Does the ‘mutable’ keyword have any purpose other than allowing a data member to be modified by a const member function?

It allows the differentiation of bitwise const and logical const. Logical const is when an object doesn’t change in a way that is visible through the public interface, like your locking example. Another example would be a class that computes a value the first time it is requested, and caches the result.

Since c++11 mutable can be used on a lambda to denote that things captured by value are modifiable (they aren’t by default):

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

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