我如何改进这种设计,迫使我声明一个成员函数const和声明变量可变? [英] How can I improve this design that forces me to declare a member function const and declare variables mutable?
问题描述
由于某种原因,我在 std :: set
中的一个类的元素上进行迭代,并希望轻微修改键,知道顺序将不会改变。
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
如果密钥被修改,它可能导致一个坏的顺序,因此在设置损坏。但我知道我的操作不会改变我的元素在集合中的顺序。
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.
暂时,这里是我的解决方案:
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
}
如何修改它(我知道我可以使用 std :: map
但我很好奇是否可以建议其他选项)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?
/ p>
Thanks
推荐答案
你不能。 Set元素需要为容器正确性的const:
You can't. Set elements are required to be const for container correctness:
它强制您意识到关键部分需要是不可变的,结构不变式将被破坏。
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<
};
如果你想保留在非关键部分表达常量的可能性,将它分成对,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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