Re: [Boost-users] [mpl][fusion] How to transform vector_c into std::vector?

Subject: Re: [Boost-users] [mpl][fusion] How to transform vector_c into std::vector?
From: Larry Evans (cppljevans_at_[hidden])
Date: 2013-11-28 12:47:36

On 11/28/13 00:37, Mostafa wrote:
> On Wed, 27 Nov 2013 21:09:04 -0800, TONGARI J <tongari95_at_[hidden]> wrote:
>
>> 2013/11/28 Mostafa <mostafa_working_away_at_[hidden]>
>>
>>> How does one transform an mpl:vector_c into a regular std::vector?
>>>
>>> Sample input:
>>>
>>> typedef mpl::vector<int, int_<10>, int_<11>, int_<12> > some_vector_c;
>>>
>>
>> Perhaps:
>>
>> std::vector<int> v;
>> fusion::for_each(some_vector_c(), [&v](int val){v.push_back(val);});
>
> Thanks, I ended up using mpl's for_each. Even though I don't quite
> understand how it works. Specifically, what does boost::get do? I
> couldn't find any documentation on it.

I assume you mentioned boost::get because it appears in the for_each
documentation here:

http://www.boost.org/doc/libs/1_55_0/libs/mpl/doc/refmanual/for-each.html

specifically in this quote:
--{--cut here--

Postcondition:	

Equivalent to

typedef begin<Sequence>::type i1;
value_initialized< deref<i1>::type > x1;
f(boost::get(x1));

typedef next<i1>::type i2;
value_initialized< deref<i2>::type > x2;
f(boost::get(x2));
...
value_initialized< deref<in>::type > xn;
f(boost::get(xn));
typedef next<in>::type last;

where n == size<s>::value and last is identical to end<s>::type; no 
effect if empty<s>::value == true.

--}--cut here--

And now I see why you asked your question because boost::get(x1)
seems to be redundant.  AFAICT, just passing th x1 or x2 ... or xn
would work.

Hence, like Mostafa, I'd like some clarification of why boost::get is
necessary in the mpl::for_each documentation.  Googling boost::get
leads to:

http://www.boost.org/doc/libs/1_35_0/doc/html/boost/get_id405862.html

which indicates the argument to boost::get is a boost::variant, which
doesn't clarify anything for me.

-regards,
Larry