我可以在boost :: adaptors :: std :: array的结果上使用boost :: copy_range来返回另一个std :: array吗? [英] Can I use boost::copy_range on the result of boost::adaptors::transformed over std::array to get back another std::array?
问题描述
此代码有效并生成正确的输出( 23
,因为 2
和 3
是两个 std的大小::vector< int>
在 array_of_vectors
中:
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;
}
}
另一方面,如果我尝试通过 boost强制
通过更改 array_of_sizes
确实是 std :: array< std :: size_t,2u>
::copy_range
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);
对此
auto array_of_sizes = boost::copy_range<std::array<std::size_t, 2u>>(
array_of_vectors | transformed(get_size)
);
我收到以下错误消息,
$ 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
另一方面,最令我惊讶的是(!),如果我强制 array_of_sizes
为 std :: vector< std :: size_t>
,例如
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)
);
有效!看起来 transformed
可以将 std :: array< T,N>
转换为可转换为 std :: vector< T>
的范围就像它会丢失对 std :: array
的编译时大小的跟踪.
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.
此错误/行为是由于什么造成的?
What is this error/behavior due to?
我只能认为由于 std :: array
的编译时已知大小而不起作用,而不是 std :: vector
的运行时间大小. transformed
可能无法解决这个问题?或者也许是 copy_range
不能?
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?
推荐答案
如果使用 -std = c ++ 2a
进行编译,则gcc 10.2会显示一条错误消息:
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 ) );
| ~~~~~~~~~~~~^~~~~
问题是 boost :: copy_range
仅知道如何构造将迭代器对作为其构造函数参数之一的容器,因此它与 std :: array不兼容
实际上没有任何构造函数-只能作为(括号)元素列表的集合来构造.
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.
使用C ++ 20的lambda,您可以编写一个替代函数,该函数使用简单的元编程来生成该元素列表:
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()>{});
}
示例.
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