Re: [Boost-users] Signals2 benchmark

10 Feb 2015

      On Tue, Feb 10, 2015 at 10:15 AM, Nevin Liber  wrote:
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On 10 February 2015 at 06:49, Michael Powell  wrote:
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#define FUSION_MAX_VECTOR_SIZE 20
I could be wrong, but isn't Fusion (or Spirit, etc) header-only?
Yes, but I'm not sure what that has to do with solving the ODR violation
problem.  Quite the opposite; it contributes to the problem.
...
...
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?
ODR, or One Definition Rule, basically says that you cannot redefine a
function or class within a program.  See
http://en.cppreference.com/w/cpp/language/definition for a more detailed
explanation, or N4296 section [basic.def.odr] to read the specifics of what
the standard says about it.  It's fundamental to C++.
ODR itself I understand. To my knowledge, you can't apply it to
template, header only libraries.

Abandon your notion of ODR. Where Fusion, Spirit, etc, are concerned,
AFAIK, it's irrelevant. These are C++ template-based, which means by
definition, their 'definition' may be different from instance to
instance, but the underlying pattern is consistent, interface driven,
etc. That's a fundamental SOLID principle.
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To appreciate where you are
coming from, I assume you have at least built boost and have worked
with it to some extent?
Yes.
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This is a huge problem with global flags.
I don't think it's the mountain you think it is.
Oh?
...
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.]
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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.
I don't see how any of that addresses the problem I mentioned.  If I use a
third party library that
#define FUSION_MAX_VECTOR_SIZE 20
and I need to
#define FUSION_MAX_VECTOR_SIZE 50
how does that not lead to an ODR violation?  This is still an issue even if
that third party library only used Boost.Fusion in its implementation.
It sounds to me as though there are secondary, or even tertiary+,
dependencies involved. You would need cooperation from your vendor, or
access to the source, in order to do what you want it seems.

HTH
...
--
 Nevin ":-)" Liber  mailto:nevin@eviloverlord.com  (847) 691-1404
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