Intended design for use within const methods?

[MHebes]

Ran into a problem. I wanted to use a plain_guarded to wrap a member variable. I have a getter to return a copy of that member variable:

class thing {
 private:
  plain_guarded<int> x_;
 public:
  int get_x() const {
    return *x_.lock(); // error! discards qualifiers
  }
};

I can't do this, because x_.lock() is not a const operation. I can const_cast x_, or make it mutable. The former is unergonomic and slightly dangerous, and the latter is dangerous because it too easily allows mutation of the underlying variable.

I suggest a const version of the lock() method (and friends) which returns a std::unique_ptr<const T, deleter> (perhaps aliased to a const_handle, like iterator/const_iterator). This would push the mutable down into the internal mutex member instead of users needing to care.

const methods that mutate an internal mutex has precedent in the std::atomic<T>::load() method, which is marked const despite mutating its internal mutex (in the case of non-lock-free instantiations).

[agserm]

Thanks for the suggestion. This has been added in 707efec, let me know if this implementation addresses your use case.

[agserm]

Closing as resolved, feel free to reopen if there are any remaining questions.