如果在从头到尾迭代时在 map 元素上调用 erase() 会发生什么?

2022-01-07 00:00:00 iterator c++ stl

在下面的代码中，我遍历地图并测试是否需要删除元素.擦除元素并继续迭代是否安全，或者我是否需要在另一个容器中收集密钥并执行第二个循环来调用 erase()?

In the following code I loop through a map and test if an element needs to be erased. Is it safe to erase the element and keep iterating or do I need to collect the keys in another container and do a second loop to call the erase()?

map<string, SerialdMsg::SerialFunction_t>::iterator pm_it;
for (pm_it = port_map.begin(); pm_it != port_map.end(); pm_it++)
{
    if (pm_it->second == delete_this_id) {
        port_map.erase(pm_it->first);
    }
}

更新:当然，然后我阅读这个问题，但我没有认为会相关，但回答了我的问题.

UPDATE: Of course, I then read this question which I didn't think would be related but answers my question.

推荐答案

C++11

这已在 C++11 中得到修复(或擦除已在所有容器类型中得到改进/保持一致).
擦除方法现在返回下一个迭代器.

C++11

This has been fixed in C++11 (or erase has been improved/made consistent across all container types).
The erase method now returns the next iterator.

auto pm_it = port_map.begin();
while(pm_it != port_map.end())
{
    if (pm_it->second == delete_this_id)
    {
        pm_it = port_map.erase(pm_it);
    }
    else
    {
        ++pm_it;
    }
}

C++03

擦除地图中的元素不会使任何迭代器失效.
(除了被删除元素的迭代器)

C++03

Erasing elements in a map does not invalidate any iterators.
(apart from iterators on the element that was deleted)

实际上插入或删除不会使任何迭代器失效:

Actually inserting or deleting does not invalidate any of the iterators:

另见这个答案:
马克勒索技术

但您确实需要更新您的代码:
在您的代码中，您在调用擦除后增加 pm_it.此时为时已晚，已经失效.

But you do need to update your code:
In your code you increment pm_it after calling erase. At this point it is too late and is already invalidated.

map<string, SerialdMsg::SerialFunction_t>::iterator pm_it = port_map.begin();
while(pm_it != port_map.end())
{
    if (pm_it->second == delete_this_id)
    {
        port_map.erase(pm_it++);  // Use iterator.
                                  // Note the post increment.
                                  // Increments the iterator but returns the
                                  // original value for use by erase 
    }
    else
    {
        ++pm_it;           // Can use pre-increment in this case
                           // To make sure you have the efficient version
    }
}

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