This code works and generates the correct output (23, as 2 and 3 are the sizes of the two std::vector<int>s in array_of_vectors:
#include <array>
#include <boost/range/adaptor/transformed.hpp>
#include <iostream>
#include <vector>
using boost::adaptors::transformed;
constexpr auto get_size = [](auto const& snip){ return snip.size(); };
int main() {
std::array<std::vector<int>,2> array_of_vectors = {
{std::vector<int>{1,1}, std::vector<int>{1,1,1}}
};
auto array_of_sizes = array_of_vectors | transformed(get_size);
for (auto i : array_of_sizes) {
std::cout << i;
}
}
On the other hand, if I try to enforce that array_of_sizes is indeed a std::array<std::size_t, 2u> via boost::copy_range by changing this
auto array_of_sizes = array_of_vectors | transformed(get_size);
to this
auto array_of_sizes = boost::copy_range<std::array<std::size_t, 2u>>(
array_of_vectors | transformed(get_size)
);
I get the following error,
$ g++ -std=c++17 deleteme.cpp && ./a.out
In file included from /usr/include/boost/range/iterator_range.hpp:13,
from /usr/include/boost/range/adaptor/transformed.hpp:16,
from deleteme.cpp:2:
/usr/include/boost/range/iterator_range_core.hpp: In instantiation of ‘SeqT boost::copy_range(const Range&) [with SeqT = std::array<long unsigned int, 2>; Range = boost::range_detail::transformed_range<<lambda(const auto:1&)
2> >]’:
deleteme.cpp:16:114: required from here
/usr/include/boost/range/iterator_range_core.hpp:842:20: error: no matching function for call to ‘std::array<long unsigned int, 2>::array(boost::range_detail::extract_const_iterator<boost::range_detail::transformed_range<<la
y<std::vector<int>, 2> >, true>::type, boost::range_detail::extract_const_iterator<boost::range_detail::transformed_range<<lambda(const auto:1&)>, std::array<std::vector<int>, 2> >, true>::type)’
842 | return SeqT( boost::begin( r ), boost::end( r ) );
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from deleteme.cpp:1:
/usr/include/c++/10.2.0/array:94:12: note: candidate: ‘std::array<long unsigned int, 2>::array()’
94 | struct array
| ^~~~~
/usr/include/c++/10.2.0/array:94:12: note: candidate expects 0 arguments, 2 provided
/usr/include/c++/10.2.0/array:94:12: note: candidate: ‘constexpr std::array<long unsigned int, 2>::array(const std::array<long unsigned int, 2>&)’
/usr/include/c++/10.2.0/array:94:12: note: candidate expects 1 argument, 2 provided
/usr/include/c++/10.2.0/array:94:12: note: candidate: ‘constexpr std::array<long unsigned int, 2>::array(std::array<long unsigned int, 2>&&)’
/usr/include/c++/10.2.0/array:94:12: note: candidate expects 1 argument, 2 provided
On the other hand, most surprisingly to me (!), if I force array_of_sizes to be a std::vector<std::size_t>, e.g.
auto array_of_sizes = boost::copy_range<std::vector<std::size_t>>( // vector instead of array
array_of_vectors | transformed(get_size)
);
it works! It looks like transformed turns a std::array<T,N> into a range convertible to std::vector<T>, just like it loses track of std::array's compile-time size.
What is this error/behavior due to?
I can only think that something is not working because of std::array's compile-time known size, as opposed to std::vector's run-time size. Probably transformed can't deal with that? Or maybe it's copy_range which can't?
If you compile with -std=c++2a, gcc 10.2 has a nice error message:
boost_1_74_0/boost/range/iterator_range_core.hpp:842:38: error: array must be initialized with a brace-enclosed initializer
842 | return SeqT( boost::begin( r ), boost::end( r ) );
| ~~~~~~~~~~~~^~~~~
The problem is that boost::copy_range only knows how to construct containers that take an iterator pair as (one of) their constructor parameters, so it is incompatible with std::array which does not actually have any constructors - it is only constructible as an aggregate from a (braced) list of elements.
With C++20's lambdas, you could write an alternate function that uses simple metaprogramming to produce that list of elements:
template<class T, class R>
T copy_range_fixed_size(R const& r) {
return [&r]<std::size_t... I>(std::index_sequence<I...>) {
auto it = boost::begin(r);
return T{(I, *it++)...};
}(std::make_index_sequence<T{}.size()>{});
}