Re: [Boost-users] [fusion] Accessing sequence elements at runtime

From: Tobias Schwinger (tschwinger_at_[hidden])
Date: 2007-03-28 04:52:29

Christian Henning wrote:
> Hi there, is it somehow possible to access a sequence element at
> runtime? 

You sure can. The 'at' function yields an L-value.

> Just image you're reading some name value pairs from file and
> you would like them to assign them to a fusion sequences. 

You must know the number of pairs you expect in advance (at compile 
time) to use the built-in Sequences that come with Fusion.

Almost philosophical stuff: We have some (still undocumented) weaker 
Sequence Concepts that would (theoretically - not that I could see that 
this use case would make a lot of sense) allow you to turn an STL 
container into a Fusion sequence.
Note however that Fusion-style iteration is terminated at compile time, 
so the sequence would be infinite and thus only work if evaluated lazily 
until the last element in the STL container.

> This way I don't know they order the values are supplied. Is that possible?
> 
> For example take the following code:

<code>

Sidenote: You might get into trouble calling constructors in static 
aggregate initialization (construction of 'any' objects in your code) if 
your code is compiled into a shared library. You might know already...

Given an 'any' with the appropriate type and a Fusion 'map', you have to 
iterate the map (hand-coded on top of Fusion iterators, if you want to 
stop once you found what you're looking for) and check for

     typeid(typename first<elem>::type) == an_any.type()

The other (possibly more efficient) alternative is to use 'switch' & 
'case' but that isn't very flexible, unless you can automatically 
generate a 'switch' cascade, see

     http://article.gmane.org/gmane.comp.lib.boost.devel/153870

for further reading.

Regards,
Tobias