How Can I Make My C Class with a Mutex Movable and Thread-Safe?

Handling Mutexes in Movable Types in C

Issue:

Due to the non-copyable and non-movable nature of std::mutex, classes holding mutexes lack default move constructors, making it challenging to create movable thread-safe types.

Solution:

To achieve thread-safe move semantics for class A holding a mutex, adopt the following strategies:

Move Constructor:

A(A&& a) {
    WriteLock rhs_lk(a.mut_); // Lock the rhs mutex
    field1_ = std::move(a.field1_); // Move the data
    field2_ = std::move(a.field2_);
}

Move Assignment Operator:

A& operator=(A&& a) {
    if (this != &a) {
        WriteLock lhs_lk(mut_, std::defer_lock);
        WriteLock rhs_lk(a.mut_, std::defer_lock);
        std::lock(lhs_lk, rhs_lk);
        field1_ = std::move(a.field1_);
        field2_ = std::move(a.field2_);
    }
    return *this;
}

Copy Constructor:

A(const A& a) {
    ReadLock rhs_lk(a.mut_); // Lock the rhs mutex for reading
    field1_ = a.field1_; // Copy the data
    field2_ = a.field2_;
}

Copy Assignment Operator:

A& operator=(const A& a) {
    if (this != &a) {
        WriteLock lhs_lk(mut_, std::defer_lock);
        ReadLock rhs_lk(a.mut_, std::defer_lock);
        std::lock(lhs_lk, rhs_lk);
        field1_ = a.field1_;
        field2_ = a.field2_;
    }
    return *this;
}

Other Considerations:

• Define a ReadLock and WriteLock type alias for convenience.
• Remember to lock both mutexes using std::lock for the move assignment operator to avoid deadlock.
• Consider using std::shared_timed_mutex in C 14 to allow for concurrent reads.
• Protect any member functions or free functions accessing the internal state of class A using the appropriate mutex locks.
• If necessary, add data member locks to store ReadLock and WriteLock objects to avoid default constructing members during copying.

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