Does the 'mutable' keyword have any purpose other than allowing a data member to be modified by a const member function?
c++, class, datamember, keyword, mutable
Solution
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
Problem
A while ago, I came across some code that marked a data member of a class with the `mutable` keyword. As far as I can see it simply allows you to modify a member in a `const`-qualified member method: ``` class Foo { private: mutable bool done_; public: void doSomething() const { ...; done_ = true; } }; ``` Is this the only use of this keyword, or is there more to it than meets the eye? I have since used this technique in a class, marking a `boost::mutex` as `mutable`, allowing `const` functions to lock it for thread-safety reasons, but, to be honest, it feels like a bit of a hack.