Re: [boost] A C++ puzzler

From: Giovanni Piero Deretta (gpderetta_at_[hidden])
Date: 2008-04-12 07:45:18

On Sat, Apr 12, 2008 at 9:28 AM, Eric Niebler <eric_at_[hidden]> wrote:
> I recently had a C++ problem and found an answer that tickled my brain.
>  In the spirit of Car Talk on NPR, I thought I'd share it in the form of
>  a puzzler.
>
>  Say that you are given a set of types, and you must find the "common"
>  type, where common is defined as follows:
>
>  - If all the types are the same, say X, then the common type is X
>  - If some types are X and some are Default (where Default is a known
>  special type), the common type is X
>  - If some types are X and some are Y (and neither X nor Y is Default),
>  then there is no common type. It's an error and you should issue a
>  diagnostic.
>
>  So for example:
>
>  [Default, Default, Default] --> Default
>  [Default, A, Default]       --> A
>  [B, B, B]                   --> B
>  [Default, C, D]             --> Error, issue diagnostic.
>
>  And here's the kicker ... do it in O(1). No O(N) recursive template
>  instantiations allowed. Assume that the maximum size of the set is known
>  ahead of time.

In your honor, the master of conditional operator tricks, here is my solution.
The easy part only allows one to retrieve the common type inside a
template function.
With some sizeof trickery you can of course retrieve it anywhere.
It is not generalized (it needs some per-type boilerplate, which
should be easily wrappable in a macro).

-- 
gpd