Re: [boost] Re: dangerous_cast<>

From: David Abrahams (dave_at_[hidden])
Date: 2002-12-04 14:04:53

"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));
     ...

-- 
                       David Abrahams
   dave_at_[hidden] * http://www.boost-consulting.com
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