为什么 vector::erase 似乎会导致崩溃？

Question

When first1.erase(std::next(first1.begin(), i)); is removed the second loop is made, its a bit strange since first2.erase(first2.begin() + 4, first2.end()); works okay

#include <iostream>
#include <vector>

int main ()
{
    std::vector<int> first1 = {0,1,2,3,4,5};
    std::vector<int> first2 = {0,1,2,3,4,5};
    std::vector<int> second;
    std::vector<int> third;

    for(size_t i = 4; i < first1.size(); ++i){
      auto child = first1[i];
      second.push_back(child);
      first1.erase(std::next(first1.begin(), i));
    }

    third.assign(first2.begin() + 4, first2.end());
    first2.erase(first2.begin() + 4, first2.end());

    std::cout << "Size of first: " << int (first1.size()) << '\n';
    std::cout << "Size of second: " << int (second.size()) << '\n';
    std::cout << "Size of first: " << int (first2.size()) << '\n';
    std::cout << "Size of third: " << int (third.size()) << '\n';
    return 0;
}

Output:

Size of first1: 5
Size of second: 1
Size of first2: 4
Size of third: 2

I expected first1/second to be the same as first2/third

you can test it here http://cpp.sh/9ltkw

Answer 1

After the first iteration of the loop

for(size_t i = 4; i < first1.size(); ++i){
  auto child = first1[i];
  second.push_back(child);
  first1.erase(std::next(first1.begin(), i));
}

i will be equal to 5 and first1.size() also will be equal to 5. So only one element of the vector is erased.

You could rewrite the loop like

for(size_t i = 4; i != first1.size(); ){
  auto child = first1[i];
  second.push_back(child);
  first1.erase(std::next(first1.begin(), i));
}

to get the expected result.

In these statements

third.assign(first2.begin() + 4, first2.end());
first2.erase(first2.begin() + 4, first2.end());

there are assigned and erased 2 elements.