std::使用视图std::ranges::views::values进行累加

Question

有没有办法编译代码，或者我必须创建自己的lambda作为std::accumulate的第四个参数？

#include <iostream>
#include <ranges>
#include <unordered_map>
#include <numeric>
namespace rv = std::ranges::views;
int main()
{
std::unordered_map<unsigned, unsigned> m = {{5, 3}};

auto values = m | rv::values;

std::cout << std::accumulate(values.begin(), values.end(), 0) << std::endl;
}

如果我理解正确的话，gcc抛出的错误基本上是说begin和end产生不同的类型，而std::accumulate不能推导出唯一的InputIterator类型。完整的编译器输出为：

main.cpp:15:65: error: no matching function for call to 'accumulate(std::ranges::elements_view<std::ranges::ref_view<std::unordered_map<unsigned int, unsigned int> >, 1>::_Iterator<true>, std::__detail::_Node_iterator<std::pair<const unsigned int, unsigned int>, false, false>, int)'
15 |     std::cout << std::accumulate(values.begin(), values.end(), 0) << std::endl;
|                                                                 ^
In file included from /usr/local/include/c++/10.2.0/numeric:62,
from main.cpp:5:
/usr/local/include/c++/10.2.0/bits/stl_numeric.h:134:5: note: candidate: 'template<class _InputIterator, class _Tp> constexpr _Tp std::accumulate(_InputIterator, _InputIterator, _Tp)'
134 |     accumulate(_InputIterator __first, _InputIterator __last, _Tp __init)
|     ^~~~~~~~~~
/usr/local/include/c++/10.2.0/bits/stl_numeric.h:134:5: note:   template argument deduction/substitution failed:
main.cpp:15:65: note:   deduced conflicting types for parameter '_InputIterator' ('std::ranges::elements_view<std::ranges::ref_view<std::unordered_map<unsigned int, unsigned int> >, 1>::_Iterator<true>' and 'std::__detail::_Node_iterator<std::pair<const unsigned int, unsigned int>, false, false>')
15 |     std::cout << std::accumulate(values.begin(), values.end(), 0) << std::endl;
|                                                                 ^
In file included from /usr/local/include/c++/10.2.0/numeric:62,
from main.cpp:5:
/usr/local/include/c++/10.2.0/bits/stl_numeric.h:161:5: note: candidate: 'template<class _InputIterator, class _Tp, class _BinaryOperation> constexpr _Tp std::accumulate(_InputIterator, _InputIterator, _Tp, _BinaryOperation)'
161 |     accumulate(_InputIterator __first, _InputIterator __last, _Tp __init,
|     ^~~~~~~~~~
/usr/local/include/c++/10.2.0/bits/stl_numeric.h:161:5: note:   template argument deduction/substitution failed:
main.cpp:15:65: note:   deduced conflicting types for parameter '_InputIterator' ('std::ranges::elements_view<std::ranges::ref_view<std::unordered_map<unsigned int, unsigned int> >, 1>::_Iterator<true>' and 'std::__detail::_Node_iterator<std::pair<const unsigned int, unsigned int>, false, false>')
15 |     std::cout << std::accumulate(values.begin(), values.end(), 0) << std::endl;
|                                                                 ^

Answer 1

gcc抛出的错误，如果我理解正确的话，基本上说begin和end产生不同类型的

确实如此。在范围方面，elements_view的iterator和sentinel是不同的类型。在C++20之前，这些类型必须是相同的，并且许多代码都是假设这一点编写的。不幸的是，我们还没有ranges::accumulate可以为您妥善处理此问题。

在此之前，还有另一个测距适配器会强制一个测距的iterator和sentinel具有相同的类型(如果已经是这样的话，则为无操作(：views::common:

auto values = m | rv::values | rv::common;

在这种情况下，它将创建common_iterators的视图，这基本上是一个variant<iterator, sentinel>，它被封装为表现得像迭代器。