Re: [Boost-users] [fusion] memory layout of fusion::vector vs boost::tuple/boost::array

8 Oct 2010


      2010/10/8 Thomas Heller 
...
alfC wrote:
...
On Oct 7, 5:13 pm, OvermindDL1  wrote:
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On Thu, Oct 7, 2010 at 5:42 PM, Alfredo Correa <
alfredo.cor...@gmail.com>
...
...
wrote:
...
On Thu, Oct 7, 2010 at 3:33 PM, OvermindDL1 
wrote:
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You do know that based on this:
...
...
http://www.boost.org/doc/libs/1_44_0/libs/fusion/doc/html/fusion/adap...
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boost::array is already adapted as a fusion container and can be used
directly, as soon as you include the proper hpp that is?
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yes, thank you, that what I wrote in my last post . I can make this
work
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#include 
#include 
...
boost::array boostarr={{1.0, 2.1, 3.2}};
vector fusionvec(boostarr); //now works
Ah, heh, got lost in the noise and I missed it.
On Thu, Oct 7, 2010 at 5:42 PM, Alfredo Correa <
alfredo.cor...@gmail.com>
wrote:
...
I still can't do the other way around:
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boost::array boostarr2(fusionvec); //doesn't work
ideas?
I know that would not work because that is still a 'custom' type as
far as fusion is concerned, and only free functions can work on it, a
constructor is obviously not a free function, so you need to write a
free function to do it (probably called 'copy', probably using
fusion's foreach to copy each element one to the other, should be
pretty well optimized out thanks to the template-magic).
no luck. I can't make it work. First I tried with "for_each" and
didn't manage to put together the code because I need to copy one
sequence from the other and the for_each doesn't keep track of the
index. So I tried with "transform", the code compiles but does nothing
to the array.
boost::array ba0;
boost::fusion::transform(ba0, bfv1, copy());
where
struct copy{
typedef void result_type;
void operator()(double& lhs, double const& rhs) const{
lhs = rhs;
}
};
ba0 is still undefined after running this.
Obviously I am getting Boost.Fusion wrong.
Thanks,
Alfredo
You will find a version using fold attached.
What it does, is that it increases the size of the boost::array passed in
as
state by one for every element in the fusion sequence and copies over the
elements. There could be a more elegant solution to the problem ;)
Thomas, thanks so much for the code. Unfortunately it doesn't work (maybe it
is my compiler?)

$ c++ -v
gcc 4.4.3
$ c++ fusion_test.cpp
$ ./a.out
terminate called after throwing an instance of
'boost::exception_detail::clone_implstd::out_of_range
...
'
  what():  attempt to access element of an empty array
Aborted
Thanks,
Alfredo
...
Cheers
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