输出迭代器适配器以计数但不复制 [英] Output iterator adapter to count but not copy

问题描述

有各种STL算法依赖输出迭代器来存储算法的结果。

There are various STL algorithms that rely on an output iterator to store the result of the algorithm.

例如， std :: set_intersection 将在输出迭代器中存储两个有序范围之间的所有公共元素然后，每输出一个元素后递增。

For example, std::set_intersection will store all the common elements between two sorted ranges in an Output iterator that is then post incremented per element outputted.

有时候，我对实际元素不感兴趣，只对输出元素的数量感兴趣。在这种情况下，复制元素会浪费内存和性能。是否有一个迭代器适配器，我可以使用它来计算和避免元素的副本？如果没有，你能建议这种适配器的通用实现吗？

Sometimes, I am not interested in the actual elements but only the number of output elements. In such cases it is a waste of memory and performance to copy the elements. Is there an iterator adapter that I can use to count and avoid the copy of the elements? If not can you suggest a generic implementation of such an adapter?

推荐答案

感谢@ecatmur的大量帮助回答和评论，我有以下解决方案，我邀请评论。我原本希望得到 boost :: make_function_output_iterator ，但似乎库中有一个错误，无法定义赋值运算符。

Thanks to a lot of help from @ecatmur answer and comments, I have the following solution, which I invite comments. I had hoped to get the boost::make_function_output_iterator working but it seems that there is a bug in the library which fails to define the assignment operator.

#include <algorithm>
#include <vector>
#include <iostream>
#include <string>
#include <cassert>

class counting_output_iterator 
{
public:  
  counting_output_iterator& operator=(const counting_output_iterator&) { return *this; };
  explicit counting_output_iterator(std::size_t& count) : m_count(count) {}
  template<typename T> void operator=(const T&) {}; //NULL op
  using iterator_category = std::output_iterator_tag;
  using value_type = void;
  using difference_type = void;
  using pointer = void;
  using reference = void;      
  counting_output_iterator& operator*() { return *this; }
  counting_output_iterator& operator++() { ++m_count; return *this; }  
  std::size_t&  m_count;
};

int main(int, char*[])
{   
    std::vector<int> arr{ 1,2,3,4 };
    std::size_t count = 0;  
    std::copy(std::begin(arr), std::end(arr), counting_output_iterator{ count } );
    assert(count == 4u);
    return 0;
}

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