[I have sent an incomplete message by mistake... here is the conclusion] On Sun, Apr 13, 2008 at 11:31 PM, Giovanni Piero Deretta <gpderetta@gmail.com> wrote:
On Sun, Apr 13, 2008 at 9:42 PM, Eric Niebler <eric@boost-consulting.com> wrote:
Giovanni Piero Deretta wrote:
conditional operator. Wish I thought of that. But it requires types to be associated with integers via a global registry, so I can't use it. Is there a way to avoid select() and result<>? It's easy with typeof, but can you do it without?
Here is a try. it probably will look more at home at an obfuscated C++ context.
It works up to 5 types. Extending it beyond is a matter of a little of pp metaprogramming.
The only O(N) template (that I can see) instantiation is template<class T> default_type::operator T();
Not a problem. It's a function template, so instantiating it would be cheaper than instantiating a class template (I think), but you're not even instantiating it. It doesn't even have a definition.
D'oh right! I assumed that generating the function signature counted as instantiation. Obviously it doesn't. I have even introduced a bug in a mindless try to eliminate all template usage: while the following:
struct C{}; struct D : C{}; assert(is_same<C,common<C, D>::type>); works as expected, this doesn't: assert(is_same<C, common<D, C>::type) The fix is simple: add the overload template<class T> true_& match(T&, T&); and change the result type of the match(...) overload to false.
And you do not pay for it if you do not use default_type (partially specializing common_type may work as an optimization).
I'll try to get rid even of this.
Even if there are very few template instantiations, compile time isn't that good. I thought that compile time integral expression computations were basically free. Maybe I'm missing something.
This solution is very clever! Why do you say that compile time is poor? What are you benchmarking against?
I have used a very scientific benchmark: I run the test and was mildly annoyed by the time it took to compile :). Anyways, there are actually N^2 applications of operator:? . The "inner loop" is always the same, but I do not know if compilers do memoization of constant expressions, so it might be worthwhile to do this optimization by hand. -- gpd