Re: [boost] A C++ puzzler

From: Giovanni Piero Deretta (gpderetta_at_[hidden])
Date: 2008-04-13 17:31:41

On Sun, Apr 13, 2008 at 9:42 PM, Eric Niebler <eric_at_[hidden]> 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{};

>
>
>  > 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
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