[boost] [fusion] how to initialize a container of non-const references to objects, from a container of objects

From: Stjepan Rajko (stipe_at_[hidden])
Date: 2007-06-09 03:30:24

Hello,

I've started to use fusion for the signal network gsoc project, and am
really amazed with how much it simplified the prototype code (it
brought down the need for preprocessing+file iteration to a bare
minimum).  Thanks for such a great library!

I recently ran into a problem, though, and am wondering if someone
could help me out.  I have a need to do something along the lines of
the following:

boost::fusion::vector<int> a;
boost::fusion::vector<int &> b(a); // first element of b refers to the
first element of a

Using fusion from CVS HEAD, this doesn't work because all of the
related constructors take a const & argument.  A similar use case (but
not exactly what I need) does work:

boost::fusion::vector<int> a;
boost::fusion::vector<const int &> b(a);

as does this: (but i'd like to avoid this approach because in my exact
case the size of the vectors varies)

boost::fusion::vector<int> a;
boost::fusion::vector<int &> b(boost::fusion::at_c<0>(a));

I tried adding non-const versions of the constructors / helper
functions to vector/vector.hpp and vector/vector_n.hpp, and after
doing so the use case I need works OK.

Unless there is an alternative way of doing what I'm trying to do,
would it be possible to add in the non-const versions of the
constructors?  I don't mind producing the patches if someone points me
to all the places where the non-const constructors should be added to
make the addition consistent, as well as to any non-obvious things
that would also need to be done (keeping in mind that currently my
scope of "obvious" appears to be pretty limited :-))

Thanks for your help,

Stjepan