What are the Uses of the `mutable` Keyword in C Beyond Modifying Data Members in `const` Member Functions?

The Purpose of the 'mutable' Keyword Beyond Modifying Data Members in Const Member Functions

In the realm of C programming, the 'mutable' keyword has drawn attention for its ability to allow data members to be modified by const member functions. However, is there more to this enigmatic keyword than meets the eye?

The primary purpose of 'mutable' revolves around distinguishing between two types of constness: bitwise const and logical const. Logical constness signifies that an object remains conceptually unchanged, as perceived through its public interface. Consider the example of a mutex guarded by a const member function. Even though the mutex can be locked within the function, it does not alter the object's public behavior, hence it falls under logical constness.

Furthermore, in modern C (from C 11 onwards), 'mutable' has gained an additional use case. It can be applied to lambdas to indicate that captured variables originally declared as values can be modified. This is significant because, by default, lambda captures are immutable, meaning their values cannot be altered.

An example of this extended functionality is demonstrated below:

int x = 0;
auto f1 = [=]() mutable {x = 42;}; // OK
auto f2 = [=]()         {x = 42;}; // Error: cannot modify a captured value in a non-mutable lambda

By designating f1 as mutable, it becomes permissible to modify the captured value of x, while f2 fails due to the restriction on modifying immutable captures. This expanded functionality provides greater flexibility in lambda expressions.

In summary, 'mutable' plays a crucial role in both differentiating logical and bitwise constness and enabling the modification of captured variables in lambdas. These capabilities enhance code flexibility and allow for elegant and efficient solutions in C .

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