How Can I Make My C Classes with Non-Movable Mutexes Both Movable and Thread-Safe?

Thread-Safe Movability with Non-Movable Mutexes in C

When dealing with movable types, such as std::mutex, in C thread-safe programming can be challenging. By default, std::mutex is neither movable nor copyable, hindering the creation of thread-safe movable objects.

To make a class containing a mutex movable and maintain thread safety, consider the following approach:

1. Enable Mutability:
Make the mutex member mutable (e.g., mutable std::mutex mut_;) to allow modification during move operations.

2. Move Constructor:
Lock the mutex of the object being moved from before assigning its members. This prevents data corruption from concurrent access.

A(A&& a) {
    WriteLock rhs_lk(a.mut_);
    field1_ = std::move(a.field1_);
    field2_ = std::move(a.field2_);
}

3. Move Assignment Operator:
Protect the move assignment using synchronization primitives. Since access from multiple threads is possible, lock both the left-hand side (lhs) and right-hand side (rhs) mutexes before performing assignments.

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;
}

4. Copy Members (Optional):
If copy members are also required, use ReadLock (instead of WriteLock) to allow concurrent reading from the object being copied from.

A(const A& a) {
    ReadLock rhs_lk(a.mut_);
    field1_ = a.field1_;
    field2_ = a.field2_;
}

5. Additional Considerations:

• Protect other member functions and free functions accessing the class state with appropriate synchronization to maintain thread safety.
• Consider using std::shared_timed_mutex or another advanced synchronization type to optimize for specific scenarios in C 14 and later.

By following these guidelines, you can ensure that your movable classes remain thread-safe even when using non-movable mutexes.

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