How Does the `mutable` Keyword Impact Constness in C and Lambda Expressions?

Mutable Keyword: Beyond Constant Member Function Modification

In C , the mutable keyword is primarily known for permitting modifications to data members within const-qualified member functions. This feature enhances flexibility by enabling const methods to alter specific members as required.

However, the mutable keyword has an additional significance. It aids in distinguishing between bitwise and logical constness. Logical constness refers to situations where an object's state remains consistent from the perspective of its public interface. A typical example is locking a mutex inside a const function to ensure thread safety.

Moreover, with the introduction of C 11, mutable can be applied to lambda expressions. This allows captured values to be modified, as they are by default immutable. For instance:

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

In this example, f1 can modify the captured x value since it is declared as mutable, while f2 cannot because it follows the default value-capture behavior. This distinction provides greater control over the modifiability of captured variables in lambda expressions.

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