How to Implement Thread-Safe Move Semantics for Classes Containing Mutexes?

Thread-safe Move Semantics for Objects Containing Mutexes

Problem:

Standard library mutexes (std::mutex) are neither copyable nor movable, leading to the absence of default move constructors for classes containing mutexes. This poses a challenge for achieving movable types in a thread-safe manner.

Solution:

To make a class containing a mutex movable while ensuring thread safety, the following steps can be taken:

class A
{
    using MutexType = std::mutex;
    using ReadLock = std::unique_lock<MutexType>;
    using WriteLock = std::unique_lock<MutexType>;

    mutable MutexType mut_;
    // Other member variables...
};

Move Constructor:

A(A&& a)
{
    WriteLock rhs_lk(a.mut_);
    // Copy or move member variables under rhs_lk protection.
}

Move Assignment:

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);
        // Copy or move member variables under combined lock.
    }
    return *this;
}

Copy Constructor:

A(const A& a)
{
    ReadLock rhs_lk(a.mut_);
    // Copy member variables under rhs_lk protection.
}

Copy Assignment:

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);
        // Copy member variables under combined lock.
    }
    return *this;
}

These thread-safe move and copy operations preserve the mutex's integrity and allow for reliable modification and access to member variables under lock protection.

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