STL Map 无Reserve和Resize的原因 / Why STP MAP can not provide function Reserve() and Resize()

今天翻论坛,看到一些帖子问为什么STL Map不提供Reserve函数和Resize函数. 这个问题从MAP的实现比较容易理解.
MAP的低层实现是红黑树,要求每个节点都有一个不重复的key值. 如果提供Reserve和Resize功能，那么就要预分配一部分节点, 那么这些节点的key值设置成什么呢? 全为0不满足节点key不能重复的要求，又不能随即设置,因为那样会导致map根据key值重新排列. 所以MAP不可能可以提供Reserve和Resize的功能.

When reading bbs today, I found some ppl asking why STL map does not provide function Reserve() and Resize(). Considering this question from the technique machenism of STL MAP, the reason could be clear.

The low level data structure of STL map is RB Tree, it requires every node of it must have a unique key. If Reserver() and Resize() is provided, then the tree need pre-allocate some nodes. But how to initialize the key of this new nodes? If they were all filled zero, then the keys are repeated; If the are other random or different keys, it will cause a much more bad result: the tree need reassign the position of the nodes according to the keys.

So map must not provide function Reserver() and Resize().