为什么vector的remove_if实际上并没有删除元素，而要配合erase使用

Erase–remove idiom

Motivation[edit]

A common programming task is to remove all elements that have a certain value or fulfill a certain criterion from a

collection. In C++, this could be achieved using a hand-written loop. It is, however, preferred to use an algorithm from the C++ Standard Library for such tasks.[1][2][3]

The algorithm library provides the remove and remove_if algorithms for this. Because these algorithms operate on a range of elements denoted by two forward iterators,
they have no knowledge of the underlying container or collection.[1][4]
Thus, no elements are actually removed from the container. Rather, all elements which don't fit the remove criteria are brought together to the front of the range, in the same relative order. The remaining elements are left in a valid, but unspecified,
state. When this is done, remove returns an iterator pointing one element past the last unremoved element.[4][5]

To actually eliminate elements from the container, remove is combined with the container's
erase member function, hence the name "erase–remove idiom".

Limitation[edit]

Erase–remove idiom cannot be used for containers that return const iterator (e.g.:

set)[6]

Example[edit]

// Use g++ -std=c++11 or clang++ -std=c++11 to compile.
 
#include <vector> // the general-purpose vector container
#include <iostream>
#include <algorithm> // remove and remove_if
 
bool is_odd(int i)
{
  return (i % 2) != 0;  
}
 
void print(const std::vector<int> &vec)
{
  for (const auto& i: vec) 
    std::cout << i << ' '; 
  std::cout << std::endl;
}
 
int main()
{
  // initialises a vector that holds the numbers from 0-9.
  std::vector<int> v = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
  print(v);
 
  // removes all elements with the value 5
  v.erase( std::remove( std::begin(v), std::end(v), 5 ), std::end(v) ); 
  print(v); 
 
  // removes all odd numbers
  v.erase( std::remove_if(std::begin(v), std::end(v), is_odd), std::end(v) );
  print(v);
 
  return 0;  
}
 
/*
Output:
0 1 2 3 4 5 6 7 8 9 
0 1 2 3 4 6 7 8 9 
0 2 4 6 8 
*/