item-16-make-const-member-functions-thread-safe

Item 16. Make const member functions thread safe

Test Codes

Notes

• classic use case for mutable
  The roots() function doesn't change the Polynomial object on which it operates, but, as part of its caching activity, it may need to modify rootVals and rootsAreValid.

    class Polynomial {
    public:
        using RootsType = std::vector<double>;

        RootsType roots() const {
            std::lock_guard<std::mutex> l(m);   // lock mutex to make sure thread safe

            if (!rootsAreValid) {
                // ... calculation

                rootsAreValid = true;
            }
            return rootVals;
        }

    private:
        mutable std::mutex m;
        mutable bool rootsAreValid {false}; 
        mutable RootsType rootVals{};
    }

• It's worth noting that because std::mutex can be neither copied nor moved, a side effect of adding m to Polynomial loses the ability to be copied and moved.

• A lesson: for a single variable or memory location requiring synchronization, use of a std::atomic is adequate, but once you get to two or more variables or memory locations that require manipulation as a unit, you should reach for a mutex.

  • Use of std::atomic variables may offer better performance than a mutex, but they're suited for manipulation of only a single variable or memory location.

• Make const member functions thread safe unless you're certain they'll never be used in a concurrent context.

References

• Race condition