CODEWITHSUNDEEP
c++11mutex
Greedo
Greedo

Reputation: 5543

Function to return a lock guard wrapping a private mutex

I have a simple class with the following declared privately:

std::mutex headMutex;  
std::unique_lock<std::mutex> usingHeadNode(){return std::unique_lock<std::mutex>(headMutex);}

my aim is to provide a simple helper function that returns a unique_lock wrapping the mutex, so that my calling code can be

auto headGuard = usingHeadNode();

to lock the mutex with the RAII technique. However compiling this raises an error:

error: use of deleted function ‘std::lock_guard<_Mutex>::lock_guard(const std::lock_guard<_Mutex>&) [with _Mutex = std::mutex]

... which looks like the constructor for the lock_guard class is missing or something? I haven't deleted anything as far as I'm aware...

Update for more context:

My simple class is a linked list, I want to add a node which involves overwriting the saved head pointer, so I believe I need to lock it to make it safe to modify. A snippet from that add function (can't share the full code as it's an assignment):

// [...]
Node * pNewNode = new Node(value);
    
//Link it in by prepending
std::unique_lock<std::mutex> pHeadGuard(headMutex); //= usingHeadNode();
pNewNode->prev = nullptr; //since we prepend and this is the new head, there is no 'previous' element
pNewNode->next = pHead; //what was the head now comes after the new node
// [...]

pHead = pNewNode; // this is why we needed the mutex

and I'd like to change the locking line to something more readable by wrapping it in a function call somehow

Upvotes: 1

Views: 484

Answers (1)

davidbak
davidbak

Reputation: 6029

Your problem is that std::unique_lock is movable but not copyable. Knowing that about the semantics you can see why your code doesn't work - and what's more, you can decide if this technique is appropriate, or, possibly, prone to error.

Upvotes: 1

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