Copying RAII object in C++

Question

I am reading Effective C++, in Rule 14: Think carefully about copying behavior in resource-managing classes, there is an example:

class Lock {
  public:
    explicit Lock(Mutex* pm) : mutexPtr(pm) {
      lock(mutexPtr);
    }
    ~Lock() {
      unlock(mutexPtr);
    }
  private:
    Mutex *mutexPtr;
};

It points out that if we construct the Lock as above, there will be a problem if we run the code below:

Mutex m;
Lock ml1(&m);
Lock ml2(ml1);

I think it may because the code may runs like below:

// ml1 constructes
lock(m)
// copy ml2, but ml1.mutexPtr and ml2.mutexPtr both point to m
ml2.mutexPtr = ml1.mutexPtr
// ml1 destructs
unlock(m)
// ml2 destructs
unlock(m)

So the m will be unlock for twice. So is it the real reason that cause the problem below? thx!

Which version of the book is this? "Item 14" in my version is about virtual destructors... — Del, Dec 11 '17 at 01:53
Effective C++: 55 Specific Ways to Improve Your Programs and Designs [Version 3] — Caesium, Dec 11 '17 at 01:58

score 0 · Answer 1 · answered Dec 11 '17 at 02:45

Yes, that is why the author is saying to be careful. If you use a recursive_mutex instead of plain mutex you can simply lock in the copy ctor and copy-assign operator, but if it's a non-recursive mutex it would likely be better to make the lock type non-copyable.

Copying RAII object in C++

1 Answers1