"Eric Woodruff" <[EMAIL PROTECTED]> writes: | "Gabriel Dos Reis" <[EMAIL PROTECTED]> wrote in message | [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... | > "Eric Woodruff" <[EMAIL PROTECTED]> writes: | > | > [...] | > | > | > Thus, given that h.storage is properly aligned, (which is the purpose | of | > | the | > | > other union member), after "new(h.storage) Foo", h.storage contains a | Foo | > | > object. Thus accessing it through a pointer-to-Foo is legal, as Foo is | the | > | > dynamic type of the object. | > | > | > | | > | This is precisely my reasoning why reinterpret_cast<> is _not_ | > | implementation defined. | > | | > | It must be the case that it is equal to the situation of having | > | | > | Foo* -> char */void* -> Foo* if Foo is the dynamic type of the object. | > | > | > After giving more thoughts to the analysis given by Anthony and | > 5.2.10/1, I think I agree with you're right. | > | | Well, for a reinterpret_cast to be valid, it must be used to cast away from | the type, and return to it.
Not necessarily: It is permitted to have the source type be identitical to the target type. But that is a minor nit. | In my example, I was only returning to it. I | question the validity of that. If new (p) Object; takes a void* and does an | implicit cast, I don't think there is any reinterpret_casting taking place | on the inside. Certainly, there is no reinterpret_cast<> happening here. However, the result of reinterpret_cast<>ing a Foo* to void* is the result of converting a Foo* to void* (that is what I understand from 5.2.10/1), therefore the two happen to have the same value there. | However, I think there must be some kind of cast to char* to | allocate the memory in number of bytes, but I'm not sure. The size of char | is not necessarily 1 byte, correct? So either way, this allocation | implementation defines what happens. I'm afraid I don't undertand this part. Could you elaborate? What do you mean by "The size of char is not necessarily 1 byte". | Again, however, I mentioned in a different post that we could always force a | reinterpret_cast<> via new (reinterpret_cast<void*> (h.storage)) Object, but | if an implementation remembers that h.storage is an unsigned char*, or even | if it doesn't, I think reinterpret_cast may only be valid to cast void* | h.storage to unsigned char* h.storage. Anthony convincingly showed that provided (1) h.storage is properly aligned (2) an object is properly constructed at h.storage the type rules allows one to reinterpret_cast<>/static_cast<> to a Foo* -- even an implementation that monitors memory access ought to to accept that usage. Or do you think I misundertstand your point? -- Gaby _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost
