映射与const键，但非const值? [英] map with const keys but non const values?

问题描述

我遇到的情况是，我希望有一个映射，该映射不允许在初始化后添加/删除键，但是允许更改值(因此，我不能简单地使映射const).即

I have a situation, where I would like to have a map that does not allow to add/remove keys after initialization, but the values are allowed to change (thus I cannot simply make the map const). Ie

/*semi-const*/ map<int,int> myMap = initMap();

myMap[1] = 2;                  // NOT OK, because potentially adds a new key
myMap.at(1) = 2;               // OK, because works only if key is present
for (auto & element : myMap) {
    element.second = 0;        // OK, values may change
}

我可以为std::map编写自己的包装器，但是我感觉这不太常见，所以我想知道是否已经存在解决方案.

I could write my own wrapper for std::map, but I have the feeling that it is something not too uncommon, so I wonder if there is already an existing solution.

地图上是否有一些标准的习惯用法，不允许添加/删除键，而值可能会更改?

ps:我知道单独的标题有点含糊，因为键在地图中已经是const了，但是我希望我的意思很清楚...

ps: I know that the title alone is a bit vague, because the keys are already const in a map, but I hope it is clear what I mean...

推荐答案

是否可以创建一个包装器，该包装器包含允许在const时更改该值的值并将其放在map中?像这样:

Could you create a wrapper that contains the value that allows the value to be mutated when const and put that in the map instead? Something like:

template<typename T>
class Mutable {
    mutable T value;
public:
  const Mutable& operator=(const T& v) const { value = v; return *this; }
  T& get() const { return value; }  
};

然后您的地图可以是类型

Then your map can be of type

const std::map<int, Mutable<int>>

实时演示.

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