为什么在 RangeV3 中 std::back_inserter_iterator 不是 WeaklyIncrementable?

问题描述

令我惊讶的是，这个类似概念的断言在 RangeV3 中失败.

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> >>());
}

这是为什么呢?

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.

这是在 RangesV3 中 back_insert 的规范方式吗?

Is this the canonical way to back_insert in RangesV3?

我在 RangeV3 中找不到 WeaklyIncrementable 文档，但在 cppreference https://en.cppreference.com/w/cpp/experimental/ranges/iterator/WeaklyIncrementable 似乎有一个签名的不同类型"可能没有为 back_inserter_iterator 定义.这可能意味着 1 或 3 件事，a) RangeV3 过度约束 copy 要求 b) copy 不是反向插入的算法，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.

找到这个 https://github.com/ericniebler/range-v3/issues/867，一个可能的解决方法是使用 range::back_inserter(v) 而不是 std::back_inserter(v).似乎某处有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 似乎有一些意想不到的(对我而言)需求.所以 RangesV3 提供了一个替换 ranges::back_inserter 的功能.

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

但是，标准中还有许多其他迭代器由于相同的原因不能正常工作，但没有直接替代品，因此可能会变得丑陋.

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，我必须调整一个新的迭代器来替换 std::ostream_iterator 创建一些人工函数，包括一个默认构造函数:

For example, I had to adapt a new iterator to replace 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;}
};

有了这个，ranges::copy(first,last,ranges_ostream_iterator(std::cout)) 工作，而 ranges::copy(first,last,std::ostream_iterator(std::cout)) 没有.

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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