std::map，指向映射键值的指针，这可能吗?

2021-12-10 00:00:00 pointers c++ map stl 
std::map<std::string, std::string> myMap;

std::map<std::string, std::string>::iterator i = m_myMap.find(some_key_string);
if(i == m_imagesMap.end())
    return NULL;

string *p = &i->first;

最后一行有效吗?我想将此指针 p 存储在其他地方，它对整个程序生命周期都有效吗?但是如果我向这个映射添加更多元素(使用其他唯一键)或删除一些其他键会发生什么，它会不会重新分配这个字符串(键值对)，所以 p 将变得无效?

Is the last line valid? I want to store this pointer p somewhere else, will it be valid for the whole program life? But what will happen if I add some more elements to this map (with other unique keys) or remove some other keys, won’t it reallocate this string (key-value pair), so the p will become invalid?

推荐答案

首先保证地图稳定；即迭代器不会因元素插入或删除而失效(当然被删除的元素除外).

First, maps are guaranteed to be stable; i.e. the iterators are not invalidated by element insertion or deletion (except the element being deleted of course).

然而，迭代器的稳定性并不能保证指针的稳定性！尽管大多数实现通常会使用指针 - 至少在某种程度上 - 来实现迭代器(这意味着假设您的解决方案可以工作是非常安全的)，您真正应该存储的是迭代器本身.

However, stability of iterator does not guarantee stability of pointers! Although it usually happens that most implementations use pointers - at least at some level - to implement iterators (which means it is quite safe to assume your solution will work), what you should really store is the iterator itself.

您可以做的是创建一个小对象，例如:

What you could do is create a small object like:

struct StringPtrInMap
{
  typedef std::map<string,string>::iterator iterator;
  StringPtrInMap(iterator i) : it(i) {}
  const string& operator*() const { return it->first; }
  const string* operator->() const { return &it->first; }
  iterator it;
}

然后存储它而不是字符串指针.

And then store that instead of a string pointer.

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