调用 erase() 后 std::map::iterator 出现问题

2022-01-10 00:00:00 iterator c++ 
// erasing from map
#include <iostream>
#include <map>
using namespace std;

int main ()
{
  map<char,int> mymap;
  map<char,int>::iterator it(mymap.begin());

  // insert some values:
  mymap['a']=10;
  mymap['b']=20;
  mymap['c']=30;
  mymap['d']=40;
  mymap['e']=50;
  mymap['f']=60;

  it=mymap.find('a');
  mymap.erase (it);                   // erasing by iterator

  // show content:
  for (; it != mymap.end(); it++ )
    cout << (*it).first << " => " << (*it).second << endl;
  return 0;
}

为什么会给出这样的输出

Why does this give an output like 

a => 10
b => 20
c => 30
d => 40
e => 50
f => 60

无论如何都不应该删除 "a => 10" ，但是如果我在 for 循环中声明 it = mymap.begin() ，一切都很完美.为什么?

shouldn't "a => 10" be deleted anyways, but if I declare it = mymap.begin() in the for loop, everything is perfect. why?

程序改编自:http://www.cplusplus.com/reference/stl/map/erase/

推荐答案

擦除 map 的元素会使指向该元素的迭代器无效(在所有该元素都被删除之后).您不应该重复使用该迭代器.

Erasing an element of a map invalidates iterators pointing to that element (after all that element has been deleted). You shouldn't reuse that iterator.

C++11起erase()返回一个指向下一个元素的新迭代器，可以用来继续迭代:

Since C++11 erase() returns a new iterator pointing to the next element, which can be used to continue iterating:

it = mymap.begin();
while (it != mymap.end()) {
   if (something)
      it = mymap.erase(it);
   else
      it++;
}

在 C++11 之前，您必须在删除发生之前手动将迭代器前进到下一个元素，例如:

Before C++11 you would have to manually advance the iterator to the next element before the deletion takes place, for example like this:

mymap.erase(it++);

这是因为 it++ 的后增量副作用发生在 erase() 删除元素之前.由于这可能不是很明显，因此应该首选上面的 C++11 变体.

This works because the post-increment side-effect of it++ happens before erase() deletes the element. Since this is maybe not immediately obvious, the C++11 variant above should be preferred.

相关文章