on Wed Oct 31 2012, Antony Polukhin <antoshkka-AT-gmail.com> wrote:
2012/10/31 Dave Abrahams <dave@boostpro.com>
on Wed Oct 31 2012, "Peter Dimov" <lists-AT-pdimov.com> wrote:
Steven Watanabe wrote:
I believe that I've seen tickets in trac indicating that at least one STL implementation checks that the element type is complete, so we can't even assume that it works in practice.
The variant could perhaps define some dummy complete type X and allocate space for vector<X>, betting on the fact that vector<Y> would have the same size. And if it doesn't, well, the heap is it then.
Yeah, I thought of that. Very interesting problem space in the end. I'm slightly tempted to think about a rewrite.
May be it would be better to write a metafunction:
template <class T> size_forward { enum ENUM{value = sizeof(T)}; };
User is free to specialize it, so Boost.Variant can use it to get sizes of types (instead of directly calling to sizeof(T))
Specialization for your case will be:
template <> struct size_forward<vector<Y>> { enum ENUM{value = sizeof(vector<X>)}; };
I think that such metafunction could be widely used (for example in Boost.Optional)
As far as I can tell, putting sizeof() inside a layer of template instantiation doesn't solve anything. But maybe I'm missing something. -- Dave Abrahams BoostPro Computing Software Development Training http://www.boostpro.com Clang/LLVM/EDG Compilers C++ Boost