小编点评

The problem with the code lies in the `copy` function. The issue with the assignment operator (`=`), the `last` parameter being passed to the function and the lack of clear initialization of `d_first` are causing unexpected behavior. The corrected code using `insert` is: ```cpp std::vector v{1, 2, 3, 4, 5}; std::vector copied{begin(v), end(v)}; v.insert(end(v), begin(copied), end(copied)); ``` In this corrected version, the elements are added to the `copied` vector using the `insert` function, which takes the start and end iterators of the source and destination vectors and inserts the elements from the source vector into the destination vector. This ensures that the elements are added to the end of the `copied` vector as intended.

正文

#include <iostream>
#include <vector>
#include <numeric>
#include <sstream>


int main() {
    std::vector v{1, 2, 3, 4, 5};
    std::copy(begin(v), end(v), std::back_inserter(v));

    for (const auto& item : v)
        std::cout << item << ' ';
}

原本目的是将vector中的元素再复制一遍，追加到其末尾，然而却发现新增的元素并不是预期中的（并且每次运行都不一样）。大概看了一下copy的实现才发现问题（copy实际源码挺复杂的，进行了不少优化，仅提供简化版本）：

template<class InputIt, class OutputIt>
OutputIt copy(InputIt first, InputIt last,
              OutputIt d_first)
{
    for (; first != last; (void)++first, (void)++d_first)
        *d_first = *first;
 
    return d_first;
}

仔细可以注意到，last是会发生变化的，所以造成了结果不符合预期。

 

可以改用insert实现：

std::vector v{1, 2, 3, 4, 5};
std::vector<int> copied{begin(v), end(v)};
v.insert(end(v), begin(copied), end(copied));