std :: map，指向map key的值，这是可能吗？ [英] std::map, pointer to map key value, is this possible?

问题描述

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;

最后一行是否有效？
我想把这个指针存放在别的地方，它会对整个程序的生命有效吗？
但是，如果我添加一些更多的元素到这个地图（使用其他唯一的键）或删除一些其他的键，将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，地图保证稳定;即迭代器不会通过元素插入或删除（除了元素被删除当然）无效。

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，迭代器的稳定性不能保证指针的稳定性！尽管通常情况下大多数实现都使用指针（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正常工作是非常安全的），您应该真正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，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;
}

然后存储而不是字符串指针。

And then store that instead of a string pointer.

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