在C 中，mutable關(guān)鍵字用於允許修改對(duì)象的特定數(shù)據(jù)成員，即使該對(duì)像被聲明為const。其核心用途是保持對(duì)象邏輯上的常量性同時(shí)允許內(nèi)部狀態(tài)變化，常見(jiàn)於緩存、調(diào)試計(jì)數(shù)器和線程同步原語(yǔ)。使用時(shí)需將mutable置於類(lèi)定義中的數(shù)據(jù)成員前，僅適用於數(shù)據(jù)成員而非全局或局部變量。最佳實(shí)踐中應(yīng)避免濫用、注意並發(fā)同步，並確保外部行為不變。例如std::shared_ptr用mutable管理引用計(jì)數(shù)以實(shí)現(xiàn)線程安全與const正確性。

In C , the mutable keyword is used to allow a specific data member of an object to be modified, even if the object itself is declared as const . This might sound contradictory at first — after all, a const object shouldn't change, right? But there are cases where you want most of an object to remain constant while still allowing certain internal values to vary. That's where mutable comes in handy.

When and Why You'd Use mutable

The main use case for mutable is when you have a class that needs to maintain some internal state that doesn't affect the logical constness of the object. For example:

• Caching : You might compute a value once and cache it, but you want to store the result without marking the whole object as non-const.
• Debug counters or logging : You may want to track how many times a const function was called, but not consider that counter part of the object's observable state.
• Thread synchronization primitives : Like mutexes inside a const object that need to lock/unlock internally.

This lets you keep your interface const-correct while still being able to make internal modifications.

How to Apply mutable

Using mutable is straightforward — just place it before the data member declaration in the class definition. Here's a basic example:

 class Example {
public:
    int getValue() const {
          callCount; // allowed because callCount is mutable
        return value;
    }

private:
    int value = 42;
    mutable int callCount = 0;
};

In this case:

• The getValue() method is marked const , meaning it promises not to change the object.
• callCount is marked mutable , so even though we're incrementing it inside a const method, the compiler won't complain.

You can only apply mutable to data members, not to top-level variables or local variables.

Common Pitfalls and Best Practices

Here are a few things to watch out for when using mutable :

• ? Don't overuse it – If everything is mutable, then const becomes meaningless. Only apply it where the modification truly doesn't change the object's externally visible behavior.
• ?? Be careful with concurrency – Even if a member is logically okay to mutate, you may still need synchronization (like a mutex) if your code is multi-threaded.
• ? Use it for encapsulated logic – It works best when the mutable state is something internal like a cache or counter that users of the class don't care about directly.

One real-world example is the standard library's std::shared_ptr , which uses mutable under the hood to manage reference counts in a thread-safe way without breaking const correctness.

基本上就這些。

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