Re: [Boost-users] multi_array and variants

Subject: Re: [Boost-users] multi_array and variants
From: Krzysztof Czainski (1czajnik_at_[hidden])
Date: 2010-11-09 05:07:43

Hi Alle,

2010/11/9 Alle Meije Wink <a.m.wink_at_[hidden]>

> Dear Kris, thanks for your reply.
>

You are welcome.

> > > template<typename T>
> > > class bisArray: public bisObject {
> > > typedef boost::variant
> > > < boost::multi_array_ref<T, 1>,
> > > boost::multi_array_ref<T, 2>,
> > > boost::multi_array_ref<T, 3>,
> > > boost::multi_array_ref<T, 4>,
> > > boost::multi_array_ref<T, 5>,
> > > boost::multi_array_ref<T, 6>,
> > > boost::multi_array_ref<T, 7>,
> > > boost::multi_array_ref<T, 8>,
> > > boost::multi_array_ref<T, 9>
> > > > bisArray_t;
> > > bisArray_t _bisArray;
>

> However what I am stuck with seems to be, in essence, still the same thing:
>
> >>> /home/amwink/programs/cpp/bis/src/bisniftiimage.cpp||In function ‘void
> bis::bisReadNifti(std::string)’:|
> >>> /home/amwink/programs/cpp/bis/src/bisniftiimage.cpp|102|warning: unused
> variable ‘testimage’|
> >>> /usr/include/boost/variant/variant.hpp||In constructor
> ‘boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13,
> T14, T15, T16, T17, T18, T19>::variant() [with T0_ =
> boost::multi_array_ref<unsigned char, 1ul>, T1 =
> boost::multi_array_ref<unsigned char, 2ul>, T2 =
> boost::multi_array_ref<unsigned char, 3ul>, T3 =
> boost::multi_array_ref<unsigned char, 4ul>, T4 =
> boost::multi_array_ref<unsigned char, 5ul>, T5 =
> boost::multi_array_ref<unsigned char, 6ul>, T6 =
> boost::multi_array_ref<unsigned char, 7ul>, T7 =
> boost::multi_array_ref<unsigned char, 8ul>, T8 =
> boost::multi_array_ref<unsigned char, 9ul>, T9 =
> boost::detail::variant::void_, T10 = boost::detail::variant::void_, T11 =
> boost::detail::variant::void_, T12 = boost::detail::variant::void_, T13 =
> boost::detail::variant::void_, T14 = boost::detail::variant::void_, T15 =
> boost::detail::variant::void_, T16 = boost::detail::variant::void_, T17 =
> boost::detail::variant::void_, T18 = boost::detail::variant::void_, T19 =
> boost::detail::variant::void_]’:|
> >>> /home/amwink/programs/cpp/bis/src/bisarray.hpp|52|instantiated from
> ‘bis::bisArray<T>::bisArray(T*, size_t, std::vector<long unsigned int,
> std::allocator<long unsigned int> >) [with T = unsigned char]’|
> >>> /home/amwink/programs/cpp/bis/src/bisimage.hpp|33|instantiated from
> ‘bis::bisImage<T>::bisImage(T*, size_t, std::vector<long unsigned int,
> std::allocator<long unsigned int> >) [with T = unsigned char]’|
> >>> /home/amwink/programs/cpp/bis/src/bisniftiimage.cpp|126|instantiated
> from here|
> >>> /usr/include/boost/variant/variant.hpp|1198|error: no matching function
> for call to ‘boost::multi_array_ref<unsigned char, 1ul>::multi_array_ref()’|
> >>> /usr/include/boost/multi_array/multi_array_ref.hpp|623|note: candidates
> are: boost::multi_array_ref<T, NumDims>::multi_array_ref(T*, const typename
> boost::const_multi_array_ref<T, NumDims, T*>::storage_order_type&, const
> typename boost::const_multi_array_ref<T, NumDims, T*>::index*, const
> typename boost::const_multi_array_ref<T, NumDims, T*>::size_type*) [with T =
> unsigned char, long unsigned int NumDims = 1ul]|
> >>> /usr/include/boost/multi_array/multi_array_ref.hpp|469|note:
>       boost::multi_array_ref<T, NumDims>::multi_array_ref(T*, const
> boost::detail::multi_array::extent_gen<NumDims>&, const
> boost::general_storage_order<NumDims>&) [with T = unsigned char, long
> unsigned int NumDims = 1ul]|
> >>> /usr/include/boost/multi_array/multi_array_ref.hpp|463|note:
>       boost::multi_array_ref<T, NumDims>::multi_array_ref(T*, const
> boost::detail::multi_array::extent_gen<NumDims>&) [with T = unsigned char,
> long unsigned int NumDims = 1ul]|
> >>> /usr/include/boost/multi_array/multi_array_ref.hpp|417|note:
>       boost::multi_array_ref<unsigned char, 1ul>::multi_array_ref(const
> boost::multi_array_ref<unsigned char, 1ul>&)|
> >>> ||=== Build finished: 1 errors, 1 warnings ===|
>
> So boost.variant now sugegsts that multi_array_ref() is called, a
> constructor with no arguments! I don't use that, and I don't even
> think it exists (because it produces a multi_array view of an
> *existing* C-style array!
>

The error sais, that the default constructor of multi_array_ref<uchar,1>
(which does not exist) is being called from the default constructor of
variant.

By default, variant cannot be empty. It's default constructor tries to use
the default constructor of the first type on the list. Therefore You have
two options:

1) Always initialize the variant with some array,
2) Add a type to the beginning of the list, which will stand for "no value":
#include <boost/none.hpp>
variant< none_t, [...] > bisArray;

Any idea why a problem with the *default* constructor would turn up?
>

The default ctor of each member is called if You don't specify another means
of initialization in the constructor.

Regards, Kris