C++ unordered_set XXX in read-only object

古城微笑少年丶 2022-06-13 03:49 55阅读 0赞

还是使用struct作为unordered\_set的数据结构，假设我们有如下代码：

struct SentimentWord {
      string word;
      int count;
    };
    
    
    size_t SentimentWordHash::operator () (const SentimentWord &sw) const {
      return hash<string>()(sw.word);
    }
    
    
    bool operator == (SentimentWord const &lhs, SentimentWord const &rhs){
      if(lhs.word.compare(rhs.word) == 0){
        return true;
      }
      return false;
    } 
    
    int main()
    {
    //...
    //...
        unordered_set<SentimentWord, SentimentWordHash> positiveWords;
        unordered_set<SentimentWord, SentimentWordHash>::iterator iter;
        iter = positiveWords.find(temp);
        if(iter != positiveWords.end()){
          iter->count++;
        }
    }

理论上说，unordered\_set中的key一旦插入是不可修改的：

> ***In an unordered\_set, the value of an element is at the same time its key, that identifies it uniquely. Keys are immutable, therefore, the elements in an unordered\_set cannot be modified once in the container - they can be inserted and removed, though.***

考虑到底层红黑树的结构，修改key将会改变整个红黑树的构造。不过，我们已经将`unordered_set`的hasher设置为用户自定义，并且只使用其中的string成员作为hash函数的输入。

为了解决这个问题，可以做如下修改：

struct SentimentWord {
      string word;
      mutable int count;//使用mutable修饰
    };

也可以使用`unordered_map`把count作为value。

## **参考** ##

[https://stackoverflow.com/questions/16452378/how-to-make-an-iterator-to-a-read-only-object-writable-in-c][https_stackoverflow.com_questions_16452378_how-to-make-an-iterator-to-a-read-only-object-writable-in-c]

[https_stackoverflow.com_questions_16452378_how-to-make-an-iterator-to-a-read-only-object-writable-in-c]: https://stackoverflow.com/questions/16452378/how-to-make-an-iterator-to-a-read-only-object-writable-in-c