How to Safely Move C Types Containing `std::mutex`?

Handling Mutexes in Movable C Types

Overview:

In C , move semantics enhance efficiency by allowing types to be moved efficiently without copying. However, the standard mutex (std::mutex) is neither movable nor copyable, posing a challenge for implementing move-enabled types that leverage mutexes.

Problem:

How can a class (A) containing a mutex be made movable in a thread-safe manner?

Solution:

To enable moveability, define lock types (ReadLock and WriteLock) as aliases for shared or unique locks. Utilize these types to protect member accesses within move-related operations.

Move Constructor:

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

• Copy members can be optimized by sharing a mutex in C 14.
• Protect other methods that modify the state of A with locking mechanisms.
• Guard against self-swap by checking for equality prior to swapping.
• Consider using read and write lock data members to avoid default construction in copy/move constructors.

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