如何改进这种强制我声明成员函数 const 并声明变量可变的设计?

2022-01-17 00:00:00 set mutable c++

由于某种原因，我正在迭代 std::set 中的类元素，并且想稍微修改键，因为我知道顺序将保持不变.

For some reason I am iterating over elements of a class in an std::set and would like to slightly modify the keys, knowing that the order will be unchanged.

std::set 上的迭代器是 const_iterators，因为如果修改了键，可能会导致顺序错误，从而导致集合损坏.但是我确信我的操作不会改变集合中元素的顺序.

Iterators on std::set are const_iterators because if the key is modified, it might result in a bad order and therefore in set corruption. However I know for sure that my operations won't change the order of my elements in the set.

目前，这是我的解决方案:

For the moment, here is my solution: 

class Foo
{
public:
    Foo(int a, int b): a_(a),b_(b) {}
   ~Foo(){}
    bool operator < (const Foo& o) const { return this.a_ < o.a_ ; }
    void incrementB() const { ++b_; } // <-- the problem: it is not const!
private:
    const int a_;
    mutable int b_;                   // <-- I would like to avoid this
}

void f()
{
    std::set<Foo> s;
    // loop and insert many (distinct on a_) Foo elements;
    std::for_each(s.begin(), c.end(), [](const Foo& s) { s.incrementB(); }); // Foo must be const. iterators are const_iterators
}

您将如何修改它(我知道我可以使用 std::map 但我很好奇您是否可以建议其他选项)来删除 mutable 和 const?

How would you modify it (I know I could use an std::map but I am curious whether you can suggest other options) to remove mutable and const?

谢谢

推荐答案

你不能.为了容器的正确性，集合元素必须是 const:

You can't. Set elements are required to be const for container correctness:

它迫使你意识到关键部分需要是不可变的，否则数据结构不变量会被破坏.

It forces you to realize that the key part needs to be immutable, or the data structure invariants would be broken.

struct element 
{
     std::string key_part; // const in the set

     bool operator<(const element&o) const { return key_part<o.key_part; }

  private:
     mutable int m_cached; // non-key, *NOT* used in operator<
};

如果您想保留在非关键部分表达"常量的可能性，请将其拆分成对并将它们存储在映射中:

If you wanted to retain the possibility to 'express' const-ness in the non-key part, split it out into pairs and store them in a map:

std::map<std::string /*key_part*/, int /*m_cached*/> mapped;

或者，更灵活:

struct element 
{
     std::string key_part; // const in the set

     bool operator<(const element&o) const { return key_part<o.key_part; }

     struct value {
         int m_cached;
         int m_moredata; //...
     } /*not in the element itself*/;
};

std::map<element, element::value> mapped;

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