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Home Backend Development C++ What is the mutable keyword in C ?

What is the mutable keyword in C ?

Jul 12, 2025 am 03:03 AM
c++ mutable

In C, the mutable keyword is used to allow the object to be modified, even if the object is declared as const. Its core purpose is to maintain the logical constants of the object while allowing internal state changes, which are commonly found in cache, debug counters and thread synchronization primitives. When using it, mutable must be placed before the data member in the class definition, and it only applies to data members rather than global or local variables. In best practice, abuse should be avoided, concurrent synchronization should be paid attention to, and external behavior should be ensured. For example, std::shared_ptr uses mutable to manage reference counting to achieve thread safety and const correctness.

What is the mutable keyword in C?

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 contrastory 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.

What is the mutable keyword in C?

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 consist of the object. For example:

What is the mutable keyword in C?
  • 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:

What is the mutable keyword in C?
 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 increasing 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 OK 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.

Basically that's it.

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