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<H1> 13.7.2 The Package System.Address_To_Access_Conversions</H1>
<H4 ALIGN=CENTER>Static Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> The following language-defined
generic library package exists: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="Examples"><TT><A NAME="I4574"></A><A NAME="I4575"></A><A NAME="I4576"></A><B>generic</B><BR>
<B>type</B> Object(<>) <B>is</B> <B>limited</B> <B>private</B>;<BR>
<B>package</B> System.Address_To_Access_Conversions <B>is</B><BR>
<B>pragma</B> Preelaborate(Address_To_Access_Conversions);</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="Examples"><TT> <B>type</B> Object_Pointer <B>is</B> <B>access</B> <B>all</B> Object;<BR>
<B>function</B> <A NAME="I4577"></A><A NAME="I4578"></A>To_Pointer(Value : Address) <B>return</B> Object_Pointer;<BR>
<B>function</B> <A NAME="I4579"></A><A NAME="I4580"></A>To_Address(Value : Object_Pointer) <B>return</B> Address;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="Examples"><TT> <B>pragma</B> Convention(Intrinsic, To_Pointer);<BR>
<B>pragma</B> Convention(Intrinsic, To_Address);<BR>
<B>end</B> System.Address_To_Access_Conversions;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="Normal"> 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 <B>null</B>. <A NAME="I4581"></A>For
other addresses, the behavior is unspecified. To_Address(<B>null</B>)
returns Null_Address (for <B>null</B> of the appropriate type). To_Address(Y),
where Y /= <B>null</B>, returns Y.<B>all</B>'Address. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Discussion: </B>The programmer
should ensure that the address passed to To_Pointer is either Null_Address,
or the address of an object of type Object. Otherwise, the behavior of
the program is unspecified; it might raise an exception or crash, for
example. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>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.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5.c</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>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. </FONT></DIV>
<H4 ALIGN=CENTER>Implementation Permissions</H4>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="Normal"> An implementation may place restrictions on instantiations
of Address_To_Access_Conversions. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>For example,
if the hardware requires aligned loads and stores, then dereferencing
an access value that is not properly aligned might raise an exception.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>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. </FONT></DIV>
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