David Abrahams <[EMAIL PROTECTED]> writes: | Gabriel Dos Reis <[EMAIL PROTECTED]> writes: | | > David Abrahams <[EMAIL PROTECTED]> writes: | > | > | "Eric Woodruff" <[EMAIL PROTECTED]> writes: | > | | > | > "David B. Held" <[EMAIL PROTECTED]> wrote in message | > | > aslftb$cr2$[EMAIL PROTECTED]">news:aslftb$cr2$[EMAIL PROTECTED]... | > | >> "Eric Woodruff" <[EMAIL PROTECTED]> wrote in message | > | >> aslbsn$nt3$[EMAIL PROTECTED]">news:aslbsn$nt3$[EMAIL PROTECTED]... | > | >> > [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? ;-)
Yeah ;-) | If the implementation is such that the same bit pattern must be used | to reference a given address, using reinterpret_cast would do | something like | | 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. Thanks for your patience. Now, imagine an implementation where the original type is remembered, then dereferencing the pointer obtained from char* -> void* -> T* may just eject you into the outer space. -- Gaby _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost
