为什么在RangeV3中std :: back_inserter_iterator无法弱增量?

问题描述

令我惊讶的是，RangeV3中的类似概念的断言失败.

To my surprise this Concept-like assertion fails in RangeV3.

#include<vector>
#include<range/v3/algorithm/copy.hpp>
int main(){
   static_assert(ranges::WeaklyIncrementable<std::back_insert_iterator<std::vector<double> >>());
}

为什么?

除其他外，这意味着我不能像使用std::copy那样使用ranges::copy算法.

This, among other things means that I cannot use the ranges::copy algorithm as I use to do with std::copy.

    std::vector<double> w(100);
    std::vector<double> v;
    ranges::copy(
        begin(w), end(w),
        std:back_inserter(v)
    );  // compilation error, concept not fulfilled.

这是RangesV3中通向back_insert的规范方法吗?

Is this the canonical way to back_insert in RangesV3?

我在RangeV3中找不到WeaklyIncrementable文档，但是在cppreference中 https ://en.cppreference.com/w/cpp/experimental/ranges/iterator/WeaklyIncrementable ，似乎存在一个未为back_inserter_iterator定义的有符号不同类型".这可能意味着1或3件事，a)RangeV3过度约束了copy要求b)copy并非用于向后插入的算法，c)我不知道如何使用RangeV3.

I cannot find the WeaklyIncrementable documentation in RangeV3, but in cppreference https://en.cppreference.com/w/cpp/experimental/ranges/iterator/WeaklyIncrementable it seems that there is a "signed different type" that is probably not defined for back_inserter_iterator. This probably means 1 or 3 things, a) RangeV3 is overconstraining the copy requirements b) copy is not the algorithm for back insertion, c) I have no clue how to use RangeV3.

找到了此 https://github.com/ericniebler/range-v3/issues/867 ，可能的解决方法是使用range::back_inserter(v)而不是std::back_inserter(v).似乎某处存在默认的可构造性要求.

Found this https://github.com/ericniebler/range-v3/issues/867, a possible workaround it to use range::back_inserter(v) instead of std::back_inserter(v). It seems that there is a default constructibility requirement somewhere.

推荐答案

ranges::copy似乎有一些意外的需求(对我而言). 因此RangesV3提供了替代品ranges::back_inserter.

It looks like there are some unexpected (to me) requierements need by ranges::copy. So RangesV3 provides a drop in replacemente ranges::back_inserter that works.

但是，该标准中还有许多其他迭代器由于相同的原因而无法工作，但是没有直接替换的迭代器，因此可能会很丑陋.

However there are many other iterators in the standard that do not work for the same reason but to which there is no drop-in replacements, so it can get ugly.

例如，我必须改编一个新的迭代器以替换std::ostream_iterator来创建一些人工函数，包括默认构造函数:

For example, I had to adapt a new iterator to replaced std::ostream_iterator creating some artificial functions, including a default constructor:

template<class T>
struct ranges_ostream_iterator : std::ostream_iterator<T>{
    using std::ostream_iterator<T>::ostream_iterator;
    ranges_ostream_iterator() : std::ostream_iterator<T>{std::cout}{} // I have to put something here
    ranges_ostream_iterator& operator++(){std::ostream_iterator<T>::operator++(); return *this;}
    ranges_ostream_iterator& operator++(int){return operator++();}
    using difference_type = int;
    int operator-(ranges_ostream_iterator const&){return 0;}
};

使用此ranges::copy(first, last, ranges_ostream_iterator<int>(std::cout))可以工作，而ranges::copy(first, last, std::ostream_iterator<int>(std::cout))不能.

With this ranges::copy(first, last, ranges_ostream_iterator<int>(std::cout)) work, whereas ranges::copy(first, last, std::ostream_iterator<int>(std::cout)) doesn't.

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