Steven Watanabe wrote:
AMDG
Larry Evans wrote:
To make x2 compile, add an extra {}, IOW
x2 = { { {1}, {1} } }
where the extran {} is for expr2::args.
So, the cost of this extra indirection is 1 more curly braces in every expr initializer list?
Exactly. If you recall I said that it changes the interface. Not that it doesn't work. In short, the existence of the "private" partially specialized version of expr would not entirely be an implementation detail.
This may also add overhead because of the extra template. On the other hand, it may compile faster because the operator() overloads don't need to be duplicated.
Oh. I just realized that this would also force expr::make to change. expr::make() takes a number of arguments equal to the arity so it needs to be in the private struct as well. I'm afraid that this would ripple across the library, and turn out to be a not so minor change, after all.
Yes, I've played with a similar design, and I've found it to cause initialization to be rather tricky. Consider the following initializations today: terminal<int>::type t = {1}; plus< terminal<int>::type , terminal<int>::type
::type u = {{1}, {1}};
That's fairly straightforward. With an extra set of braces with each expr, it becomes the rather unwieldy: terminal<int>::type t = {{1}}; plus< terminal<int>::type , terminal<int>::type
::type u = {{{{1}}, {{1}}}};
That hurts my eyes. -- Eric Niebler Boost Consulting www.boost-consulting.com