当你修改 std::set 的元素时会发生什么? [英] what happens when you modify an element of an std::set?

问题描述

If I change an element of an std::set, for example, through an iterator, I know it is not "reinserted" or "resorted", but is there any mention of if it triggers undefined behavior? For example, I would imagine insertions would screw up. Is there any mention of specifically what happens?

解决方案

You should not edit the values stored in the set directly. I copied this from MSDN documentation which is somewhat authoritative:

The STL container class set is used for the storage and retrieval of data from a collection in which the values of the elements contained are unique and serve as the key values according to which the data is automatically ordered. The value of an element in a set may not be changed directly. Instead, you must delete old values and insert elements with new values.

Why this is is pretty easy to understand. The set implementation will have no way of knowing you have modified the value behind its back. The normal implementation is a red-black tree. Having changed the value, the position in the tree for that instance will be wrong. You would expect to see all manner of wrong behaviour, such as exists queries returning the wrong result on account of the search going down the wrong branch of the tree.

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