Re: [boost] A C++ puzzler

From: Eric Niebler (eric_at_[hidden])
Date: 2008-04-13 15:42:38

Giovanni Piero Deretta wrote:
> On Sat, Apr 12, 2008 at 8:38 PM, Eric Niebler <eric_at_[hidden]> wrote:
>>  Giovalli:
> 
> Giova*ll*i  ?!?!?!

Whoops, sorry. :-P

>> 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.

> 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?

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com