template <class ForwardIterator, class UnaryPredicate>
  ForwardIterator remove_if (ForwardIterator first, ForwardIterator last,
                             UnaryPredicate pred);

std::remove_if函数删除容器中满足pred的元素，返回最后一个没被移除的元素后面一个迭代器（An iterator to the element that follows the last element not removed.），但是容器的大小是不变的。
假设std::remove_if返回的迭代器是pend，虽然容器的大小不变，但是从pend到容器最后一个元素的值是unspecified state的（The relative order of the elements not removed is preserved, while the elements between the returned iterator and last are left in a valid but unspecified state.）。

// remove_if example
#include <iostream>     // std::cout
#include <algorithm>    // std::remove_if
#include <vector>

bool IsOdd (int i) { return ((i%2)==1); }

int main () {
  std::vector<int> myints1{1,2,3,4,5,6,7,8,9};            // 1 2 3 4 5 6 7 8 9

  auto pend1 = std::remove_if (myints1.begin(), myints1.end(), IsOdd);   // 2 4 6 8 ? ? ? ? ?
                                                 // ^       ^
  /* 奇数被移除了，所以只剩偶数了 */
  std::cout << "the range contains:";
  for (auto p=myints1.begin(); p!=pend1; ++p)
    std::cout << ' ' << *p;
  std::cout << '\n' << std::endl << "The myints contains: \n";
  
  /* 奇数被移除了，剩余的偶数从索引0开始填充，std::vector后面没被填充的元素值可能是原始位置的值，但不保证 */
  for(auto const &it: myints1){
   std::cout << it << ", ";   
  }
  std::cout << std::endl;

  std::cout << "-------------------------------------------\n";
  
  std::vector<int> myints2{1,3,5,7,9,11,13,15,17};

  auto pend2 = std::remove_if (myints2.begin(), myints2.end(), IsOdd); 
  
  /* 所有元素都是奇数，都被移除，因此打印为空 */
  std::cout << "the range contains:";
  for (auto p=myints2.begin(); p!=pend2; ++p)
    std::cout << ' ' << *p;
  std::cout << '\n' << std::endl << "The myints contains: \n";

  /* myints2中的所有元素都是奇数，都被移除了，因此原容器myints2中的元素都没被替换，因此输出的值可能还是1, 3, 5, 7, 9, 11, 13, 15, 17,但并不能保证值一定是这样 */
  for(auto const &it: myints2){
   std::cout << it << ", ";   
  }
  std::cout << std::endl;

  return 0;
}

输出：

the range contains: 2 4 6 8

The myints contains: 
2, 4, 6, 8, 5, 6, 7, 8, 9, 
-------------------------------------------
the range contains:

The myints contains: 
1, 3, 5, 7, 9, 11, 13, 15, 17,