that Address does not necessarily have the same representation as an access type. For example, an access value might point at the bounds of an array when an address would point at the first element. Or an access value might be an offset in words from someplace, whereas an address might be an offset in bytes from the beginning of memory. 

Implementation Permissions

6
An implementation may place restrictions on instantiations of Address_To_Access_Conversions. 
6.a
Ramification: For example, if the hardware requires aligned loads and stores, then dereferencing an access value that is not properly aligned might raise an exception.
6.b
For another example, if the implementation has chosen to use negative component offsets (from an access value), it might not be possible to preserve the semantics, since negative offsets from the Address are not allowed. (The Address attribute always points at “the first of the storage elements....”) Note that while the implementation knows how to convert an access value into an address, it might not be able to do the reverse. To avoid generic contract model violations, the restriction might have to be detected at run time in some cases. 

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