From: David Abrahams (dave_at_[hidden])
Date: 2002-12-04 14:45:30
Gabriel Dos Reis <gdr_at_[hidden]> writes:
> David Abrahams <dave_at_[hidden]> writes:
> | "Eric Woodruff" <Eric.Woodruff_at_[hidden]> writes:
> | > "David B. Held" <dheld_at_[hidden]> wrote in message
> | > news:aslftb$cr2$1_at_main.gmane.org...
> | >> "Eric Woodruff" <Eric.Woodruff_at_[hidden]> wrote in message
> | >> news:aslbsn$nt3$1_at_main.gmane.org...
> | >> > [snip]
> | >> >
> | >> > > holder<Foo> h;
> | >> > > new (h.storage) Foo;
> | >> >
> | >> > What is the meaning of that syntax?
> | >>
> | >> This is placement new syntax. It means construct a Foo at the address
> | >> h.storage, without allocating any memory.
> | >>
> | >
> | > So the type really is of Foo, which has to mean that casting h.storage back
> | > to a Foo* using reinterpret_cast is covered by the standard.
> | No, the standard only guarantees that you can do a round-trip
> | cast. The pointer didn't start out as a Foo*. The fact that it has the
> | same address as a Foo* doesn't mean anything. Just for example,
> | something like the following is a perverse but legal reinterpret_cast
> | implementation:
> | if is_pointer<source_type> and is_pointer<dest_type>
> | return (dest_type)(
> | (unisgned)src
> | ^ sizeof(remove_pointer<source_type>::type)
> | ^ sizeof(remove_pointer<dest_type>::type));
> And why isn't that applicable to the trip
> char* -> void* -> Foo*
You mean aside from the fact that sizeof(void) is illegal? ;-)
If the implementation is such that the same bit pattern must be used
to reference a given address, using reinterpret_cast would do
p ^ sizeof(char) ^ sizeof(void) ^ sizeof(void) ^ sizeof(Foo)
and, assuming sizeof(void) is well-defined the resulting pointer would
have been xor'ed with sizeof(char) and sizeof(Foo), so it would point
into outer space.
-- David Abrahams dave_at_[hidden] * http://www.boost-consulting.com Boost support, enhancements, training, and commercial distribution