>
> #define FUSION_MAX_VECTOR_SIZE 20

I could be wrong, but isn't Fusion (or Spirit, etc) header-only?

> So, without rebuilding the entire world, how do I increase the maximum size
> of a Fusion vector without an ODR violation??

I'm not sure what you mean, ODR violation?

To appreciate where you are
coming from, I assume you have at least built boost and have worked
with it to some extent?

> This is a huge problem with global flags.

I don't think it's the mountain you think it is.

Once you build the Boost libs in an iteration, or application life
cycle, unless there's been a Boost patch or upgrade, you generally
never need to touch it again.

Everything else is header-only. Yes, you sometimes declare fully or
partially specialized templates (or at least, I do), but this is a
'trivial' part of using template header-only resources.

> [Note:  I am not criticizing Boost.Fusion here, as they really didn't have a
> choice in a pre-variadic template world.]
>
>>
>> You'd almost certainly use namespaces or template parameters to make
>> symbol unique the two implementations.
>
> Again, how do you do this if you are using compile time switches?  Please
> post some sample code showing the technique you envision for users.

AFAIK, compile time switches are injected via whatever project headers
you are using. In a Microsoft Visual C++ world, for example, that is
the *vcsproj* file, to my knowledge, usually via project settings. But
once these are established, you focus can (and should) be on the
problem solution / application at hand.