On Sat, Apr 12, 2008 at 9:28 AM, Eric Niebler <eric@boost-consulting.com> 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