Re: Object layout guarantees and manual pointer adjustments

From:

Francis Glassborow <francis.glassborow@btinternet.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Mon, 20 Apr 2009 13:49:04 CST

Message-ID:

<-rSdnbqs5r1I-nHUnZ2dnUVZ8s-dnZ2d@bt.com>

Pavel Minaev wrote:

On Apr 18, 3:42 pm, Francis Glassborow
<francis.glassbo...@btinternet.com> wrote:

I cannot imagine how a system might work where the value of sizeof(T)
was the result of summing two or more non-contiguous chunks of storage.

I think the following code must always work for all types T (given
enough memory is available)

int main(){
void* mem_ptr = operator new( sizeof(T));
T* T_obj = new (mem_ptr) T;

}

Now explain, if you can, how that could work if the base memory for T
could be non-contiguous.

By allocating any additional memory during the evaluation of the
expression "new T", and not counting that memory for the purpose of
sizeof().

Effectively that would be using dynamic memory for part of an object. I
doubt that such is allowed to a conforming implementation. I am also
pretty certain that no existing implementation does any such thing.

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